Running rich, negative long term fuel trim

jbacklund

Original poster
Member
Dec 16, 2012
118
Okay, next issue... my car is running a little bit rich. Scanning it and reading about -8% LT when idling. When at around 25% throttle and 50kpa MAP LT fuel trim is just -1%. Pressing the throttle a little bit more and at higher MAP I get -4% to -8%. Values are not that much off so the check engine lights is not on and no DTC thrown.

I have been driving for a while so the LT Fuel trim is stable.

When I first bought the car it had DTC P0172, but after a reset it didn't come back.

For the record, what I've changed so far is spark plugs, air filter, engine oil, O2 sensor. I have also cleaned the throttle body and what was possible inside the intake manifold.

I've soon run two bottles of injector/fuel system cleaner through the system as well. Fuel pressure at idle is 55 psi. MAP sensor seems to be reading accurate pressure. ( http://gmtnation.com/f25/manifold-absolute-pressure-during-different-throttle-positions-6265/ )

Any ideas? What else could I check? Not so keen on pulling out the injectors so I'll try all other alternatives.
 

CaptainXL

Member
Dec 4, 2011
2,445
Turn off the truck and unplug the MAP sensor. This will force the PCM to use default tables based on engine rpm, throttle position and vehicle speed. Start it back up and see if it runs any better. Check fuel trims again.

Also when you have it disconnected get a vacuum guage connected and see what the manifold reading is. Should be 18-22 in mercury at idle fully warmed up.
 

jbacklund

Original poster
Member
Dec 16, 2012
118
CaptainXL said:
Turn off the truck and unplug the MAP sensor. This will force the PCM to use default tables based on engine rpm, throttle position and vehicle speed. Start it back up and see if it runs any better. Check fuel trims again.

Also when you have it disconnected get a vacuum guage connected and see what the manifold reading is. Should be 18-22 in mercury at idle fully warmed up.

Thanks for the info. Unplugged the MAP sensor connector, but truck ran very bad and with the reduced engine power light on. Was not really possible to check if the fuel trims were any different.
 

CaptainXL

Member
Dec 4, 2011
2,445
jbacklund said:
Thanks for the info. Unplugged the MAP sensor connector, but truck ran very bad and with the reduced engine power light on. Was not really possible to check if the fuel trims were any different.

Ok, so you disconnected it before starting the truck? If so, then you got a problem like a vacuum leak or something. That's why I asked to connect a vacuum gauge. Do that and you should have your answer.
 

jbacklund

Original poster
Member
Dec 16, 2012
118
Yes it was disconnected before I started it up. Will get back with the values for the vaccum.
 

limequat

Member
Dec 8, 2011
520
Meh, I think you're looking for problems where none exist. 8% LTFT isn't a big deal on a stock truck. That's why the trims are there: to accommodate build variances and part wear over vehicle life.
 

CaptainXL

Member
Dec 4, 2011
2,445
Running too rich – High negative fuel trim corrections can be caused by MAF sensor problems, high fuel pressure, leaking fuel pressure regulator diaphragm, faulty evaporative emissions components, leaking injectors, defective O2 sensors, exhaust leaks/pinholes before the O2 sensor, coolant temp sensor problems, and base engine issues such as low compression and incorrect camshaft timing.

http://www.easterncatalytic.com/education/tech-tips/fuel-trim-can-be-a-valuable-diagnostic-tool/
 

jbacklund

Original poster
Member
Dec 16, 2012
118
Still working on it.

MAP good, stable vaccum between 18-22 inhg. Compression not the best, all cylinders between 160-170. There at least within range and no big difference.
Injectors cleaned with Ultra Sonic cleaner.
Exhaust backpressure tested, no excesssive backpressure.

I've read a lot and what I've understood sometimes even though the backpressure is good you might have a partially clogged cat. Symptoms will then be running rich with idle misfires. (Which I have). Hard to say but it feels a little bit bogged down when climbing hills. Hard to say because it's been the same since I bought it and have not tried any other TB.

So the conclusion with the background from when I bought the car (P0172 and P0128) I'm pretty certain that I might be the cat.

Already changed everythin that might have caused the cat to clogg up.

Have just loosened the cat from the exhaust manifold, next will be the at the other flange.

When everythinh is off I'll run it and see if the trims will be better. Will reset it first with new PCM flash.

I'll keep you guys updated.
 

gmcman

Member
Dec 12, 2011
4,658
OP, I would let the truck sit overnight then perform another compression test, a dry one. Then after each cylinder you test, put about 5-10 drops of motor oil through the plug hole and repeat the test, this is the wet test and note the changes.

Which spark plugs did you use? Important.

Have you ever changed the fuel filter?

Just make sure your compression tester is fully seated and with a good O-ring, 160 is kinda low.

When all back together after the dry and wet compression tests, with a scan tool, unplug a coil pack harness one at a time and record the RPM drop for each cylinder. Hopefully we can rule out a bad or weak cylinder.
 

tricguy007

Member
Dec 7, 2011
131
I have a similar issue my number 2 cylinder is misfiring at idle -11 fuel trims I've done all the regular tune up stuff swap the plug to number 3 cylinder still no change swapped coil to number 3 cylinder still no change could it be I feel a fuel injector or an intake gasketwhat I've changed fuel pressure regulator spark plugs fuel filter ext. I've checked compression about 200 on cylinder 2 compared to cylinder 3 is 200 also well driving down the highway my fuel trims are -3 I've even done an injector cleaning service
 

jbacklund

Original poster
Member
Dec 16, 2012
118
gmcman said:
OP, I would let the truck sit overnight then perform another compression test, a dry one. Then after each cylinder you test, put about 5-10 drops of motor oil through the plug hole and repeat the test, this is the wet test and note the changes.

Which spark plugs did you use? Important.

Have you ever changed the fuel filter?

Just make sure your compression tester is fully seated and with a good O-ring, 160 is kinda low.

When all back together after the dry and wet compression tests, with a scan tool, unplug a coil pack harness one at a time and record the RPM drop for each cylinder. Hopefully we can rule out a bad or weak cylinder.

It was not the CAT, removed it but still had instantly negative fuel trims.

Thanks for the advice with wet test. I will retake the compression tests to confirm and also make the leakdown test.

I use the recommended iridium plugs, recently replaced. I also have a new fuel filter.

tricguy007 said:
I have a similar issue my number 2 cylinder is misfiring at idle -11 fuel trims I've done all the regular tune up stuff swap the plug to number 3 cylinder still no change swapped coil to number 3 cylinder still no change could it be I feel a fuel injector or an intake gasketwhat I've changed fuel pressure regulator spark plugs fuel filter ext. I've checked compression about 200 on cylinder 2 compared to cylinder 3 is 200 also well driving down the highway my fuel trims are -3 I've even done an injector cleaning service

Did you pull the injectors and sent those in for proper service?
 

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