[Solved] Fuel trim

On the interstate my short fuel trim SHRTFT1(%), is consistently at -20.3.
I believe this is causing it to downshift. What could be the cause of this and how do I fix it? What’s the normal trim % supposed to be?

This is a continuation of my last thread

 

There's a couple threads on this which may help you:

https://www.tundras.com/threads/1st-gen-4-7-long-term-fuel-trim-normal-percentage.106586/

https://www.tundras.com/threads/1st-gen-4-7-long-term-fuel-trim-normal-percentage.106586/

I can't directly answer your question due to lack of knowledge. My basic understanding of fuel trims is - and take this with a grain of salt, my memory is shit - but depending on what the upstream O2 sensor reads in the exhaust, the short-term fuel trim will be adjusted. If the short term stays at a specific % for a long time, the ECU will update the long term to match that %. I want to say excess oxygen in the exhaust gasses is one of the triggers to increase fuel. I don't remember if negative FT means it's seeing too much oxygen or too little. Maybe too little, so it's dialing back on fuel delivery?

 

The guys in that thread are talking about long term, I need any info I can get on the short term trim.

 

I'm no expert, but I don't see why the STFT would cause it to downshift or behave any differently. With that said, it is weird it's consistently off by that much. What is the value for bank 2? What is the LTFT for bank 1 and 2? Has it thrown a too rich code?

 

No codes at all for this, had a cylinder 5 misfire code about a week ago. New Denso ignition coils seemed to fix that. I’ll check the other bank and the long trims tomorrow and let y’all know.

 

Image #1
This is what it pretty much stays at during normal interstate driving

Image #2
This is what it does after I come down a long hill on the interstate, after it flattens out the trims will stay like this until I get to the next hill. Then it returns back to Image #1 state and starts bogging down under load again.

LONG FT 1 never changes from 0

 

When was the last time you changed plugs or upstream O2?

Seems like you may have a small vacuum leak or exhaust leak on bank 1.

In the meantime pull your negative battery cable then cycle your key like you're going to start the engine. This should reset your short and longterm trims. Drive around for a couple days and see if you see out of whack numbers again.

 

I'm pretty sure long FT never changes unless short FT is consistently at a value at least X% plus/minus away from the current long FT. If short FT remains consistently higher or lower than long FT, ECU will apply the short FT as the new long FT.

Short FT is variable. Long FT becomes whatever the average/typical short FT is, if I recall correctly. You shouldn't see long FT move, I don't think?

If I'm right on this and what I wrote above - again, grain of salt, my memory is shit - logically, it'd be indicative of either you getting a bad air flow reading upstream (i.e. MAF not reading flow properly), or an introduction of oxygen between the MAF and the the combustion stream (i.e. a manifold leak, vacuum leak from anywhere in that range, or similar way for oxygen to enter the stream). One other possibility would be bad reading from upstream O2 sensor, but I'd only suspect that if your STFT was showing high or low on a single bank, not both.

If you did have an intake manifold leak, you theoretically should be hearing a sucking or hissing sound while the engine is idling. And again, given the intake manifold and exhaust manifold gaskets are all using separate pieces per-bank, likelihood you'd be seeing both banks simultaneously with high STFT would be super low.

 

A healthy engine has STFT oscillating slightly around 1 (0.98 to 1.02, 98 to 102%, -0.02 to 0.02, air-fuel ratios depending on how the sensor reading is presented)) in its perpetual look out for the best AFR. If ECU detects STFT leaning away from 1.0 it adjusts LTFT, so STFT would return back to 1. LTFT has multipliers for low, mid and high loads. This is why it jumps instantly when you stomp on the pedal. LTFT in a healthy engine is as close to 0% as possible.

 

Also what brand are your O2 sensors and your MAF? How old are they and how many miles?

 

It was the upstream driver side o2 sensor, bank 1 sensor 1. 8 hours into my 12 hour drive today the codes finally popped for it. P0300, P0130, and P0133. Stopped at an advance and got a brand new Denso, changed it in the parking lot. Truck drove perfectly for the last 4 hours back home.

 

Interesting. I swore in your other thread you said you replaced the O2.

Advance doesn’t show Denso O2 for me. I can’t even get em shipped. I wonder why GA is different from wheee you are. Aggravating.

Oh well. On the plus side, I wasn’t wrong about the three possible causes.

 

Yep, just walked in a told him year and motor, exact match denso popped up on screen. I think it was $95 after tax.