2008 Tahoe LTZ - 9 MPG

[Rule Andrews]

I cant seem to figure this issue out. The gas mileage in my 08' Tahoe is getting worse and worse. I got a new fuel sending unit installed last year and was getting around 14 mpg. Its just been getting progressively worse. The last tank I got 9 mpg! The truck also shakes a little at idle. The RPM gauge is stable, but there is definitely a chugging/shaking.

The only check engine light I have on is a P0449 (Evap Vent Solenoid) which shouldn't be causing this issue.

A month ago there was a strong smell of gas in the rear wheel well. I got under there, looked around with a flash light and mirror and couldn't find a thing. I checked the fuel pressure with the key in the on position and my gauge read 55 psi, at idle 44 psi and when shutting off the engine, no drop at all.

Changed air filter.
Checked for sticking caliper.
Tire pressure is good.
Cleaned throttle body and mass flow air sensor.
The PCV valve and hose is fine.

Does any one have any idea or some other things to try?

Thanks!

 

[kbuskill]

I cant seem to figure this issue out. The gas mileage in my 08' Tahoe is getting worse and worse. I got a new fuel sending unit installed last year and was getting around 14 mpg. Its just been getting progressively worse. The last tank I got 9 mpg! The truck also shakes a little at idle. The RPM gauge is stable, but there is definitely a chugging/shaking.

The only check engine light I have on is a P0449 (Evap Vent Solenoid) which shouldn't be causing this issue.

A month ago there was a strong smell of gas in the rear wheel well. I got under there, looked around with a flash light and mirror and couldn't find a thing. I checked the fuel pressure with the key in the on position and my gauge read 55 psi, at idle 44 psi and when shutting off the engine, no drop at all.

Changed air filter.
Checked for sticking caliper.
Tire pressure is good.
Cleaned throttle body and mass flow air sensor.
The PCV valve and hose is fine.

Does any one have any idea or some other things to try?

Thanks!

When was the last tune up?

How many miles are on the truck?

 

[Scottydoggs]

how many miles on her? ever do a tune up?

 

[Rule Andrews]

Its got about 130 miles on it. I put new plugs in last year.

 

[iamdub]

Sounds like a dead or weak cylinder. When did you have it scanned for codes last?

Do you know anyone with a scan tool that has live (real time) data capabilities?

 

[mikeseay]

Maybe a leaky injector.


Sent from my iPhone using Tapatalk

 

[Teeroy78]

Hey @Rule Andrews

Is yours a Flex Fuel model? If you have a scan tool (I use BlueDriver for now, works well), check the Alcohol Fuel % and short / long term fuel trims for both banks. If they are out of whack, that could cause some roughness at idle and poor fuel consumption. At least it seems to be for me until I get my trims properly reset.

I cant seem to figure this issue out. The gas mileage in my 08' Tahoe is getting worse and worse. I got a new fuel sending unit installed last year and was getting around 14 mpg. Its just been getting progressively worse. The last tank I got 9 mpg! The truck also shakes a little at idle. The RPM gauge is stable, but there is definitely a chugging/shaking.

The only check engine light I have on is a P0449 (Evap Vent Solenoid) which shouldn't be causing this issue.

A month ago there was a strong smell of gas in the rear wheel well. I got under there, looked around with a flash light and mirror and couldn't find a thing. I checked the fuel pressure with the key in the on position and my gauge read 55 psi, at idle 44 psi and when shutting off the engine, no drop at all.

Changed air filter.
Checked for sticking caliper.
Tire pressure is good.
Cleaned throttle body and mass flow air sensor.
The PCV valve and hose is fine.

Does any one have any idea or some other things to try?

Thanks!

 

[Rule Andrews]

Thanks for all the replies. Cylinder's and injectors are good.

It is flex fuel. The readings for the short and long shift are:

ShrtFT1 (%) 1.6
LongFT1 (%) -6.3
ShrtFT2 (%) -0.8
LongFT2 (%) -7.0

I'm not too sure what these numbers mean...

Another thing that has been happening, is the wheels bind in 2wd some times. Maybe its actually in 4wd when it shows in 2wd...This might have something to do with it?

 

[fathoe]

I cant seem to figure this issue out. The gas mileage in my 08' Tahoe is getting worse and worse. I got a new fuel sending unit installed last year and was getting around 14 mpg. Its just been getting progressively worse. The last tank I got 9 mpg! The truck also shakes a little at idle. The RPM gauge is stable, but there is definitely a chugging/shaking.

The only check engine light I have on is a P0449 (Evap Vent Solenoid) which shouldn't be causing this issue.

A month ago there was a strong smell of gas in the rear wheel well. I got under there, looked around with a flash light and mirror and couldn't find a thing. I checked the fuel pressure with the key in the on position and my gauge read 55 psi, at idle 44 psi and when shutting off the engine, no drop at all.

Changed air filter.
Checked for sticking caliper.
Tire pressure is good.
Cleaned throttle body and mass flow air sensor.
The PCV valve and hose is fine.

Does any one have any idea or some other things to try?

Thanks!

Sorry to revive an old thread, but i found that there are 2 super common dumb issues that occur to ruin mpgs (plus a third more major thing.

1. I noticed my 1st bank had vibrating spark plugs. I thought the boots were loose so i put them on a little firmer. Closed the hood and went down the road. Opened the hood again and they were still vibrating. Pulled the boots off and spun them by hand with a socket. To make a long story short, the entire bank had backed out till its last couple threads. Whenever i did them last i made them finger tight plus 90 degrees with a ratchet. Make them tight, but not overly tight the head is still alluminum

2. Broken exhaust bolt creating a exhaust leak leaning out that bank, which in turn the ecm throws more fuel and makes the bank run rich, but the sensors make it look lean/in spec. This can cause a code over time, but there is a certain threshold on when it will throw a lean code, so for a month or two it could look fine on the dtc side, but not be so fine because the computer is hiding it from you. My exhaust manifold bolt was broken at the most rearward flange hole so i have to pull the engine to extract it. If your case is similiar to mine, you can get a thicker gasket and some orange permatex to potentially seal that cylinder for longer just to help ease your gas bill.

3. Transmission pwm failure/tcc solenoid failure/misinterpretation.
After I performed the top 2 repairs, my mpg went from 9.8 to 12.8 with the same driving. Nowhere close to what i want to be (considering those were majority highway miles [should be somewhere near 16mpg for hwy speeds of 85mph]). I do notice that whenever i am at high way speeds the 4l70 wont engage full slip, it will only slip partially, keeping rpms around 200-400 lower then full lockup (im not super well versed in transmissions), which in turn probably tanks hwy mpgs. Driving my moms charger with a five speed 5.7 (i also know this is hardly comparable) it engages like around a 80% slip at 70-80 mph, being at 1200 rpm vs full lockup being around 2500 rpm in 5th gear with no throttle applied. Also on the 6l80 tahoes (09-14) in 6th gear there seems to be a larger slip difference then the 4l70 tahoes.

My tahoe has been through a transmission already, and 2 different pwm tcc solenoid replacements, still throwing a tcc pwm code.