2000 C2500 stuck in closed loop.

[j cAT]

Originally Posted by 20GMC00

Both LTFT are constantly at 25%. I'll be getting some for sale signs today. Thanks for the help.

the positive 25% fuel trims both banks is a vacuum leak or low fuel pressure.

if the fuel trims drop off on the higher above idle RPM then it is a vacuum leak .

do the smoke test much faster to find the leak.

also do a reset on the PCM so that the fuel trims can re-learn . sometimes people get confused with the fuel trims because of the PCM memory still responding to the past .

[Blue Bowtie]

If this is a central injector pod (spider) type injection system, verify that the pod seal is installed correctly. You may be able to get a reasonably good look at it from the top, without having to pull the upper intake. There should be the orange quad ring type seal visible all the way around the pod opening in the upper intake.

Don't bother asking me why I know this can be an issue...

[20GMC00]

When I spay carb cleaner around the intake, I can hear it backfire a little when I spray between the distributed and the intake. I tried the smoke test and there were no visible leak with that.

[20GMC00]

Damn autocorrect.

[j cAT]

Originally Posted by 20GMC00

When I spay carb cleaner around the intake, I can hear it backfire a little when I spray between the distributed and the intake. I tried the smoke test and there were no visible leak with that.

do the smoke test again. pressurize the intake a few PSI.

use condom on the TB air intake bore . inject smoke into the PCV hose to the TB base intake manifold .

[20GMC00]

What psi do you recommend? Thanks.

[j cAT]

Originally Posted by 20GMC00

What psi do you recommend? Thanks.

like 3to 5 psi .put a plug on the exhaust pipe also as the smoke will blow out from there . 3 psi should be good with the exhaust plugged up.

with what you said it does appear to be an intake leak somewhere on the top of the engine.

[20GMC00]

I'll try both methods again in a little while. Thanks.

[20GMC00]

I tried the smoke again. Found nothing. Tried carb cleaner again. Didn't find anything there, not even a backfire in the intake like before. Watched the fuel trims and both stayed at 25% and the 02 sensors never peaked at all. I'm lost. Good idea on the condom usage by the way.

[20GMC00]

These codes came up after I replaced the spider so I screwed up somewhere. Will pull the plenum off tomorrow and see what I find. Also, where the connector for the spider plugs in, how for up should the spider be? Right now the 2 tabs on the connector go down and touch the plenum before they latch. Thanks.

[j cAT]

Originally Posted by 20GMC00

These codes came up after I replaced the spider so I screwed up somewhere. Will pull the plenum off tomorrow and see what I find. Also, where the connector for the spider plugs in, how for up should the spider be? Right now the 2 tabs on the connector go down and touch the plenum before they latch. Thanks.

yes the spider install could cause the leak .. injector seals are also a common screw up when replacing .

I would find out where its messed up by smoke test because the carb cleaner does indicate this is leaking..

the wiring connectors touches the manifold does not seem correct.
see if you can get a video on this install.

[20GMC00]

I just got the plenum off. It was leaking smoke right in front of the distributor. When I got it off, I noticed the gasket was smashed level back there but it stuck down everywhere else. Also there is oil back there. Is that normal? Thanks.

[20GMC00]

Got a new gasket put on it and it ran like new.....for a couple days. Now I'm getting P0174 by itself. It starts right up and when I put it in gear, it hardly wants to move and is backfiring in the intake. If I put the pedal a ways down it suddenly clears up and takes off like new. How could it be lean on just on bank?

[20GMC00]

Also the STFT is at 99.2% for both rear O2 sensors. I think I'll just run it the way it is. Tiring of messing with it.

[j cAT]

Originally Posted by 20GMC00

Also the STFT is at 99.2% for both rear O2 sensors. I think I'll just run it the way it is. Tiring of messing with it.

if it did run good for several days not the 174 code then the gasket is messed up.

installing an intake manifold use bolts with no heads on them, to drop down on block , then this will prevent the gasket getting messed up .. also follow the torque procedure. could be the intake is distorted.

I do not recommend any silicone sealers . takes too long to cure .

time to smoke test again. runs good OFF idle is an indicator of the vacuum leak.