Suggestions for steady drop in MPG?

[Mike534x]

Hello everyone!

I've been having an issue with my Avalanche for the past year or so. When I purchased it originally with 79k in 2013, I would average roughly 14-15 around town/city driving and 18-19 on the highway. As of late, within the last 2-3 years the fuel economy has tanked so I average around 10 to 11 MPG...with summer blend and roughly the same maybe 1-2 MPG less with winter blend. Highway driving is roughly the same, probably about 17.5-18 give or take. Not sure if this matters, but I did keep it in storage during the last two winters, and brought it back out for the remainder of the year to drive around when doing work on the Envoy.

Yeah its a brick so I should not expect stellar MPG, but the drop seems to be big enough to look into. I'm not too lead footed either with it so I'm trying to see where a good place to start would be.

Thanks!

What I've done so far;

Tune up with AC Delco Iridiums at 89k
Swapped the wires 2 months ago when I started to have a rough idle which immediately cleared up
Cleaned the MAF Sensor. I was going to clean the throttle body but it looked fairly clean without much build up.
Replaced the Coolant Temp Sensor when I did the water pump/belts.
Ran a tank with two bottles of the Techron concentrate plus. If BG44k is worth buying, I'll try throwing that into the tank.
No engine lights/codes


Specs:

05 Avalanche Z71 (1500) currently at 101,779k
5.3 Flex
3.42 gearing
4x4

 

[Chickenhawk]

How does it perform on the highway? Does it seem to accelerate strongly or does it seem slower to accelerate? It could be your cat is starting to get plugged. (Running with a bad misfire or long-term low coolant temp can do that.)

Other than that, all I can suggest is to have a look at the oxygen sensor readings with a good scan tool. Also, what percentage of ethanol is in your local gasoline? This can easily reduce city mileage that much if they suddenly introduce 10% ethanol.

 

[littleblazer]

Hows your fuel trims?

 

[Mike534x]

How does it perform on the highway? Does it seem to accelerate strongly or does it seem slower to accelerate? It could be your cat is starting to get plugged. (Running with a bad misfire or long-term low coolant temp can do that.)



Other than that, all I can suggest is to have a look at the oxygen sensor readings with a good scan tool. Also, what percentage of ethanol is in your local gasoline? This can easily reduce city mileage that much if they suddenly introduce 10% ethanol.

It drives great on the highway, the 3.42 gears don't really help much with acceleration but it has 0 hesitation once you get on it to make a pass. Before I changed the plugs it had no codes as far as a misfires go, but it did have a slight "miss" when idling that cleared up with the new plugs. The coolant temp sensor was still good, but I figured with it being a cheap part I saw no harm in replacing it while I did the water pump. I was wondering if the the 02 sensors could be getting old/slow, but it doesn't have any codes pending or stored. I can hook up torque and provide what its reading if that helps. All the stations around here us 10% ethanol, the nearest station without it is a good few hours away.

Hows your fuel trims?

Haven't had a chance to hook up Torque to take a look with being gone for the weekend, I'll post them soon!

 

[littleblazer]

It drives great on the highway, the 3.42 gears don't really help much with acceleration but it has 0 hesitation once you get on it to make a pass. Before I changed the plugs it had no codes as far as a misfires go, but it did have a slight "miss" when idling that cleared up with the new plugs. The coolant temp sensor was still good, but I figured with it being a cheap part I saw no harm in replacing it while I did the water pump. I was wondering if the the 02 sensors could be getting old/slow, but it doesn't have any codes pending or stored. I can hook up torque and provide what its reading if that helps. All the stations around here us 10% ethanol, the nearest station without it is a good few hours away.




Haven't had a chance to hook up Torque to take a look with being gone for the weekend, I'll post them soon!

If they're massively positive it may be time for intake gaskets. That made a huge difference on the 03.

 

[Redbeard]

As Chickenhawk mentioned check your o2 sensor. I have the 4.2 engine (2004) and when mine went out the milage dropped to what yours did to 10 mpg or so virtually overnight from a common 15-16 mpg running around in town. IIRC the sensors were replace 2 or 3 years ago making them last over 10 years, so as you mention they just might be getting "slow".

 

[Mike534x]

Hey everyone, after a long delay I have the screenshots of the readings Torque gives me for the 02 sensors. I also took some of my Short and Long term fuel trim's as well.

First image is

Bank 1 Sensor 1 and Bank 1 Sensor 2

Bank 2 Sensor 1 and Bank 2 Sensor 2

Fuel Trim Bank 1 Long Term + Fuel Trim Bank 1 Sensor 1


Last one I mistakenly checked everything so its grouped Fuel Trim Bank 1 Short Term, Fuel Trim Bank 2 Long Term, Fuel trim Bank 2 Sensor 1 and Fuel Trim Bank 2 Short Term.

 

[gmcman]

What is your cold tire pressure? Check before the sun hits them.

 

[Mike534x]

I'll have to recheck them, but I always keep them filled to 40 psi. Although I'm wonder if my MAF is bad, or possibly the crank sensor? I do notice the truck still has a bit of a "rough" idle, not mis-fire like rough but its enough that it feels "off" if that makes sense. Shutting the truck down each time results in a "sputter/shake". I threw some redline fuel system cleaner in to help clear out any carbon/gunk in the pistons. The truck has a slight "ping" also.


If brand/type matter; they're Bridgestone Dueler A/T Revo 2's. They aren't too aggressive, but not going with these again haha.

