2004 Tahoe LT 5.3 Flex Fuel AWD - Both Banks Lean & Random Misfire

[dat_boy_greedo]

Hey Guys!

I’m new here so apologizes if this has been answered previously or if this is some obvious fix.

So I just picked up this 04 Tahoe for almost dirt cheap knowing it had some issues but its pretty clean for a Michigan car and I’ve always wanted one of these so I said screw it, its probably something simple and ill have it fixed in no time.

Fast forward a couple months the problems still persist and I’m at a loss.

So currently the truck is still showing lean on both banks with a random misfire. It does have a CAT code as well as both of the wheel speed sensors codes so I know those need to be replaced. CEL, ABS, and TC lights are on. The truck runs fine at idle and just driving it normal no problems show up until I really get on the gas, once it gets over 3200-3400 rpm the truck fall on its nose like it has no power and the random misfire starts. Intake hoses look good. I haven’t been able to detect any vacuum leaks or anything like that. Does have a slight exhaust leak on the driver side. I have not come across anything to this point that to my knowledge would cause a lean condition on both banks along with the random misfire.

Seeing as it was lean on both banks and it was having fuel pump issues my first guess was a weak fuel pump. I replaced the old original fuel pump with a new Delphi unit. Made sure the wiring looked good, lines looked good, and buttoned everything up. Problem still persisted. I cleaned the MAF, nothing changed. Unplugged the MAF to troubleshoot further, nothing changed. Replaced the MAF on a whim, this is where I got my first change in performance for a little bit. It seemed like it had more power. I did a couple pulls on the freeway to get the rpms up and there wasn’t a misfire or anything for a good 30 minutes of messing around (doing pulls from 60 to 90) truck seemed to have more power and then it started to act up but in a different way then normal. Where it would start random misfiring before (around 3300-3400 rpm) it just started falling completely on its face and where it used to feel down on power it felt like there was no power and would actively lose mph until I let off the throttle but I didn’t get the random misfire sound or anything, almost like the power was just disengaged from the driveline. Again driving normal it would act fine but if I got on it then it would start to act up. This happened for about 5-10 minutes, basically until I pulled up to work.

I am at a loss because honestly I thought that this would be a simple fix. It turns out that these trucks might be a little more complicated than I first thought lol and I work so dang much that I just don’t have a ton of time to spend trying to trace this down.

To anyone who answers I really appreciate you taking the time to try to help me out.

 

[Roilux]

Does "lines looked good" include checking fuel line pressure? Was the fuel filter also replaced?

Side note: When my throttle body failed, in addition to codes I noted ABS and TC lights came on but no codes for those, only the throttle body. Replaced throttle body and all was back to normal.

 

[nonickatall]

Replacing the fuel pump makes sense if it's broken. However, the fuel pressure might still be incorrect afterward, as it's controlled by the fuel pressure regulator on the rail. I would check or replace that. It only costs a few dollars and is quick to replace.

On the other hand, it doesn't make sense to keep replacing parts; the diagnosis should be the first step.

You need to read the fault codes and see what the relevant sensors are reporting, such as the mass airflow sensor, the fuel pressure sensor, and the lambda sensors and others.

By the way, your problem sounds typical of clogged catalytic converters. The vehicle runs at low RPMs because enough exhaust gas can pass through the catalytic converters. But at higher RPMs, misfires occur due to the backflow of exhaust gas into the cylinders. The cylinder then can't draw in enough fresh air to produce proper combustion.

 

[dat_boy_greedo]

Does "lines looked good" include checking fuel line pressure? Was the fuel filter also replaced?

Side note: When my throttle body failed, in addition to codes I noted ABS and TC lights came on but no codes for those, only the throttle body. Replaced throttle body and all was back to normal.

So pressure was checked at idle. My gauge was not long enough to check while driving. At idle the pressure looked good. As far as the fuel filter I just found the fuel filter on the truck while doing the fuel pump. When looking for parts to do the job I looked for the fuel filter and they said the truck didn’t have an external filter just a filter inside the pump. So know that I know there is one that could be done but was not done along with the fuel pump.

 

[dat_boy_greedo]

Replacing the fuel pump makes sense if it's broken. However, the fuel pressure might still be incorrect afterward, as it's controlled by the fuel pressure regulator on the rail. I would check or replace that. It only costs a few dollars and is quick to replace.

On the other hand, it doesn't make sense to keep replacing parts; the diagnosis should be the first step.

You need to read the fault codes and see what the relevant sensors are reporting, such as the mass airflow sensor, the fuel pressure sensor, and the lambda sensors and others.

By the way, your problem sounds typical of clogged catalytic converters. The vehicle runs at low RPMs because enough exhaust gas can pass through the catalytic converters. But at higher RPMs, misfires occur due to the backflow of exhaust gas into the cylinders. The cylinder then can't draw in enough fresh air to produce proper combustion.

Fuel pump was going out. Once the tank got lower than a quarter tank if I was driving a decent distance the pump would start to fail and I would have to pull over wait for 2-10 minutes and start it back up, sometimes I would have to knock on the tank a couple times with a rubber mallet I kept in there, so it needed to be done.

