1998 Lexus GS400 V8 - LTFT -20% but drives totally fine

Alrighty, I've been chasing this problem for about two years now. Long term fuel trims gradually creep more and more negative after resetting the ECU. Once they reach -25 the CEL comes on, I reset the ECU, then they return to around -5% and as the weeks go on start creeping negative again.

The numbers are identical for both cylinder banks.

Car doesn't smell rich, doesn't misfire, and is currently getting the best MPG I've ever had (not saying much given it's a V8) at a combined average of 19-ish. Power feels excellent. Super smooth and tons of pull. I've had vacuum leaks and stuff and can usually tell an immediate difference with this motor. When it's healthy, it feels soooo strong, and right now it feels incredibly healthy.

MAF sensor has been cleaned several times and replaced and cleaned again with no apparent change.

O2 sensors are original, but again both banks are reporting identical numbers. It seems unlikely that they'd both fail in identical fashions, but it's not impossible I guess.

Voltage while driving fluctuates as expected.

Now here's the interesting bit, the LTFT decreases toward zero depending on throttle position. Floored, it hits -5.5%. Idle, it stays rock steady at -20% as of today. It reacts in a linear fashion with throttle input. About half throttle drops it to around -11%.

Short term trims bounce around +/- 7-ish percent as expected.

Fuel injectors, fuel pump, intake/TB gaskets, vacuum lines, PCV, all new as of two years ago and I just ran a bottle of fuel system cleaner with no change. Engine air filter is brand new. Intake pipes all in great condition.

I'm thinking of having a shop stick the sniffer thing into the tailpipe just to verify fuel mixture.

As the site title suggests, I don't want to just start throwing parts at it, particularly since I'm on a college budget. How likely is it that the O2s really have failed perfectly in sync? Or is there something else I should check?

UPDATE: I think I have traced the problem to the MAF plug. It seems like every time I mess with that plug, whether it's for cleaning the sensor, checking the air filter, or whatever, the fuel trims correct themselves for a short while and then gradually slide rich again.

That's the one common denominator I can come up with from all the times I've gotten the fuel trims to read normally for a bit. It seems to always be after messing with the MAF plug.

Any suggestions for cleaning/fixing the pins in the plug? Or maybe some high conductivity substance I can cram in there? Anything short of cutting the MAF plug off and wiring on a brand new one?

Do a pin drag test on connector pins. The ones that are loose just depin them and use a pick to push the internal spring inwards to retighten the drag. Otherwise purchase a connector plug and solder and shrinktube the wires together. The harness pigtail you should be able to get from a parts store otherwise the dealership.

Can't really help you but am having very similar issues with my 2008 Mazda 6. LTFT at -20, STFT at 0 when at idle. When I increase RPM, the LTFT goes to 0 and the STFT stays at 0 - does not change at all. I have a thread in here as well titled 2008 Mazda 6....

The common item between our two problems is I also cleaned my MAF. I don't know that they are related but we have similar issues and both touched the MAF so I want to follow your issue as well. Best of luck on this!

Tom

Tom yours should be a vacuum leak. You will need to smoke test intake or use propane/ water to find leak

Edit ....
After reading your novel tom this looks like a o2 sensor issue or injector problem.

For 02 sensors check for oil in conectors if there is then clean them out with electrical conector cleaner (make sure key is off and no power going to connector first)

You will need to graph the o2 sensor reading otherwise you could misdiagnosis this issue.

tjscanlon wrote: Can't really help you but am having very similar issues with my 2008 Mazda 6. LTFT at -20, STFT at 0 when at idle. When I increase RPM, the LTFT goes to 0 and the STFT stays at 0 - does not change at all. I have a thread in here as well titled 2008 Mazda 6....

The common item between our two problems is I also cleaned my MAF. I don't know that they are related but we have similar issues and both touched the MAF so I want to follow your issue as well. Best of luck on this!

Tom

An easy test is to try unplugging and re-plugging your MAF sensor about ten times, and then see if the short term starts trying to cancel out the long term. That’s what mine does when I mess with the plug, and then after a while it just settles back to its rich reading until I mess with it again.

In my case having two cylinder banks makes finding the location of the error a lot easier. If both my upstream O2s are reading identical then I know to look earlier in the system for the problem. Assuming you have only one upstream O2 and four cylinders, it’s a little harder to pinpoint where the problem is occurring.