 

[Wooluf1952]

I may have missed it, but without a CEL, the rough idle could be bad motor mounts.

 

[Mike534x]

I didn't even consider motor mounts, wasn't sure if they were known for going bad.

 

[Wooluf1952]

Motor mounts are known to fail. Also, it is recommended to use OEM replacements. After market mounts a often too hard.

 

[Mike534x]

Motor mounts seem to be good, I was out there with a buddy and had the brakes to the floor while revving the engine in "D". Not much movement under the hood.


I picked up 2 bottles of Redline fuel cleaner, threw in the appropriate amount (might have added a little more then needed) and drove it her for the an hour on the highway. The mileage has gone up a little bit, and the shut down is a getting much smoother. I'm thinking there must be some carbon build up. Wondering if I should run some seafoam through the intake and let it hot soak for an hour and see what happens. It wasn't driven much last year, and sat for most of winter with a start up or too each week to keep the battery charged from what my dad told me. So thats where my thinking on the carbon is coming from.

 

[Mooseman]

The mounts may "look" OK and still hold the engine but if the fluid inside has leaked out or they have work hardened, they will not dampen that specific 10Hz vibration at idle.

 

[Mike534x]

Is there another way to test the mounts if they're bad?

 

[Mooseman]

This is the Avalanche right? Please disregard everything I said. The mount issue is for the 4.2

 

[Mike534x]

It is! Lol That's why I was wondering if the V8 mounts were known to go bad. No worries though!

Thats why I was thinking maybe carbon build up, after running redline through it for a week it seems to have helped it quite a bit. So I'm thinking of maybe running seafoam through the throttle intake and see what if there's any changes.

 

[Maverick6587]

Does the truck hesitate even slightly on a cold start? I had that issue with my 4.2 and it ended up being the crank sensor. I only ask because you said it sputters/shakes when you shut it off.

I'd be interested to see your O2, fuel trims, MAF, and MAP readings as you shut it off.

 

[Mike534x]

No hesitation, it starts up relatively easy. I picked up a Delco Crankshaft Position Sensor to throw in and see what happens, yeah the shut down is a bit tricky to describe.

When you shut it off, it almost feels like you're stepping on the gas pedal while turning the key off to rev the engine as it shuts down but without the RPM change. I would record it but you can't exactly feel what its doing haha. The redline treatment helped a bit, its not as harsh as it was before but still noticeable.

I did pull the throttle body off to clean it, along with the opening that leads into the intake. MAF sensor was cleaned thoroughly (I was debating if this was going bad), and the vapor canister purge valve that sits on the intake was replaced for another problem I was having with filling up at the pump (still working on fixing that). There is still a noticeable "miss" at idle, even just sitting with it in park you can feel it through the seat. Its not a bad one like I mentioned, but its enough to feel the small vibrations through the seat, where as it was buttery smooth several months ago.

I can try grabbing all those readings for you in about a day! I haven't driven it in a few days while the Solution Finish is curing on the body cladding. I'll be out there replacing my fault impact sensor so I can grab everything needed then.

So far I'm thinking;

Intake Manifold Gaskets, maybe there's a small leak that's causing it?
MAF Sensor
Upstream O2 sensors (both fronts were reading .7 in my screenshots above from Torque
????

 

[Maverick6587]

Make sure that you unplug the battery for 30 minutes after cleaning the throttle body. When you replace the crankshaft position sensor you HAVE to run a CPAS (crank) relearn. I attempted going a month without running the CPAS and I thought it "relearned" by itself but, it did not. You'll need a Tech 2 or a higher-end scanner to run the CPAS relearn.

If you had a leak on the intake manifold gasket, you would probably see a P0171, a lean code. You could make sure those bolts are tightened to 89 inch-pounds and no more than that.

I've had really good success using Berryman B-12 Chemtool Carburetor, Fuel System and Injector cleaner. It's a toss-up between that and Sea Foam.

Also, make sure that you use only Mass Air Flow cleaner when cleaning the MAF sensor. Anything else like brake cleaner is too harsh on the probes of the MAF sensor and can cause damage or shortened life.

I see that you changed out the spark plugs. You might also want to do a voltage and amperage test on the ignition coils. I've seen that a few times where they get old and put out spark but, at low amperage/voltage and cause misfires.

 

[TJBaker57]

to run a CPAS (crank) relearn

CASE Relearn is what you meant, right?

 

[Maverick6587]

@TJBaker57 HA. Yes, CASE relearn. Thank you

 

[Mike534x]

Well if a CASE relearn is required, I'll just send it back and not worry about it for now. No worries, I used CRC MAF Sensor cleaner. I know better then to use brake cleaner on anything sensitive like that! Haha.

I did try Seafoam months ago, but didnt seem to help. Haven't tried it through the intake yet. I'll give Berryman a shot once I run this tank empty.

Plugs were done around 80k (currently at 102k), and wires were done about 3-4 months ago give or take. Is there a proper way to test the coils? So just use a multimeter? I wanted to try pulling one of the plugs just to see if there was any abnormal carbon built up or fouling.

 

[Maverick6587]

Yeah, you can just use a multimeter to test voltage and amperage. This is a lot easier and quicker than testing amperage.

Here's a good video that explains how the coil-over system works. They use the resistance of the coil to test the coils. If you want to skip everything else, the coil testing starts at 9 minutes and 8 seconds.

If you still want to verify amperage this is a good video and he used a pretty cheap multimeter to show the process.