Towing heavy objects up the Black Hills

Reprise

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#1
Ok. So almost everyone who's a regular here has probably seen my posts regarding my setup, so I won't post that yet again.

Some observations during this trip to Montana, with about 5000# of trailer & 'stuff' in tow...

Today covered basically the state of South Dakota; I woke up in the east end and am writing this from the west end (past the Badlands). Needed to get through SD today to stay on track for Friday arrival at my destination.

- I'm using about 1/2 qt of oil per day. I expected to use some, based on history - the only 'surprise' is how quickly it's being consumed.

Today was a long towing day, and I just added a qt this morning before starting out. So I'll be interested to see the results of tomorrow's level check. Getting the feeling that when the engine goes above 3,000 RPM, I could drive behind the Voy and watch the blow-by (I don't see a thing out of the tailpipe, and as a reminder, I've got about 145K on the engine (only the last 12K is 'mine', though)). Unfortunately, at 65mph, the engine is running just shy of 3K on the tach. More on this later. BTW - speed limit in SD is 80mph. I'm not supposed to go more than 65mph due to the China Bombs on the new trailer.


- I noticed this morning that I'm actually losing a little coolant. Before this trip, my coolant level was steady as the proverbial rock.
Glanced at it yesterday before starting out, and I noted it had dropped a little bit (but nothing substantial).
Glanced at it this morning, and I'd lost a little more. Wound up making a detour to WallyMart for a jug of DexCool premix. Shoulda brought some from home instead of assuming that I wouldn't have any cooling issues (towing in 90-100F heat). When I added it to the reservoir, I wound up not having to add much (and while I'm thinking of it - just *where* is the 'full hot' indicator on the bottle? I can see the 'full cold', but I didn't have it to add, then.

BTW, my coolant temps have been pretty good for the most part. A couple of episodes where I was at 220 / 230F max - but for the most part, I'm generally a shade past 200 to about 215 (my normal is 195, IIRC). Wondering if I've got a leak, a potential head gasket issue (hate to even mention that), or is it expected to lose coolant under severe conditions (gonna get more severe in the next day or two; I'm at about 3000ft elevation now, and my destination is over 5000ft.)

And finally, my trans temps. All over the place. Part due to ambient; part due to load, and perhaps part due to cooler size. And part due to my desire to experiment.

When I started out a few days ago, I drove to the REI in Madison, WI to pick up a Nat'l Parks pass (pays for itself after about the 3rd place visited).
I made that 70mi trip in 'D'. Temps by the end of the drive had risen to just shy of 260F (nothing burnt or broke, but I hope to never see 260F again).
Fluid is Valvoline Dexron Full Synthetic (12qts worth). My external cooler, for those who don't remember, is a Derale stacked plate - but it's the little one (10 plates), and the stock cooler was bypassed by the mech who installed the rebuilt trans (who told me, when I wanted to get a bigger cooler to compensate - "you'll never need one bigger than that".) I think I'm starting to disagree with his statement.

After keeping the trans in 3rd since that little 'experiment', my temps were steady as a rock - less than 180F (my non-towing temp is 155-160F). So - not bad, considering. Until today. Still in 3rd, I managed to hit 230 during the hottest part of the day (and after an extended side trip to the WM, as mentioned above.) I did stop at a rest area and let the Voy cool off for about 45min or so.

Afterward, by dropping my speed and maintaining a rock steady RPM, I was able to get the fluid temp down to a more manageable 185 or so. The funny thing was...as the day went on, ambient got cooler and I was maintaining a locked throttle - I was able to navigate some of the steepest climbs of the day - with the fluid temp going all the way down to 164.x - *during* the climbing. The 'usual' is about 175F or a little higher (which I mentioned above) - up to maybe 188F.

So... I'm wondering... what's my next steps...
- Get a bigger cooler? I don't mind doing this, but I want to be sure it'll make a difference before spending the $. And yes, I'll sell my current one if someone wants it.

- Tow less weight? This would be impossible w/o taking a bath on the trailer (I don't have payments, and got a very good deal - but it would be bad).
- Time to step up to a Silvy / Sierra? I could see perhaps towing bigger / heavier later on - but not for a couple of years.