As far as just replacing parts I have only touched things that have thrown codes besides the fuel pump which like I said was dying. I did have a MAF sensor code, tried cleaning it and it did nothing so I replaced.

And I knew a clogged cat could cause a misfire but I didn’t think would cause a lean condition on both banks. I assumed that the lean condition was causing the misfire and assuming that the cat wouldn’t cause the lean condition I have been overlooking it to be fair.

As far as reading the codes I have, like I said I had MAF code, I addressed it. The only other code the was relevant would be the CAT code. No fuel pressure sensor codes or anything like that just both banks lean and the random misfire. Then we get to the abs stuff (wheel speed sensors etc)

The pressure sensor is on the list along with the fuel filter.

 

[nonickatall]

You've already done some important things, but codes don't necessarily tell you the cause, they tell you the symptom.

For example, if you have a problem with unmetered air, meaning the mass airflow sensor is measuring too low a mass because not all the air is passing through it, then you might end up with a lean mixture. The engine detects this via the O2 sensors and tries to compensate by injecting more fuel, but the O2 sensors never reach their target value. The error code you then get might be "O2 sensor out of range" or "mixture too lean," even though it's actually unrelated.

Your "mixture too lean" message actually fits more with a vacuum leak, meaning a leak in the intake tract downstream of the mass airflow sensor, or with insufficient fuel pressure.

I would suggest you measure the fuel pressure, or even replace the fuel pressure regulator directly. A clogged fuel filter could also be a possibility, but as far as I know, our cars don't even have fuel filters... although your '04 Tahoe might actually still have one. If so, that could certainly be the cause.

And I would do a smoke test to check for leaks in the intake system. That would also explain your "too lean" problem.

The misfires at high RPMs are more indicative of a clogged catalytic converter, but could also be caused by a very lean mixture.

 

[nonickatall]

So pressure was checked at idle. My gauge was not long enough to check while driving. At idle the pressure looked good. As far as the fuel filter I just found the fuel filter on the truck while doing the fuel pump. When looking for parts to do the job I looked for the fuel filter and they said the truck didn’t have an external filter just a filter inside the pump. So know that I know there is one that could be done but was not done along with the fuel pump.

I checked, and it seems GM installed fuel filters up until 2004, but then they stopped doing that. The fuel filter is located in the fuel line under your driver's seat...

If you have one, I would definitely replace it first...

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please. If we can't see it, it does not exist.

You are already receiving sage advice from the knowledgeable folks on this Forum.

Purchase a fuel pressure gauge with a long enough hose from Harbor Freight, tape it to your windshield, and go for a drive. This way, you can make a final determination on fuel pressure at speed and acceleration.

It seems that you have a cat issue, and you need to get that fixed now, before it takes out more of your truck's equipment. Either delete it or replace it.

 

[Roilux]

... However, the fuel pressure might still be incorrect afterward, as it's controlled by the fuel pressure regulator on the rail. I would check or replace that. ...

My '01 had the regulator on the fuel rail. My '04 Flex has it in the fuel tank as part of the fuel pump. The change was made between '03 to '04 model years.

 

[Roilux]

... The fuel filter is located in the fuel line under your driver's seat...

If you have one, I would definitely replace it first...

Co-signed. Also, this tool will ease working in a cramped space to disconnect fuel line from the filter.

Amazon.com

 

[Fless]

Fuel pump was going out. Once the tank got lower than a quarter tank if I was driving a decent distance the pump would start to fail and I would have to pull over wait for 2-10 minutes and start it back up, sometimes I would have to knock on the tank a couple times with a rubber mallet I kept in there, so it needed to be done.

As far as just replacing parts I have only touched things that have thrown codes besides the fuel pump which like I said was dying. I did have a MAF sensor code, tried cleaning it and it did nothing so I replaced.

And I knew a clogged cat could cause a misfire but I didn’t think would cause a lean condition on both banks. I assumed that the lean condition was causing the misfire and assuming that the cat wouldn’t cause the lean condition I have been overlooking it to be fair.

As far as reading the codes I have, like I said I had MAF code, I addressed it. The only other code the was relevant would be the CAT code. No fuel pressure sensor codes or anything like that just both banks lean and the random misfire. Then we get to the abs stuff (wheel speed sensors etc)

The pressure sensor is on the list along with the fuel filter.

Hopefully the new fuel pump is the correct one for the Flex Fuel system. And the '04s don't have anything monitoring fuel pressure, so there won't be a DTC for that.

Most, if not all, of the Flex Fuel SUVs in 2004 will have an external fuel filter on the driver's side rail, close to the FF sensor (fuel composition sensor).

And my '04 L59 FF has the fuel pressure regulator on the DS fuel rail, controlled by vacuum. What's the running fuel pressure?

These engines really like the OE MAF if you can get one or keep yours clean and working; sometimes the aftermarket MAFs don't play well. You can tell with a good bidirectional scanner, reading the MAF PID at hot idle to see if it roughly matches the engine displacement in liters.

Also, what fuel is in the tank? E10, E15, E85? Does that ethanol content agree with what a scanner says the ethanol / alcohol content is?