Why 13 MPG?

[Royalwapiti]

I have been consistently getting 13mpg, with air on. 4x4 EXT 88,000 miles

New plugs, new coil paks, new air filter, cleaned throttle body. I have a cross country trip planned and i need to figure this out in the next week or so. I would expect 16-17mpg.

I plugged in my Innova reader and gather some freeze frame data. Driving conditions today are 90 degrees, AC was on. This was after a 20 mile drive, it was idling.

Fuel Sys 1 CL
Calc Load % 43.1
ECT f 199
STFT B1 % 0.7
LTFT B1 % -10.1
MAP in Hg 13.5
RPM 621
Speed 0
Spark Advance 2
IAT f 149
MAF lb/min 0.906
TPS% 16.0
O2S B1 S1(v) 0.085
STFT B1 S1% 3.1
O2S B1 S2 (v) 0.815
STFT B1 s2% NA
OBD Sup OBD OBD2
MIL 0

My fan winds up loud, at startup and when AC is running and engine is warm, then it is intermittent, but even so, when the fan is winding loud my acceleration is terrible. When fan is winding I push the pedal and nothing happens, rpms increase and I sit there.

I have no reason to think it is a CatConverter, but may try taking out the O2 sensor to see if performance picks up.

The only mod is addition of a tranny cooler.

Any tips or suggestions?

 

[Blckshdw]

Is your MPGs based on your own calculations or what the DIC is telling you? Is this over a full tank, or whenever you happen to look at/calculate it? Is this city driving, highway, or some percentage of mix?

I know when I drive around on surface streets with the AC on, my mileage tanks as well, somewhere in the upper 13's. Without it, I usually average 15-16 on my 20 minute commutes, which are surface streets and a couple of major arteries that are 45 mph speed limits.

 

[Royalwapiti]

And yours is a 2WD. Holy crap, I need to get rid of this thing.

I don't have the fancy DIC. This is mixed driving half a tank was the last test, with me doing the math. The usage on this tank consisted of two 60 miles round trips which included city driving and interstate speeds and the rest misc city miles. With AC on.

So you are telling me that 13 is normal with AC on? Our TB usually sits all summer, we don't drive it daily. My wife will use it daily in the winter due to snow. And for the last couple years in September I have driven it on 3500 mile roundtrip hunting vacations. I pull an aluminum trailer with 550 pound ATV on it. I got 13mpg on those trips as well. So I was assuming I woud get better without the trailer. I am planning another trip in the next few weeks. I have owned this TB since new but as I said my wife drives it in the winter and I never paid any attention to mileage.

Is there anything in the numbers I quoted in my original post above that I can try? Not sure but wondered if the IAT temp was high, considering it is ambient air blowing through system. I guess it would depend on where the sensor is at intake.

 

[Sparky]

13 with AC on doing only city driving I can see happening. Highway the MPG can pick up dramatically as getting these 4600 pound bricks moving takes some effort.

Towing of course will bring down the mpg. I was averaging 19-20mpg 90% highway on mine, then I towed for half my tank, and my mpg for that tank was down to 15-16.

FWIW, That 90% highway would get me 19mpg, 100% highway (and pretty flat roads) got me all the way up to nearly 23mpg. Drop down to 75% highway and my mpg was 17-18. Now I do a bit less than that and get 16-17mpg.

Check your glovebox RPO codes for your axle ratio, I'm curious. GU6 is 3.42, GT4 is 3.73, and GT5 is 4.10.

 

[Blckshdw]

with Sparky, when I run the AC just around town in traffic, that's when I get 13 MPGs. Highway it picks up, although if I'm cruising, I tend to just put the windows down. If I'm careful on the highway, I can get up to 21-22 MPG if I stick to the sweet spot (between 55 and 60 for me) and maintain constant speed. Your gear ratio could have a lot to do with it as well.

OP, now that you mention it, your IAT does seem high. Were you sitting there idle for very long before you grabbed that freeze frame data? My IATs are usually 10-20* above ambient while driving, and although I haven't paid attention to them at a stop, I think it would take a while to heat soak the intake so the IATs got that high.

