engin_ear

iTrader: (1)

'99 GS400, 170K miles. I've been monitoring my Long Term and Short Term fuel trims as I drive, and I'm seeing the Long Term trims dancing around as fast as the Short Term trims, I was under the impression that the long term trims were supposed to be more stable (long term, duh) but mine are clearly not stable, they change very much with every change in throttle position. For those of you that work with this stuff all the time, what is the rate of change SUPPOSED to be for long term trims?

I've replaced most of the vacuum hose clamps I can get to, or that seemed too old/loose, under the hood. I may still have a vacuum leak elsewhere, so i may get a smoke test done. But my question is what do the Long Term trims look like when the car has no problems at all? Maybe I have NO problem, or maybe I have an ECM problem...?

Thanks

TrueGS300

To determine that things are good. At idle you should see the STFT going positive/negative back back and fourth +or- 2-3. And LTFT should be pretty stable at positive 7 or less for the age of the car. That would be well good enough to not throw codes but not like a perfectly new car which should be down below positive 5 LTFT. When driving, you should only be looking at cruising speeds. Heavy acceleration, WOT, or sporadic pedal inputs are not looked at because the computer starts dumping fuel etc. But I like to see similar readings of below Positive 8 LTFT when cruising.

What I believe you are referring to when you're driving around and seeing instant large changes are various fuel trim cells though. I'm not too familiar with exactly how Toyota does theirs. But I know GM well and I imagine they are all similar as all cars use similar o2 based data. There can be upwards of 20 different fuel trim cells that the computer uses under all sorts of different conditions. Say you are cruising at 50mph and watching LTFT. It happens to be in cell 5. But you want to go 55mph. So you lightly accelerate. You see a big instant change in LTFT. But it's because for that particular driver input, the car just moved to cell 12 or whatever it would be and is going off the usual learned data for that trim cell. Obviously in the example cell 12 which is only used at this amount of acceleration is going to be different than cell 5 which just sits nice and smooth without additional fuel demand.

engin_ear

iTrader: (1)

Thanks TrueGS300!
I didn't know about this cell thing...sounds like there is not one LTFT value, but a whole bunch depending on conditions. At cruising on highway, my LT trims are reasonable, somewhere between 3 and 9. Once I stop the car though, the trims (this morning for example) went to 21.9 for both Bank 1 & 2 and sit there until I press on throttle again. Yesterday's high number was 17.9. The ST trims seem to be fine, they're usually between -3 and +5. I did replace fuel pump and filter and replace a bunch of vacuum clamps. I'm not throwing the lean code anymore, but things didn't seem normal.

TrueGS300

Sounds like you're lean. At idle engine vacuum is the highest and will be where vacuum leaks are the most prominent. Have you tried using an alternate fuel source to find leaks? Propane is the method by the book, but everyone mainly uses a spray like starting fluid or brake clean. Watch your fuel trims and spray around the intake, throttle, pcv, etc until you see the trims go way negative. When you see a change, focus your sprays more and more to the exact place that makes the trims go negative. You may find that its a cracked or disconnected line, a throttle gasket, an intake gasket etc. I've had EVAP system purge valves stuck open causing a lean condition due to the unmetered air constantly leaking in. Lots to check!

dwoods801

I replaced every piece of hose twice and finally traced the problem to a weak fuel pump. I had always thought a car that ran had a good fuel pump, but I guess they can get weak, and cause lean conditions across the rpm range. It was an easy pump to replace once I figured it out. Ordered a pump of eBay for only $50, and took care of it.

TrueGS300

Just sucks that checking fuel pressure on these cars is a PITA. Hate when manufacturers don't give a test port. Have to try and access the banjo bolt from the feed line to the rail under/behind the intake.

A tip though, usually when a lean condition is cause by a weak fuel pump it is seen at higher rpm/driving when fuel demand is greater and the LTFT goes lean. Lean conditions at low rpm/idle are caused by vacuum leaks when the vacuum from the engine is the greatest. A scanner with live data and watching fuel trims is really the only way to figure out vacuum leaks these days. Unless you can obviously hear it and find it that way.