03 Z71 Tahoe

[biggsteve97]

I have an 03 Z71 Tahoe with 194k on the clock. Starting in about February of last year I noticed my fuel mileage starting to drop and a loss in power below 3000 rpm and the exhaust sounded weird at idle. I have been getting 17-17.8 mpg on my daily drive since I bought the Tahoe in 2006. I changed spark plugs first, no change in mileage or power. Changed wires, no change in mileage or power. I took the cold air kit off and put the stock air box back on, no change in mileage but power came back up some but only for a few months. Changed the fuel filter, no difference. Changed both primary oxygen sensors, no change in mileage or power. Checked fuel pressure and it was 54 psi, same as it was when I bought the Tahoe. Thought maybe intake gaskets, found no vacuum leak around intake or throttle body. about 2 weeks ago I put the cold air kit back on and no change in mileage or power below 3000rpm but power did come up above 3000rpm. about a week ago I cleaned the MAF sensor with MAF sensor cleaner that the gm techs said to use. Mileage came up from 12.8 to 13.9 with a slight increase in power below 3000rpm. The weird exhaust sound went away and an intermittent miss with a consistent stumble at idle has replaced the sound. One thing I have noticed over the last few months is when I am crusing at 35-45 mph in overdrive and the torque converter locks in, the engine rpms drop down to around 1100-1300rpm and the enigne sounds like it is majorly bogged out. If i take it out of overdrive and put it in regular drive for all speeds under 50mph, the engine doesnt lug along but my mileage drops down to 8-9 mpg. I am a loss as to where to go from here. I am almost to the point of selling it.

[AMERICAN PICK-UP]

I have had several of these rigs they get around 10-11 mpg in town about 15-16 on the hightway. average about 12.5 mpg.

Originally Posted by biggsteve97

I have an 03 Z71 Tahoe with 194k on the clock. Starting in about February of last year I noticed my fuel mileage starting to drop and a loss in power below 3000 rpm and the exhaust sounded weird at idle. I have been getting 17-17.8 mpg on my daily drive since I bought the Tahoe in 2006. I changed spark plugs first, no change in mileage or power. Changed wires, no change in mileage or power. I took the cold air kit off and put the stock air box back on, no change in mileage but power came back up some but only for a few months. Changed the fuel filter, no difference. Changed both primary oxygen sensors, no change in mileage or power. Checked fuel pressure and it was 54 psi, same as it was when I bought the Tahoe. Thought maybe intake gaskets, found no vacuum leak around intake or throttle body. about 2 weeks ago I put the cold air kit back on and no change in mileage or power below 3000rpm but power did come up above 3000rpm. about a week ago I cleaned the MAF sensor with MAF sensor cleaner that the gm techs said to use. Mileage came up from 12.8 to 13.9 with a slight increase in power below 3000rpm. The weird exhaust sound went away and an intermittent miss with a consistent stumble at idle has replaced the sound. One thing I have noticed over the last few months is when I am crusing at 35-45 mph in overdrive and the torque converter locks in, the engine rpms drop down to around 1100-1300rpm and the enigne sounds like it is majorly bogged out. If i take it out of overdrive and put it in regular drive for all speeds under 50mph, the engine doesnt lug along but my mileage drops down to 8-9 mpg. I am a loss as to where to go from here. I am almost to the point of selling it.

[Special-Ed]

fuel choice will make a difference as well

my suburban z71 4.10 gears
with e-85 get 10 city 14 highway... roughly
with 10% it gets 11 city and 16 highway... roughly
with 100% gas 14 city and 19 highway... roughly

[biggsteve97]

Originally Posted by AMERICAN PICK-UP

I have had several of these rigs they get around 10-11 mpg in town about 15-16 on the hightway. average about 12.5 mpg.

Well that may be but it does not explain why from 2006-late 2012 I averaged 17.5mpg on my daily drive and now I am only averaging 13mpg. I use QT mid grade and have since 2009 because I drive past one every day on my way to work. I did try premium about 2 months ago for several tanks and made absolutely no difference.

[Special-Ed]

Originally Posted by biggsteve97

Well that may be but it does not explain why from 2006-late 2012 I averaged 17.5mpg on my daily drive and now I am only averaging 13mpg. I use QT mid grade and have since 2009 because I drive past one every day on my way to work. I did try premium about 2 months ago for several tanks and made absolutely no difference.

actually it can... my point is they ay have switched fuel blends at your gas station...

they may of had 100% and switched to 10% ethenol

most people dont understand this fact that low mid and hi does not mean they add or take away ethanol

[biggsteve97]

Originally Posted by Special-Ed

actually it can... my point is they ay have switched fuel blends at your gas station...

they may of had 100% and switched to 10% ethenol

most people dont understand this fact that low mid and hi does not mean they add or take away ethanol

The QT's around here have had 10% ethanal for longer than I have owned the Tahoe.
I understand on the ethanol. Learned the hard way. My Tahoe is E85 compatiable and I tried it back in 2007 for a couple of tanks because it was almost $2 cheaper. It ran smoother but barely achieved 9mpg both tanks. I gave up on that crap after that.

