Cruise control down hill jerk help needed

jeffro312

Original poster
Member
Oct 4, 2012
442
East Haven,Ct
So this has happened since I purchased truck never really cared to figure it out.

As title says if I'm cruising usually around 65-75 (don't think speed is a factor). And a downhill comes up to where I guess speeds rolling would be greater then set speed on cruise control. The truck jerks until either I speed up using cruise or coast using cruise or turn it off.

Jerking would be equivalent to me hitting gasor brake pedal repeatedly

Does it all the time.



Also this same jerk can be felt at speeds of 105mph except that was a slightly more severe jerk to where I not expecting the jerk almost hit my head on steering wheel. My experience with governers was when u reach max governed speed it gradually slows down. My truck pretty much felt like it shut engine off completely and tapped brakes.


Not sure if these are related figured I'd mention it

In case your curious why I'd drive a rainier that fast I simply wanted to see when the governer kicked in and I sure did find out
 

CaptainXL

Member
Dec 4, 2011
2,445
Couple questions. Has the transmission ever been serviced?
Did this just start happening after any maintenance was done?

Also some possibilities to do/check...

- Remove and clean throttle body
- Check Transmission fluid level
- Change transmission fluid and filter
- Perform Dealer transmission flush
- Perform update to PCM
http://forums.trailvoy.com/showthread.php?t=79773
(Bill noted this on the OS thought I would bring it here)

These are just a few things to ponder.
 

jeffro312

Original poster
Member
Oct 4, 2012
442
East Haven,Ct
Sorry forgot to mention its an 04 with 181k.

Place I bought it from turned out to be a bunch of scum. Told them I won't buy car till they replaced cracked exhaust manifold so they replaced it snapped a bolt in head and refused to fix it

Here in ct state only req 30 day warrenty on used cars over x amount of yes old
 

CaptainXL

Member
Dec 4, 2011
2,445
That's quite a bit of miles. Especially if the trans has only had that one pan drop (which only changes half of the fluid). I wouldn't really do too much. The dealer can perform a trans flush but given the mileage they might not touch it.

I am not even sure that's what the problem is.

In the mean time I would just suck out the fluid in the pan through the dipstick tube and fill it back up with Dex VI and some fluid conditioner. If the symptoms improve then you know the trans is the culprit. Then start saving for a rebuild. That's probably the smartest thing to do.
 

jeffro312

Original poster
Member
Oct 4, 2012
442
East Haven,Ct
Fluid wasn't that bad when I changed it and I just changed it to change it since I wasn't aware of previous maintenece at 161k when I bought it did all fluids
 

Tmn8tr

Member
Jan 22, 2013
12
jeffro312 said:
Fluid wasn't that bad when I changed it and I just changed it to change it since I wasn't aware of previous maintenece at 161k when I bought it did all fluids

If we weren't in different states I would think we went to the same stealership to get our trucks. My TB does the same thing, as well as feels like it's missing when sitting at a stop light. I was thinking about this the other day while it was doing it, and reached down and put it in neutral and it smoothed out. I said to my self: "Self, if this is really missing it wouldn't care what gear it was in, would it?" Being that mine is a former rental vehicle, I'm starting to notice lot's of signs it has been abused, since it only has 70k. I may be heading for the transmission shop in the next couple of months.
>T
 

CaptainXL

Member
Dec 4, 2011
2,445
jeffro312 said:
Fluid wasn't that bad when I changed it and I just changed it to change it since I wasn't aware of previous maintenece at 161k when I bought it did all fluids

You should treat it as if it has only had that one pan drop (which only changes half of the fluid). Not good at 181K. It's supposed to be done every 30-50K? with a severe duty schedule. That's assuming the truck was beat to crap before you got it. In this case at 161K I would have flushed the trans to get rid of all the fluid.

Of course you probably already know that when someone buys a used vehicle over 50K miles and the maintenance is unknown all of the scheduled maintenance items should be done asap (either before driving off the lot or by the buyer). What this does is gives you a starting reference so that you know when to change stuff next. Otherwise your just shooting in the dark. Hope everything works out.
 