 

[BlazingTrails]

When mine was doing that 12-13 mpg it turned out that catalytic converter was clogged. Went from 12 to 18-19 mpg

That may not be your problem tho

 

[{tpc}]

You say the fan is winding loud? Does it sound like a jet engine about to take off kind of loud? On my wifes envoy, when the fan clutch went out, it sounded like a jet engine trying to take off, and suffered a very similar symptom, you hit the gas and it didn't want to go anywhere.

I would look at the fan clutch. I would also look at the temp gauge and see if your thermostat is maybe acting up. Recently I replaced this and we found a few more mpgs.

 

[Royalwapiti]

Thanks for the replies. I distinctly remember getting 17 average a few years ago.

On glovebox sticker I did find a GU6, so must have 3.42 gearing.

I have thought about thermostat but my temps have always maintained the right levels, with scanner yesterday engine temp ranged from 190 to 199(f), high was at stop lights or parked. It seem like an easy job but a bitch moving the alternator to get at it...

ITA stayed about the same 138-150, it was 90+ degrees (F) here yesterday.

I have long been suspcetive of fan because it is like a jet much of the time I drive it, then it will shut down, sound normal and acceleration is good great again. Then at the next light it might be roaring again. So i didn't know if it was just overworked from some other sensor telling it to kick on. Is there a way to test what tells the PCM to turn it on? isn't that related to the IAT?

CatC might be the easiest test for next step, remove O2 sensor, though I have always maintained the correct temp, possibly something else gave me a rich condition to clog it.

We do not have emission testing in the state I live, can i just remove it? Or will that cause other sensor or PCM issues?

 

[{tpc}]

The fan clutches are notorious for going bad. There should be some information around how to test it, I believe I've seen a youtube video on it.

I'm pretty sure they can come and go, so the fact that it happens at one moment and not the next, could be an indication of that. Ours was diagnosed and replaced by the dealer when it went. It has happened a few times on the one we currently have, so its probably going to go out again on ours at some point.

 

[djthumper]

You fan clutch is starting to sound suspect. Where is the needle on the temp gauge.

 

[Royalwapiti]

You fan clutch is starting to sound suspect. Where is the needle on the temp gauge.

Gauge is steady eddy at 210. ECT is 190 to 200

 

[triz]

On the highway you'll get much better mileage as long as your not fighting the wind the whole time. I cant remember exactly what I got the last time in drive to Colorado but I would fill up after 350 miles with gas still in the tank.

If going cross country make sure to check your tire inflation, and a bottle of Techron concentrate to help clean those injectors.

 

[djthumper]

Gauge is steady eddy at 210. ECT is 190 to 200

At least so far your thermostat is fine.

 

[{tpc}]

Most recently, when the second one (fan clutch) on the envoy started to act up on my wife, I took the car for spin to check it out. Sure enough, felt loss of power, fan roaring, just as the first time.

I stopped the truck, shut it off and popped the hood. I tried manually moving the fan, and it felt "engaged" or at least hard to move by whatever that fluid is in the clutch assembly. I just kinda moved it back and forth. Not at all sure of what, if anything it was doing, but after messing with it for a bit, it "loosened" up.

I got back in the truck, assured myself we would be buying another fan clutch assembly and started it up. The roaring was gone, and the power was back. That was about a year ago I think, and we haven't had issue with it since.

Like I said, I saw a youtube about how to test it while running. Personally I think its dangerous, and I would advise against it, hence why I haven't gone out of my way to find the video of it. I'll leave the messing around with the fan up to you.

 

[DocBrown]

There is no reason you shouldn't get at least 18 mpg at 70mph with the A/C on.

I went through all this same troubleshooting only to end replacing the CAT. The CAT on GMT360/370s are notorious for going bad between 80-120k miles. No CEL, nothing else obvious except poor mileage and really poor towing performance if you tow. Usually caused by bad thermostat. It doesn't have to be far off, just enough that over a period of time it causes meltdown.