[69gmcc10]

If you saw a difference when you cleaned the MAF I would keep looking there. The MAF plays a vital role in the engine, it can cause a plethora of various codes including misfire codes and O2 sensor codes. There are several tests that can be ran to give you a hunch if the sensor might be bad that you can look up and run. If you do replace the MAF go new and dont be tempted by a cheap refurbished one. I bought a refurbished sensor once and it was worse that the one I was replacing and I hear that is a common problem.

I would also check to see if you might have a cat starting to fail. Run the engine for a few and see if there is a difference in exhaust pipe temp from side to side in front of the cat.

[biggsteve97]

Originally Posted by 69gmcc10

If you saw a difference when you cleaned the MAF I would keep looking there. The MAF plays a vital role in the engine, it can cause a plethora of various codes including misfire codes and O2 sensor codes. There are several tests that can be ran to give you a hunch if the sensor might be bad that you can look up and run. If you do replace the MAF go new and dont be tempted by a cheap refurbished one. I bought a refurbished sensor once and it was worse that the one I was replacing and I hear that is a common problem.

I would also check to see if you might have a cat starting to fail. Run the engine for a few and see if there is a difference in exhaust pipe temp from side to side in front of the cat.

I wondered about the MAF sensor. After cleaning the MAF I noticed that the aluminum ring that is between the sensor uprights and the honeycomb screen has what looks like pits in it. Like aluminum wheels have after the clear coat has worn off and they get exposed to water for an extended period of time. Plus there is a continuous stumble at idle and intermittent miss that wasn't there before i cleaned the MAF sensor. So I haven't ruled it out yet. There are no codes and the service engine light has been on since 2009.

As far as the cats go, well back in 2009 I put on a set of long tube headers that came with a custom y-pipe and "cats"...they aren't really cats though but look like cats. They are actually resonators so they are straight thru, no restriction.

[69gmcc10]

Do you know anyone else with a LS engine that would let you borrow there MAF for 15 min? Take yours off and install theirs and see if the problem goes away.

[biggsteve97]

Originally Posted by 69gmcc10

Do you know anyone else with a LS engine that would let you borrow there MAF for 15 min? Take yours off and install theirs and see if the problem goes away.

Actually I tried that. I have an 02 Silverado with the 4.8 and I borrowed The MAF for a week. I did not notice the intermittent miss but the continuous stumble was still there. It did have a little more power below 3000rpm but no change in mpg though.

[69gmcc10]

Other than dirty injectors I dont know what else to suggest.

Lets see if someone smarter will chime in, that shouldn't be too difficult.

[biggsteve97]

Originally Posted by 69gmcc10

Other than dirty injectors I dont know what else to suggest.

Lets see if someone smarter will chime in, that shouldn't be too difficult.

I was hoping not to have to deal with the injectors but havent ruled that out. I really wish I had a live data reader. If they didnt cost so much, I have needed that more times than not.

[biggsteve97]

Something else that it does occasionally is if I am backing into the driveway or a parking spot and doing it quickly and I snap the throttle down the motor falls on its face and the rpms drop down below 400 rpm. it just shakes violently and struggles to move. Eventually it will come out of it slowly if I hold the throttle pedal or push down more. However if I lift off the throttle then step back on to it slowly its fine. This only happens in reverse.

[gmclowsierra]

Check the vacuum line on the pressure regulator. It night have a hole in the reg. Letting in fuel directly into the intake. Pull the line look for fuel. If its there replace the reg.

[Boog]

Originally Posted by gmclowsierra

Check the vacuum line on the pressure regulator. It night have a hole in the reg. Letting in fuel directly into the intake. Pull the line look for fuel. If its there replace the reg.

Exactly. They're not expensive and super easy to replace.

Look in the HowTo Faq index just above.

[stueber68]

I have the same problem with an 03 with 125k on it and I am leaning towards a bad coil now.

[biggsteve97]

I checked the vacuum line for the regulator and no fuel in the line. No cracks in the line either.

[biggsteve97]

Update: Well the past week the temp here has not been over 60 degrees and the power is mostly back mileage hasn't changed much though. Isn't the air intake temp sensor in the mass air flow sensor? Is there something else Im missing?