- Tow at night? Awkward, to say the least. Same for "don't climb hills". Illinois is as flat as a pancake, but the rest of the country isn't.

For now, I'll just limit my speed / RPM. I'm kind of stuck until I get back from this trip, anyway.

Closing for now, as they're about to close the place I'm writing this from (I've had very limited internet access the last few days). So I'll put an e-mail watch on this in order to see replies there. Hope everyone is doing OK. Talk at you soon!
 

cornchip

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#2
I practice what I say in my mind before shifting into gear.....3 not D. My recent drive to Wisconsin made me fully aware of where my transmission is headed if I continue to use 4th. My trip out without the trailer was done in 4th at 75mph most of the way. My TCC was already slipping at speed and throwing 1870 and 894 above 55mph. When I arrived I checked the fluid level and noticed it wasn't burnt, but it reeked of lining material (TCC disc or 4th band...not sure). I towed back with the trailer in 3rd all the way at 65mph and had no issues with the exception of a new code...741. I have learned to just slow it a bit on extremely long grades near the top to keep the 741 away (cruise still works with 741 'on' and shouldn't). As far as transmission temps go, it never goes beyond 180 even with a B&M 11" square stacked plate cooler routed after passing through the radiator cooler first. I estimate my towed weight to be approximately 2250 lbs and extremely draggy for comparison.

My damage is done....I'm just living with the lack of a truly functional OD for now. Fortunately the bulk of my commuting is local country roads at 50mph in 3rd gear and we have a second vehicle better suited for highway use if need be. I've noticed no real drop in fuel mileage (it's always poor! with the 4.2). The 4L60e is just a piss poor unit IMHO. I look forward to replacing this truck with something more durable in the future.

Hope you make it back!
 
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#3
Finally, wi-fi access again -- if only for tonight! :woohoo: (on edit: fell asleep writing, so posting 'next morning')


So after a few more days towing, including today, here's the 'secret' of keeping the 4L60E happy during towing...

Keep the trans from dropping down to 2nd as much as possible during climbs, and especially stay below 4K RPM.

There are techniques I polished over the last few days which I could go into, but it all boils down to the above.
One thing I discovered that I'll mention now is 3rd gear lockup. I can't 'command' it at will, but when you've dropped out, and only initially jump 200 RPM instead of 1000--it's a godsend.

Last Friday, I was in the opposite end of Montana ('kitty corner') from where I needed to be by EOD - a 450+ mile drive. I made it in time, but it was a 11+hr drive, climbing for significant portions, especially at the end.

The good news there is that I was largely able to regulate the trans temps - most of the time I was under 200F - and the few times I wasn't, it wasn't up there for long. If you can get a long stretch after a climb, keeping the tach to under 3K (which for my LWB w/ 3.73, is about 65mph), the fluid will cool back down very nicely--assuming you have a good / large enough external cooler.

That being said, there are exceptions to the rule, and today yesterday was one of them.

After heading on downhill grades most of the day, I looked for a nice place to put the trailer last night (for those who RV / TT - I've been using a smartphone app called 'RV Parky' - which works whether or not you have wi-fi / cellular service (it'll even work w/o GPS location, but if it can grab that signal, you've got everything you need (it has preloaded content, so works even w/o GPS signal). Highly recommended, if you're looking for something new / better to find parking spots than what you have now.

Anyway, after almost running out of gas--long & somewhat funny story, which I'll save for another time--I found the best rated place near where I was that had full hookups, wi-fi, etc. I'd been dry camping since Friday, thanks to NPS--and tonight, I was going to get (halfway) back to 'civilization'.

Was in Helena, right off I-15. My destination was right off I-90 -- so if I had gotten back on 15, I probably would've had an easier time. Instead, Google Maps told me to take US12 there, so I did. Because of that, from what I can tell, I climbed 2800 ft in about 8 miles (idea for future: dash mount altimeter.) I wound up stopping at the summit -- although I didn't know it at the time -- and shutting the Voy off for 15min, as the trans temp had risen to 258 (!) As it turns out, I was able to get it cooled back down from 230-ish when I restarted the truck, all the way back down to 175 or so by the time I got down the other side of the summit.