Playsinsnow

Member
Nov 17, 2012
9,727
Hope my cruise still works with that many miles. IIRC doesn't the owners manual advise AGAINST using the cruise control uphill or down? Prob get better mileage on the uphill/downhill stretches too. Doesn't it take "power" to keep a vehicle at a constant speed when gravitational pulls aid acceleration? power being fuel or brakes or hubs that need to be replaced. My cruise control will roll over the set speed down a real "hill", so Im used to checking myself using the cruise
 

jeffro312

Original poster
Member
Oct 4, 2012
442
East Haven,Ct
Playsinsnow said:
Hope my cruise still works with that many miles. IIRC doesn't the owners manual advise AGAINST using the cruise control uphill or down? Prob get better mileage on the uphill/downhill stretches too. Doesn't it take "power" to keep a vehicle at a constant speed when gravitational pulls aid acceleration? power being fuel or brakes or hubs that need to be replaced. My cruise control will roll over the set speed down a real "hill", so Im used to checking myself using the cruise

Usually not a huge hill and only going down normally cars go into like a coast down hill when set. Mine jerks like its trying to slow it self down. Only if cruise is on. I can coast down a hill normally no cruise with no problem
 

Robbabob

Member
Dec 10, 2012
1,096
FWIW...

My Dakota ('01 with 210K) will downshift to slow my truck when I exceed the speed set point by X mph. Scared the crap outta me that first time!
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
jeffro312 said:
Usually not a huge hill and only going down normally cars go into like a coast down hill when set. Mine jerks like its trying to slow it self down. Only if cruise is on. I can coast down a hill normally no cruise with no problem
Sorry I didn't reply earlier. This is a well-known problem with the middle years like 2004-2005. GM even had a TSB out for it. Discussed a few times on Trailvoy, where I mentioned my experience.

Bucking/surging while using CC going downhill [Archive] - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

A PCM update *may* solve the problem. But this wasn't a recall because it wasn't a safety issue - just an annoyance. Dealers aren't required to deliver free fixes for mere annoyances.

Google for this if you're curious, but I already have and the details of the fix were never published in a place you can see them for free.

Make : CHEVROLET Model : TRAILBLAZER Year : 2004
Manufacturer : GENERAL MOTORS CORP.
Service Bulletin Num : 3329 Date of Bulletin: JAN 01, 1901
NHTSA Item Number: 10017201
Component: VEHICLE SPEED
CONTROL:CRUISE CONTROL
Summary:
SURGE, CHUGGLE OR BUCK WHILE DRIVING DOWNHI
LL (APPROXIMATELY 1500 - 3000 FEET ABOVE SEA
LEVEL) AT HIGHWAY SPEEDS WITH THE CRUISE CONTROL ON.
 

jeffro312

Original poster
Member
Oct 4, 2012
442
East Haven,Ct
the roadie said:
Sorry I didn't reply earlier. This is a well-known problem with the middle years like 2004-2005. GM even had a TSB out for it. Discussed a few times on Trailvoy, where I mentioned my experience.

Bucking/surging while using CC going downhill [Archive] - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

A PCM update *may* solve the problem. But this wasn't a recall because it wasn't a safety issue - just an annoyance. Dealers aren't required to deliver free fixes for mere annoyances.

So your saying there's nothing mechanically wrong that I should worry about too much as its just an annoyance as I assumed it was to begin with and why i never checked into it till curiosity got me and all this talk of transmission issues are false as I assumed as rest or driving is normal

Thanks roadie you always have a way of brightening my day haha thanks again.
 

Shdwdrgn

Member
Dec 4, 2011
568
Since I'm currently working on and issue which *appears* to give the same symptoms, lets compare notes...

If this is the same issue as mine, it's a pretty distinctive set of conditions. While on the highway, if you run the engine over 2700rpm, then take your foot completely off the gas (like you would do while going down a long hill), the engine will suddenly decelerate quickly like you've run out of gas. Then when the engine drops below 1000rpm, the engine speed suddenly kicks back up again. If the engine races back over 2700rpm again while still coasting (which could happen if the cruise causes the transmission to downshift to maintain speed), you can get into a cycle where it feels like someone is hitting the brakes/gas/brakes/gas...