 

[BlazingTrails]

 

[Royalwapiti]

Thanks guys,

I started removing the O2 sensor last night, so I could test the CAT, or zat least see if performance picks up with it out. I stripped it, even thou I used the correct slotted socket from the tool loan program. So I took the heat sheild partially off and will put a pipe wrench on it tomorrow when I get back home.

On the new sensor, do I want to use some loctitie and copper anti-sieze and if it has the loctiite on it, should I add the anti-sieze?

I plan using anti-sieze on the studs and nuts for the heatshield. I

If I go the route of a new CAT any certain brand? I will probably hire this done as I am preparing for my trip. Costs?

Thanks

 

[Sparky]

I went with the magnaflow myself. If your pipe is OK you could maybe get a universal just welded in, might be cheaper than replacing the whole pre-fit assembly.

My cat lasted to almost 200k miles. Only reason it was replaced was my pipes rusted out so I went with the pre-fit unit. Which then I ordered the wrong one and got the EXT unit, so the shop had to shorten the pipe

 

[DocBrown]

I bought one from NAPA. The guy behind the counter even thought it was over priced and gave me a shop discount. It saved me about $70. The CAT worked fine, but the end of the pipe ended up rusting out at the flange a few years later. I think it had a lifetime warranty but I was not about to go through the amount of work it took the first time. I ended up using a patch pipe. Bottom line is that I would not get one from NAPA again.

 

[Royalwapiti]

Thanks for the feedback.. Tonight I will try the pipe wrench on the O2 sensor.

 

[Royalwapiti]

I noticed a slight increase in power with the O2 sensor out. It is hard to judge with the added roar of the leak. How significant of an increase should be enough to replace CATc?

 

[FrankR12]

So how is it that these threads just stop without a solution / answers? This is like reading a pretty good mystery novel and you turn the page and realize the publisher never put the last 2 chapters in.....GEEEEZE

 

[Royalwapiti]

Well. I had the cat checked at the muffler, it's good. While there they checked fuel pressure too, it was 60.

I put a Lime tune in it in prep for a long trip thus week. Will be burning the highest fuel I can find that is ethanol free.

Will report back.

 

[Royalwapiti]

I have the Lime tune in. The last tank I filled was 290 mile at 20.5 gallons 14.14 mpg. City Highway mix. I have only put that many miles on since my return on Sept 26th from Arizona. Like I said it sits most of the time until the snow flies.

I think a sensor or two is messed up somewhere that tells the pcm to do some thing and it kills mileage. But others have changed all the sensors plus some and their mileage didn't improve.

 

[BlazingTrails]

That sound about right to me. My truck is top notch and gets 15 combined. I can get 22-23 hwy, but only around 10 city. it takes a lot to move 5k lbs brother.

I usually get around 300-330 miles per tank.

 

[Sparky]

Mine's been down in the 15-16 range since I stopped driving quite as much highway. I still cruise at 55 for a few miles but I have a lot more stop and go with my new commute to work since moving.

 

[Royalwapiti]

I am amazed, last week I finally got a chance to drive mine far enough on a tank with highway miles only. I got 21.7 mpg. That's a mix of Freeway doing 70, two lane highway doing 55. I about crapped when the trip was over, I went 250 miles and was just barely below half a tank. The week before with my wife driving city trips to work, got 12mpg.

The limequat tune has 0% torque management and she likes to step on on it. I tend to baby it.

 

[Blckshdw]

Glad to hear you're seeing a big improvement!

 

[DocBrown]

That's pretty nice. Still, it seems odd to me to have to do a tune to get that kind of an improvment. I used to get 19 on the freeway with mine, it was an EXT and I have to use E-10 to boot. I didn't have a tune either.

 

[Royalwapiti]

True Doc, but these forums are full of posts about poor fuel mileage.

The kicker for me is I have a company car, so I drive that whenever possible and not the TB. When I hit the road to go anywhere I take the vehicle with free gas and other expenses.