P0171/4 and misfire

[lkrasner]

2002 Tahoe 5.3 Flex.

In the colder months, I get a bit of a rough idle and a P0171/4 (Fuel system too lean banks 1 and/ or 2). Occasionally, especially if I let it idle more than a minute or so I get a random misfire code as well. I can't find a vacuum leak for the life of me spraying crab cleaner at stuff, the MAF sensor is cleaned and tested good. Problem persisted through a fuel pump change (was not starting low on fuel and eventually began to die above half throttle or so).

So where should I be looking? It seems to run fine at anything above idle, and is better when it is warmer out.

 

[SLCHOE]

You need to know your LTFT at idle for both banks. If over 10+% it's excessive and adding 10+% more fuel to keep it running properly than normal. This means you have an issue.

P0171/P0174 combined with misfire codes on these vehicles usually leads to intake manifold gasket replacement. There are ways to verify this beyond spraying carb-clean and looking/listening to the engine for a change. This involves a scan tool and watching live data parameters while inducing changes to the air/fuel ratio and watching the readings of sensors, watching the reaction or lack thereof of said sensors and also correction strategies directed by the PCM for those changes. Without a scantool carb-clean diagnostics only take you so far.

 

[lkrasner]

You need to know your LTFT at idle for both banks. If over 10+% it's excessive and adding 10+% more fuel to keep it running properly than normal. This means you have an issue.

P0171/P0174 combined with misfire codes on these vehicles usually leads to intake manifold gasket replacement. There are ways to verify this beyond spraying carb-clean and looking/listening to the engine for a change. This involves a scan tool and watching live data parameters while inducing changes to the air/fuel ratio and watching the readings of sensors, watching the reaction or lack thereof of said sensors and also correction strategies directed by the PCM for those changes. Without a scantool carb-clean diagnostics only take you so far.

Yes, it is over 10% at idle. I have a scan tool, but I don't have a smoke machine to properly look for leaks.

How can I diagnose the intake gasket leak? And if it is bad, how difficult is it to replace?

 

[SLCHOE]

Yes, it is over 10% at idle. I have a scan tool, but I don't have a smoke machine to properly look for leaks.

How can I diagnose the intake gasket leak? And if it is bad, how difficult is it to replace?

First off determine if it is indeed a vacuum leak. If you have a scantool that probides live data, watch your STFT/LTFT for both banks. Pull up your data for LTFT on both banks (graph the data if you can). Let the engine idle for a minute or two. If the LTFT is still high watch the data as you stab the throttle to about 4000 rpm/hold it for 1 second and let go.

If the fuel trim went to zero, you have a vacuum leak. If the fuel trim rose as the rpm went up, you have a dirty MAF or fuel delivery issue (fuel pump/filter/etc).

 

[lkrasner]

First off determine if it is indeed a vacuum leak. If you have a scantool that probides live data, watch your STFT/LTFT for both banks. Pull up your data for LTFT on both banks (graph the data if you can). Let the engine idle for a minute or two. If the LTFT is still high watch the data as you stab the throttle to about 4000 rpm/hold it for 1 second and let go.

If the fuel trim went to zero, you have a vacuum leak. If the fuel trim rose as the rpm went up, you have a dirty MAF or fuel delivery issue (fuel pump/filter/etc).

Fuel trim is around 25% at idle, goes down to 13 at 3-4k rpm, then comes back up when returning to idle.

However, just found spraying carb cleaner around the gasket drops the fuel trim to 0 and changes the idle slightly

 

[afpj]

Look at post 38 on the following thread http://www.tahoeyukonforum.com/threads/rough-idle-help.72188/page-4

Professor Slchoe linked some awesome videos. All you wanted to know about fuel trims.

 

[SLCHOE]

However, just found spraying carb cleaner around the gasket drops the fuel trim to 0 and changes the idle slightly

That is pretty great evidence of a intake manifold gasket leaking right there.

Look at post 38 on the following thread http://www.tahoeyukonforum.com/threads/rough-idle-help.72188/page-4

Professor Slchoe linked some awesome videos. All you wanted to know about fuel trims.

LOL.

 

[David Paul]

Thanks Isaias for this information.
I was able to confirm the need for a new intake manifold gasket.
David

 

[SLCHOE]

You're welcome!