In retrospect, I don't think I could've done anything differently. There comes a point where you can only lift your foot so much / so many times to force the trans to upshift again. In my case, after about 15sec of instantaneously going back / forth between 2nd / 3rd gear no matter how gentle I was with the pedal, I decided to grit my teeth and hope the end of the climb was just around the corner (it wasn't, by a long shot). Ironically, I had just poured a quart of oil in a few minutes earlier. Betting that's gone now when I check the level this morning.

In closing today's post, I'll reply to @cornchip 's assertion that the 4L60E is a 'piss poor' unit...in light of what's available now, including the GM/Ford 10-speed that seems to be all the rage -- yes, better choices are available, *now*. 'Hindsight is 20/20', and all that. FWIW, I *still* like (respect?) the 4L60e. That's all I'll say for now - no sense making this post into 'ode to the 4L60e' ...lol.

I'll admit, I'm starting to think about what a couple of extra gears in a 6L80 could provide for towing (and yes, @cornchip , longevity...lol.) And I'm probably going to bypass swinging through Colorado to Grays Peak (highest place on the Continental Divide; approx 12,9xx ft elevation) for now. Not going to assume unnecessary risk for the sake of saying 'I did it' / "It's possible". The little Envoy that could (did) is going back to the garage first for 'stage 2'. :book: It's handled 8K elevation w/ 5K in tow as-is; no sense potentially burning up this trans as an experiment.

Thinking seriously about a short hop to Yellowstone and hope I can get a walkup spot for the trailer this time of year ('futility', from what I hear), even though it kills me to spend the gas to come out here a second time when I'm practically there now.

More later. Onward!
 

cornchip

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#4
Been to Colorado to Jeep and ride dual sport bikes over most of the mountain passes. Has to be one of my most favorite things to do. Such an amazing area to visit.

In closing today's post, I'll reply to @cornchip 's assertion that the 4L60E is a 'piss poor' unit...in light of what's available now, including the GM/Ford 10-speed that seems to be all the rage -- yes, better choices are available, *now*. 'Hindsight is 20/20', and all that. FWIW, I *still* like (respect?) the 4L60e. That's all I'll say for now - no sense making this post into 'ode to the 4L60e' ...lol.
The 4l60e had such a long build life that I find it hard to forgive the small things that could have been avoided. It's a pattern of thinking that will always haunt GM....and likely with the newer transmissions. Comes down to penny pinching I suppose at the manufacturing level. Piss poor might be a over reaction and I'll leave it at that. I still like my GMT360 despite it's flaws.
 

littleblazer

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#5
The 4l60e came to be from the 700r4... how gm replaced the turbo 350 and 400 with that crap is still beyond me. The 4l80e is one hell of a trans though. (The red truck loses od and 4th but considering it had 210k of being matted I give it a break. With only 1 fluid change and pushing around 6k plus all the time it's like it was mildly towing its whole life.) Just think, it could be a dodge. :raspberry:

For the coolant loss, how is your water pump if I missed that part. Mine lost a little for a while then finally took a dump.
 

shovenose

Well-Known Member
#6
I think when serviced every 30-60k, kept cool, driven gently, never used for towing/hauling, and luck, a 4l60e will last forever. the problem is, these vehicles are designed for towing, hauling, etc. so they should have used a stronger transmission.
 
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#7
Online again tonight (but still far from home). Reading a bit more about towing in general and starting to think I just need 'more truck'. But for now, I'm planning the following for 'phase 2' -

- Install the TBSS intake, throttle body and fuel rail I got from a guy on Craigslist a few months back (I'd need to get new injectors.) Supposedly, the TBSS intake is one of the best ones you can put on an LS-style engine, especially as a 'bang for buck' option. Estimated I might gain 30-40 HP from this alone (!)

- Pick up a new cam / lifters from Texas Speed (they have a custom / recommended grind for tow vehicles that still keeps the engine very streetable -- no / very little lope -- although they told me it'll need a dyno tune afterward, no matter what). This runs about $500 (excluding dyno time), and I'm pretty sure I could install it. IIRC, they estimated addn'l 50+ HP with this grind.

