New Hoe Owner. First debacle

[Verd]

Picked up a 04 Tahoe with about 180k on it for a good price. Always wanted one, and my S10 got creamed during this years winter storms in front of my house so i figured why not. The truck has its issues that need addressing and that's fine.

So the other day i was driving down the parkway doing about 70ish, and the check engine light popped. When I purchased it I got a P0174 but no check engine light was on. Now after checking it since I have also P0171. I have researched it and I checked the usual suspects, intake gaskets being one of them, and they seem fine and actually the truck runs fine by all accounts so this had me perplexed.

I tossed on my fuel pressure gauge and I am showing 35 psi with the key on and 50-55 with the engine running. I believe this is too low and could be why I am throwing lean codes. Then come to learn the 5.3 model i have does not have a serviceable fuel filter so my question is, is it a safe bet to order myself a fuel pump? Any suggestions I check anything else before going for the money? Thanks ahead of time. I know this question is asked a lot but it seems the solutions have varied.

 

[HiHoeSilver]

Picked up a 04 Tahoe with about 180k on it for a good price. Always wanted one, and my S10 got creamed during this years winter storms in front of my house so i figured why not. The truck has its issues that need addressing and that's fine.

So the other day i was driving down the parkway doing about 70ish, and the check engine light popped. When I purchased it I got a P0174 but no check engine light was on. Now after checking it since I have also P0171. I have researched it and I checked the usual suspects, intake gaskets being one of them, and they seem fine and actually the truck runs fine by all accounts so this had me perplexed.

I tossed on my fuel pressure gauge and I am showing 35 psi with the key on and 50-55 with the engine running. I believe this is too low and could be why I am throwing lean codes. Then come to learn the 5.3 model i have does not have a serviceable fuel filter so my question is, is it a safe bet to order myself a fuel pump? Any suggestions I check anything else before going for the money? Thanks ahead of time. I know this question is asked a lot but it seems the solutions have varied.

Your fuel pressure is OK. Have you done the simple thing and changed O2 sensors? Any exhaust leaks?

 

[Verd]

Your fuel pressure is OK. Have you done the simple thing and changed O2 sensors? Any exhaust leaks?

Ahh the 02's that reminds me. Do I replace just the 2 upstream? or do I need to change all 4?

 

[HiHoeSilver]

I think the lean codes are from the post cat ones because they're measuring the amount of air in the exhaust, that's why if you have leaks, they see mostly air and throw the code. On the other hand, it's totally possible that the sensors are working properly and your problem lies elsewhere. I'm not sure if there's a way to test them...

 

[rockola1971]

I think the lean codes are from the post cat ones because they're measuring the amount of air in the exhaust, that's why if you have leaks, they see mostly air and throw the code. On the other hand, it's totally possible that the sensors are working properly and your problem lies elsewhere. I'm not sure if there's a way to test them...

Isnt throwing any codes for 02 sensors so I wouldnt replace them. The post cat 02 sensors would NOT cause a lean condition since these 02's have nothing to do with information to the PCM for fuel injection control. They serve no purpose other than to tell you your cats are going or gone bad. You pre cat 02 sensors are where the PCM gets lean/rich information from those 02 sensors. Any scanner will show the 02 sensors switching correctly and within spec. They should be since there are no codes for them. Something else is wrong. Fuel pump, intake manifold leak, etc. Air is getting into the system after the MAF or not enough fuel is getting to the cylinders.(pump, regulator, etc.)

 

[HiHoeSilver]

Isnt throwing any codes for 02 sensors so I wouldnt replace them. The post cat 02 sensors would NOT cause a lean condition since these 02's have nothing to do with information to the PCM for fuel injection control. They serve no purpose other than to tell you your cats are going or gone bad. You pre cat 02 sensors are where the PCM gets lean/rich information from those 02 sensors. Any scanner will show the 02 sensors switching correctly and within spec. They should be since there are no codes for them. Something else is wrong. Fuel pump, intake manifold leak, etc. Air is getting into the system after the MAF or not enough fuel is getting to the cylinders.(pump, regulator, etc.)

Thanks for the correction. Could it be a dirty MAF?

 

[rockola1971]

Dirty MAF would throw a code too. Since he isnt getting any codes its something after MAF but but well before pre cat 02's. Pre cat 02s are seeing the lean condition which is why the P0171 and 174 were being set. This tells me the 02's are doing what they are suppose to be doing. They see a overly lean condition which is caused by unmetered air getting into the intake stream (after the MAF) or engine is starved of fuel. Both will cause a lean condition.
This problem is going to take some serious methodical troubleshooting starting with the easiest. Checking intake gaskets with some WD40, etc. Spray it on the gasket areas and see if idle jumps up.

But first I would clear all codes and see what comes back.

 

[Verd]

Isnt throwing any codes for 02 sensors so I wouldnt replace them. The post cat 02 sensors would NOT cause a lean condition since these 02's have nothing to do with information to the PCM for fuel injection control. They serve no purpose other than to tell you your cats are going or gone bad. You pre cat 02 sensors are where the PCM gets lean/rich information from those 02 sensors. Any scanner will show the 02 sensors switching correctly and within spec. They should be since there are no codes for them. Something else is wrong. Fuel pump, intake manifold leak, etc. Air is getting into the system after the MAF or not enough fuel is getting to the cylinders.(pump, regulator, etc.)

Ok so ruling out 02's (which I dont think that is the culprit either tbh). Like I said I checked fuel presure and HiHoe said that was fine too. Everyone is saying Intake gasket then, I just am not seeing where the leak is. As a side note I cleared the code today and drove it about 30 miles and it has yet to come back yet which is twice as confusing.

 

[rockola1971]

Ok so ruling out 02's (which I dont think that is the culprit either tbh). Like I said I checked fuel presure and HiHoe said that was fine too. Everyone is saying Intake gasket then, I just am not seeing where the leak is. As a side note I cleared the code today and drove it about 30 miles and it has yet to come back yet which is twice as confusing.

You may not even be able to see the leak on the intake gasket(s). Especially if its toward the bottom which would be impossible to see standing there next to a fender. For it to be a leaking intake gasket it would have to be at both heads since you had a lean condition on both right and left bank. But I would give it a week of driving to collect data and see if any codes come back. While you are "waiting" go through your intake snorkel between the MAF and the Throttle Body and make sure all clamps are tight and no holes, tears, rubs in the snorkel.(intake pipe)
Let us know how it acts after a week of driving and that pesky service engine soon (SES) light.

 

[Verd]

You may not even be able to see the leak on the intake gasket(s). Especially if its toward the bottom which would be impossible to see standing there next to a fender. For it to be a leaking intake gasket it would have to be at both heads since you had a lean condition on both right and left bank. But I would give it a week of driving to collect data and see if any codes come back. While you are "waiting" go through your intake snorkel between the MAF and the Throttle Body and make sure all clamps are tight and no holes, tears, rubs in the snorkel.(intake pipe)
Let us know how it acts after a week of driving and that pesky service engine soon (SES) light.

Thanks! will do. Appreciate the insight. In the meantime I am gonna see what I can do about the leaking transfer case. never a break.