What would cause long term trims to be so high but not short term?

cp2295 · Mar 13, 2014

Got a 98 4.0 ohv. Every time I look at the long term fuel trims both banks are around 15-20% (sometimes around 12%). But every time I see the short term ones they are from -5%-5%. I've never seen them any lower or higher and they usually hover around 0 so why would the ltft be so high? I notice when the truck is warm it's hard to start sometimes (not every time) always starts instantly when it's cold.

It kinda sputters when it starts like I stop turning the starter and then it takes it a second and then it revs way up to 2200 rpms

ab_slack · Mar 13, 2014

I don't have definitive answer. Just a WAG from my experience regulating things with computers, perhaps the trims are additive and the long term trim is developed by a bias in the sort term trims?

Another way to think about it, the long term reflects the memory of the system while the short term is the current adjustment relative to the nominal long term operating point.

Like having a fine adjust on top of the big adjustment.

You may be tempted to think, okay the long term trim is 15% so the trim it is using right now (short term) should be something close to 15%. But if it is additive, if the short term is 1%, you add 15% + 1% = 16%. So the operating point is basically around that 15% long term.

In such a regulation scenario I would expect exactly what you describe with the short term hovering close to zero. In fact the long term could develop in a bias in the short term. Like if the short term is consistently trimmed positive, the long term will start to become more positive until eventually the short term is sometimes above zero and sometimes below.

So that would be my guess, but perhaps someone has a definitive answer.

cp2295 · Mar 13, 2014

Oh okay that makes good sense, do you know what would cause the percentages to be up around there? It's a lean mix so maybe tiny vac leak I can't find? Fuel pump being weaker? I notice when I throw it into neutral it takes a while to drop rpms down to 750

BRUTUS_T_HOG · Mar 13, 2014

Short term fuel trims are adjusted off long term trim. Of you have +5 long term and -5 short term you have 0 total trim. If it stayed that way long enough the long term trim would reset to 0.

Long term trims are what the computer remembers "ok at this throttle and load I had to add 5% more fuel" short term trim is what is happening now "cp2295 fixed that vacuum leak so now I have to remove 5% fuel, I'll remember next time"

ab_slack · Mar 13, 2014

cp2295 said:
Oh okay that makes good sense, do you know what would cause the percentages to be up around there? It's a lean mix so maybe tiny vac leak I can't find? Fuel pump being weaker? I notice when I throw it into neutral it takes a while to drop rpms down to 750

My answer was very speculative, seemed logical to me as I do stuff like that but an entirely different field, but logic can be a good way to screw up with confidence...but it looks like Brutus who answered more clearly seems aligned with my speculation.

As for why the percentages are where they are? I have no clue. For all I know they may be typical as a vehicle ages for it to move a certain way.....Or the percentages are very typical and optimal, but it is the initial state (0% long term) is conservative to insure it runs much like when the computer gets bad O2 sensor input it goes into open loop mode which is less efficient but insures it runs.

I could start running all sorts of speculation and even come up with logic, but I really have nothing concrete to offer. There are too many possibilities, but in practice, there are things that typically happen. I just have no experience so I'll refrain from tossing anything more speculative out there.

cp2295 · Mar 13, 2014

Okay well thank you for clarifying my original question that makes a whole bunch of sense now, before I was just really confused.. I always thought stft were how much the computer compensates right now and ltft is the avg.. It is that way but I see now stft is however much it has to compensate off of that avg, not off of zero

What would cause long term trims to be so high but not short term?

cp2295

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