- Perhaps a set of headers (I had purchased a chromed set of the 'shorty' type (no downtubes; terminate at the collector), then decided to return them for same item but ceramic coated; the reseller said the vendor came back with 'backorder', and they just refunded my $. I've since found out the shorties aren't considered to be very good (but I don't want to spend $$$$ for a full custom exhaust at this point - and it *would* be custom - no one makes a non-TBSS solution any longer.)

- Bite the 4wd bullet and pick up a 4.10 axle for the rear while dumping the front hardware.

- Find someone to put together the circuit board to enable tow/haul mode - I've been told the shift tables already exist in my ECU - it's just a matter of building the actuation circuit on a board and connecting it to the right ECU pinout (B-71, IIRC; I've got it recorded on my laptop) A lot of F-body guys have done this as a cheap mod, since they also have the 4L60 - that's where I got the info from.

- I'll call this 'phase 3', but the next thing after that would be forced induction - and while I've always wanted a blower, a turbo would provide a side benefit of assisting with high altitude scenarios (one reason why the V6 EcoBoosts with 'Max Tow' pkg do so well and are so popular - plus they have a ton of aftermarket support). Not to mention a turbo setup is generally 1/2 the price of a blower or less. I'd start with low boost (7-8 lbs, perhaps) but build with the idea that it could be turned up (maybe to 18-20lb). Not sure I could engineer all of this on my own, however. Thankfully, I have a local mechanic who put a twin-turbo LS3 (with a built 4L60e, boys!) - in a 1st-gen Monte. I should've taken a pic when he showed it to me, but I was too busy drooling at the time (remember, I have a serious soft spot for 1st-gen Monte Carlos, having owned one when I was younger).

Obviously, all this costs money and time (and more $ if I have someone else put it in). And I'm sure there's a school of thought that says "cut your losses and get a 3/4 ton - problem solved". That may be what I have to do, in the end. But (and this should be no surprise to most of those here)... I really, really like my LWB Voy, and it's got a LOT of life left in it. Maybe just not the kind of life I'd like it to live...LOL. Right now, I'm in the 'drive under 50mph with flashers while climbing' club - and I just want to get out of that club...lol.

Anyway... to cover a few responses here:
- @cornchip - If you were backpedaling on your 4L60 comments b/c of me - don't. I don't work for GM and don't need to defend them. Rant away - all of us have, including me. No offence taken. We just happen to disagree, although I'll freely admit that GM cheapened the 4L60 from a parts / materials perspective, as time went on (plastic accumulator pistons come to mind, among other things.)

- @littleblazer - my WP was replaced last summer as part of my fan clutch replacement (strictly 'preventative maint' / 'let's not have to go in there again' later stuff; e.g.; the old water pump still had life left.) Replaced it with a Gates, along with all new hoses, belts, t-stat (and the thermal fan clutch, of course), as well as coolant replacement. Just as with the old pump, the truck was steady as a rock, temp-wise, and NEVER lost coolant until I started towing this beast over mountains the last couple of weeks (I could / did tow 'local' just fine - but my area is pancake-flat, compared to where I am now.) Ironically, I looked up the casting number on the old pump, and it was an AutoZone part - so it was not the first WP replacement this motor had seen. Also, for reference, I do have the rear HVAC (but don't use it; I've always wanted to go in and take it out, like Paul Bell did / wrote about a couple of years ago) The parts to do it were used in the TBSS - he even gave the part #s, bless him.