If that description, especially the RPM range, matches the conditions you are seeing, then I know what the problem is, and a dealer update of your PCM will not fix the problem, but a PCM update is required.
 

jeffro312

Original poster
Member
Oct 4, 2012
442
East Haven,Ct
Shdwdrgn said:
Since I'm currently working on and issue which *appears* to give the same symptoms, lets compare notes...

If this is the same issue as mine, it's a pretty distinctive set of conditions. While on the highway, if you run the engine over 2700rpm, then take your foot completely off the gas (like you would do while going down a long hill), the engine will suddenly decelerate quickly like you've run out of gas. Then when the engine drops below 1000rpm, the engine speed suddenly kicks back up again. If the engine races back over 2700rpm again while still coasting (which could happen if the cruise causes the transmission to downshift to maintain speed), you can get into a cycle where it feels like someone is hitting the brakes/gas/brakes/gas...

If that description, especially the RPM range, matches the conditions you are seeing, then I know what the problem is, and a dealer update of your PCM will not fix the problem, but a PCM update is required.

Well not sure if its the same as for me to create problem no feet on pedals are involved in simple set cruise control to 65 and when a downhill road comes when gravity takes over and truck seems to try rolling faster then set speed it jerks hard enuf we all jerk forward in are seats. If I am using pedals no cruise I can simply let off gas and car will coast normally down hill as if I'm in neutral
 

CaptainXL

Member
Dec 4, 2011
2,445
Shdwdrgn said:
If that description, especially the RPM range, matches the conditions you are seeing, then I know what the problem is, and a dealer update of your PCM will not fix the problem, but a PCM update is required.


Nonsense. Since there is a PCM update (as Roadie indicated) the dealer can fix it. If you are referring to doing a PCMforless tune then please enlighten us with the details. I am skeptical that a tune alone can fix it. PCMforless in fact downloads the latest PCM calibration and tunes off of that. All fixes from GM would be integrated into that calibration.
 

Shdwdrgn

Member
Dec 4, 2011
568
I should have specified... the issue I'm talking about is the deceleration fuel cut-off... supposed to help economy by cutting off the fuel when you're coasting, but in playing with the settings I've found its very easy to get into cycle where it bounces back and forth. The RPM values (2700/1000) that I gave are what was stock on my truck, and since I have 3.42 gears and oversized tires, I would actually have to hit 72mph in 3rd gear before DFCO could ever turn on... so I never saw this when my truck was stock. On the other hand, if you have 3.73's or 4.10's it would be a lot easier to hit this range. The stock settings should make it very difficult to get into a brake/gas cycle like this, but I've seen it downshift at really weird times while the cruise was enabled, so it wouldn't surprise me if this happened.

I'm currently trying to find working values for this mode where it can turn on during normal cruising, and not start cycling while decelerating. The thing is, to fix this you need someone with access to reprogram your PCM... and if you're going to put in the money for that anyway, you may as well get a full tune done while you're at it.
 

jeffro312

Original poster
Member
Oct 4, 2012
442
East Haven,Ct
Anyways I don't think its a big enuff deal to pay money to fix I simply just turn it off in rare instances I'm cruise controlling down a hill. Thanks for all the advice i just got curious and fig I get some opinions.



Also nobody stated if when hit max speed of 105 the total shut down feeling is it normal. Like I seriously thought my tranny just dropped that's how severe it "governed" it
 

Shdwdrgn

Member
Dec 4, 2011
568
jeffro312 said:
Also nobody stated if when hit max speed of 105 the total shut down feeling is it normal. Like I seriously thought my tranny just dropped that's how severe it "governed" it

The speed limiter is set at 104mph. When you hit this, it completely turns off the fuel, which would cause engine braking. I've never hit it myself, but it does sound like it would give quite a jolt.
 

TGFB87

Member
Dec 27, 2011
14
I can say that the same exact cruise issue happens on my 03 TB with the V8. Been happening for as long as I can remember.
Its a bit annoying, but it only happens on long decents at highway speeds. I just tap the brakes, coast, then resume when the long descent is over.
 

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