The pessimist in me is starting to think 'head gasket(s), under stress'. I suppose if I do the 'phase 2' mods I mention above, that'd be a good time to pull the heads and confirm. If I'm right about it being the gasket(s), eventually they'll get worse on their own. I also need to collect a sample of the engine oil for OLA to check for coolant contamination (my OLM is now at 34%, so it should be ready to change when I hit home (of course, with all the replacement oil I've put in lately, I should just ignore the OLM - I've effectively changed the oil already... ::wry grin::

- @shovenose - remember, I've essentially got a '4L60e plus' - as it was rebuilt almost 2yrs ago with new / strengthened parts w/ towing in mind (and has worked flawlessly since, although I only started actually 'towing' about 3mos ago). It originally failed because I put a TransGo kit in, and took so long to do it that I lost about 2+ quarts of fluid besides the 5 that normally drain out - so when I just dumped in 5 new quarts, thinking that was all it needed - and then took it on the expressway two days later - I essentially killed it. Only took about 2 minutes to do it, too - I pulled in an auto parts store less than 2mi from the off-ramp, and could smell the burnt fluid even before I parked. Took the stick out and saw copious amounts of smoke exiting the tube. Toasted my 3rd clutches, of course. Anyway - I think the current trans 'issues' are due to a too-small cooler for my application (I have the small 10-row stacked plate Derale, that was installed along with the rebuilt trans - bypassing the OEM rad cooler entirely. Pre-tow temps have been generally in the 150 - 160F range over the past two years - so the Derale has been doing its job until recently.
Besides the weight, the ambient temps are playing a role - I've noticed an easier time in upper Montana, where temps were in the 60F-75F range, than (back) in S. Dakota, where the temps have been in the high 90s in the afternoons (during my outbound trip, when I started this post, my outside display was showing 98-100F temps). With that said, I'm starting to research if I could retrofit a 4L80 into the Envoy. I'd love a 6L80, but am guessing I'd have a real problem getting it to mate up with my 2003 ECM.

Finally, in closing tonight's post, an aside about weather here in S. Dakota... they do *not* mess around. Was in a restaurant tonight and not only heard the 'emergency alert' tone / info announced over the restaurant's PA system from the TV that it was broadcast from (that was a first in itself), I actually saw the patrons at different tables perk up, stop talking, and LISTEN to the announcement info - and from all appearances, these were 'city folk', not farmer types. My trailer is actually shaking a bit from the winds of the latest storm (not the same one from earlier). Tomorrow I get to go on the roof of the trailer and see what happened as a result of the two 'ping pong-sized' hailstones that hit it (they were admittedly pretty loud).
Anyway, I'm sure the storm will pass over w/o flipping over my trailer (and hopefully, I don't see big hail dents in the Envoy tomorrow morning!)

Later!
 
#8
Just a thought. Why not swap in an LQ4 6.0 iron block? More torque and only 100 lbs heavier.

Give that intake a go and see how much it makes a difference. What you need is torque.
 
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#9
What you need is torque.

Was just thinking that this morning as a 3/4 ton Silvy w/ a long 5er followed me onto I94 this morning - then passed me effortlessly. Didn't see / hear any signs of being a Dmax, but I *did* see the nice size boat he had hooked up to the 5er (double tow). SMH in amusement.

All the HP in the world is nice, but torque is what gets the mass rolling. I need an engine transplant or a new tow vehicle (TV)
 
#10
The only problem with a 6.0l is you'll pass everything but a gas station. I've been told in the past the 5.3 is better than a 6.0l ot 5.7 for towing... SMH at them because it's not the case but to each their own.
 
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Finally back home. Except I left the power brick for the laptop in the trailer, so typing this on my phone. :: grrr ::

Searched on CL for an LQ4. Found a guy selling one w/ a TH400 for $700. Only problem there is that it's an early version, so it has iron heads & a 'long tail' crank.

Got him down to $400 for the block & rotating assy, but he won't move from there. Oh, well.

Looking up the specs on the LM4 and LQ4, it doesn't look to me like I'd be gaining *that* much addnl TQ (about 50ft/lb, IIRC.)

Found someone selling a Procharger (blower kit) from a 6.0L. $3500, and HP / TQ roughly double on 8lb pulley (even more w/o underdriven one) A little rich, but TBH, I might spend close to that anyway with the LQ4 build?

@littleblazer - Re: MPG - I avg'd between 7-9.5 MPG over approx 2800mi and change. The 6.0 can't be much worse than that, can it? LOL. And if I climb the grade w/o putting the flashers on, 'problem solved', right? :wink:
 
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#12
Finally back home. Except I left the power brick for the laptop in the trailer, so typing this on my phone. :: grrr ::

Searched on CL for an LQ4. Found a guy selling one w/ a TH400 for $700. Only problem there is that it's an early version, so it has iron heads & a 'long tail' crank.

Got him down to $400 for the block & rotating assy, but he won't move from there. Oh, well.

Looking up the specs on the LM4 and LQ4, it doesn't look to me like I'd be gaining *that* much addnl TQ (about 50ft/lb, IIRC.)

Found someone selling a Procharger (blower kit) from a 6.0L. $3500, and HP / TQ roughly double on 8lb pulley (even more w/o underdriven one) A little rich, but TBH, I might spend close to that anyway with the LQ4 build?

@littleblazer - Re: MPG - I avg'd between 7-9.5 MPG over approx 2800mi and change. The 6.0 can't be much worse than that, can it? LOL. And if I climb the grade w/o putting the flashers on, 'problem solved', right? :wink:
We get 10 no trailer... the truck is also a bit heavier so theres that as well.

As for the power gained, look at dynos on each. Area under the curve is what you want not peaks. I'd bet the average on the 6.0l is a lot better.
 
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#13
As for the power gained, look at dynos on each. Area under the curve is what you want not peaks. I'd bet the average on the 6.0l is a lot better.
Well, I searched for the corresponding graph. Best I could find was a 2007 LQ4, and some generic Gen III (?) 5.3L (couldn't find a specific LM4 or LH6 chart):
Thoughts?
I know (?) how to read a graph -- and they're constructed differently -- but isn't the LQ4 a bit 'peaky', compared to the 5.3 ? It was my understanding that a 'flatter' torque curve was better? Here we see (2nd graph) that the 5.3 has delivered 90% or better by 3500rpm, and doesn't start really falling off until after 5K, after max HP is reached.

Meanwhile, the 1st graph showing the late LQ4 has a... oh, nevermind... I can see it's delivering *more* than the 5.3 from pretty much idle, onward. :bonk:

LQ4:

1529683935986.png






Some unknown 5.3:


 
#14
Well, I searched for the corresponding graph. Best I could find was a 2007 LQ4, and some generic Gen III (?) 5.3L (couldn't find a specific LM4 or LH6 chart):
Thoughts?
I know (?) how to read a graph -- and they're constructed differently -- but isn't the LQ4 a bit 'peaky', compared to the 5.3 ? It was my understanding that a 'flatter' torque curve was better? Here we see (2nd graph) that the 5.3 has delivered 90% or better by 3500rpm, and doesn't start really falling off until after 5K, after max HP is reached.

Meanwhile, the 1st graph showing the late LQ4 has a... oh, nevermind... I can see it's delivering *more* than the 5.3 from pretty much idle, onward. :bonk:

LQ4:

View attachment 85115






Some unknown 5.3:


The 5.3 will feel faster. The 6.0 will deliver the same performance loaded or unloaded.
 
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#15
Well, as it turns out, I found a 2003 Sierra 1500HD that I liked / didn't have to spend 5 figures on (a little work for me to do on it to get it up to 'Reprise Safe Towing Spec'). Once the work is done and I'm securely towing with it, the Envoy goes up for sale, as I don't have the room for 3 vehicles, long-term.

If anyone is interested, PM me, as it'll be posted here before anywhere else (& cheaper than anywhere else.) It'll be awhile before I put the ad up, though (probably a couple of months, as I'm still using the Envoy to tow the trailer.) This forum is full of posts describing what I did to maintain it during the past 3yrs. Will make a great vehicle for someone that needs a 3rd row (or if you use the cargo area to haul stuff, like I do).

We drove the Sierra back up to my neck of the woods today (purchased it, ironically, in the SW side Chicago neighborhood I used to live in).

Taking that 60mi trip on the tollway home, I'm already 'comfy' with the reliability of the truck (took me a bit to trust the Envoy; the Sierra is giving me that 'good' feeling exponentially faster.) 1st mod - removed the tape striping (as a test) from the passenger side door that was put on...lol. Finish that w/ the remainder of the truck later, and follow up w/ some Goo Gone.

Need to make sure I can fit the truck in the garage - it's six inches shy of 20' long. Have some storage cabinets along the back wall - they're gonna have to be relocated, as the Sierra is nearly 3ft longer than the Voy - which just barely fits.

Nothing good comes w/o effort... :wink:
 
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#16
Well, I found my coolant leak, with the use of a pressure tester kit (thanks, AAP!)

Looks to be coming from the area of where the (newish) upper lower hose joins to the WP neck / t-stat outlet. But that's 'good' news!

"How so?", you say?

Well, it ain't the <not gonna say it...fingers crossed> , or the radiator, or the head / block mating surface / gasket (would be 'no' visible leak), or a freeze plug (???) or the overflow bottle. The only better outcome would be is if it were the cap itself that failed.

Yeah, I'll have to take apart half of the front side of the engine bay to get to it - but I've been there before. It'll be faster, this time.
Now off to get some oil (time to change, anyway), some extra coolant (on sale & I need it for the Sierra, anyway), and another drain pan.
Oh, and I won't need to worry about sending an oil sample off for UOA re: coolant.

It's a great day! :wooot:
 
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#17
Postscript to the last update above... my coolant started leaking again (or, maybe more precisely, never really stopped leaking, even after I fixed the first one.) Hopefully, I've now got it resolved once & for all.

Some thoughts...

When towing, the driveline undergoes stress (which varies by vehicle & load, but the fact that it's stressful is inviolate, IMO)

I had earlier mentioned that during my trip, I sometimes saw coolant temps rise (over longer periods of time, and nowhere near as frequently or as high as the trans fluid). But I did see them rise, to perhaps 2-3 notches north of the midpoint (210?) - so this would be about 220-230F, if I correctly recall my Torque app showing them when this would happen. Nowhere close to 'boiling over' - but higher. And somewhere I'd never seen them before in the three years I've owned the Envoy.

Heat creates expansion, generally. I think we'll all agree on this, as well (and I'm sure there's something in the world that doesn't, but let's limit this to a discussion of IC engines & their requisite composite liquid cooling systems, OK ? LOL)

On today's pressure testing session (and BTW, I plan on getting one for my permanent toolbox), I found that my original fixed leak (from the t-stat housing) was still intact (had to retorque & I also put some blue locker on the two bolts, so they wouldn't move again on 'the next tow').

However, this second session revealed two *new* leaks at hose clamp locations that hadn't shown up earlier, and the hoses *hadn't been removed*, previously. The hoses / connections in question (lower main to t-stat, and upper main to WP neck) *had* been themselves nudged / moved a bit during the 1st leak fix attempt, however.

Probably more correct to say that the *worst* leak had been fixed, and after that, coolant then began escaping from the remaining leaks - until the t-stat housing had been fixed, it was the path of 'least resistance'. Afterward, the remainder showed up.

So...lessons learned & food for thought, forward...

- You can have a solid cooling system, as I did -- but if you're on a tow, and noticed a significant coolant temp elevation, along with a loss of a little coolant the next day at the reservoir -- SOMETHING CAME LOOSE, due to expansion. Take a few moments to check / tighten down / retorque connections as needed (or, even better, attach your pressure tester while you've got an overnight cold engine, & check it out - another reason why I'm going to get one for my 'road' toolkit).

- When using your pressure tester, any drop from 15psi in less than 2 min time INDICATES YOU (STILL) HAVE A LEAK someplace. Consensus that I've been able to find seems to be that you should hold 15psi on the gauge for a good 5-10min or so, minimum. 'Indefinitely' is not feasible; a lowering of ambient temps alone (say, overnight) will cause the pressure to drop.

Basically, you should get tired of standing & staring at the gauge, before you see the needle move at all. :yawn:

Fingers crossed that I've seen the last of my coolant leaks... :rolleyes:

As of now, the only money I've had to spend to fix this has been for additional coolant. Oh, and a new 16qt sealable drain pan (really, a big bottle / jug) - but that was more for convenience & to save practically new coolant for reuse, as well as for future uses (like the Sierra, which is coming up soon).
 
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