Fuel trim question: I’ve got a 97 1FZFE. Set my Ultra Gauge to monitor fuel trims. Long term fuel trim is ALWAYS +8 to +11%. Short term fuel trim is always +/-5%, and typically less than +/-3%. If I had a vacuum issue, I’d suspect the LTFT would drop significantly when I hit the gas pedal hard, but it doesn’t really change. I can’t detect a vacuum leak and pretty much everything is new or rebuilt, except injectors and knock sensors. Injectors were rebuilt and flow tested by Witchhunter. Fuel pump, strainer, fuel filter, fuel pressure regulator, fuel pump relay, and igniter are all new. Any ideas of where to look?
Fuel Trim (1 Viewer)
- Thread starter Indiana80
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Any exhaust leaks before the upstream O2 sensor?
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Maybe…I had an OTRAMM exhaust system installed by a local independent shop about a year ago. They struggled with it. They installed a new Magnaflow y-pipe then installed the OTRAMM cat back system. It was hitting the frame in several spots. Long story short, I pulled the OTRAMM system off and reinstalled back to front per OTRAMM’s instruction. When I got everything about buttoned up, I decided I should check y-pipe to manifold joints. They were both loose. I ran it like that for maybe 500-1000 miles without knowing it was loose. After I snugged those bolts (took about 4 or 5 turns), it was quieter, but I wonder if the gaskets might be toast. They were new Toyota donut gaskets. I’ll double check that area for leaks with some soapy water. Thanks!
Yeah, I guess if there is unmetered air coming in before that O2 sensor it will cause the ECU to add more fuel. I've read, but not confirmed, anything read by the downstream O2 sensor won't effect fuel trim even if you have exhaust leaks. You may get a CEL but it should not impact the way the truck runs.
Schrodingers Box on YouTube has some good detailed stuff on fuel trim but I don't recall him ever getting to "exhaust leak" as a cause. So, I can't say that I completely know what I am talking about.
Schrodingers Box on YouTube has some good detailed stuff on fuel trim but I don't recall him ever getting to "exhaust leak" as a cause. So, I can't say that I completely know what I am talking about.
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Thanks. Also, I know ethanol blended fuel (which I use) plays a part in the LTFT. Maybe also the 33” tires I’m running?
When did it tart doing this?
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Not sure. It’s been like this quite awhile.
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I did find what looks like a thicker vacuum line under the upper intake that looks original and crusty. Pretty disappointed b/c I paid a mechanic to pull intake about a year ago and he agreed to replace all the vacuum lines while under there. I’ll post a pic of the vac line- it runs off the blue vacuum switching valve that the charcoal canister hose runs to. It’s a short rubber hose that runs out of the vacuum switch valve toward front of truck and
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In partsouq diagram lookup looks like it’s called a fuel vent hose, which makes sense to me since it runs to charcoal canister. Wondering if this old vent hose could cause a fuel starvation problem or a vac leak?
a vacuum leak would increase your LTFT values. and the values you are reporting are in line with what I've seen.
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Thanks, Landtank. So you see +10% LTFT as normal on these trucks with 33” tires and heavy aftermarket bumpers?
tires, bumpers and such have no bearing on your fuel trim. Its a discrepancy between the calculated base injector duty cycle and the reading from the front O2 sensor. That error is a culmination of all the variables in the system that are used to calculate the base duty cycle along with the air's density and temperature.
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Gotcha. Thank you. Would you keep digging to try to figure out why LTFT is always around +10 or call it good?
I would normally say it depends what your MPG looks like.
Although, if you have a vacuum leak and are sucking in unfiltered air, I would fix it no matter what. Especially if it's just changing out some hoses.
Although, if you have a vacuum leak and are sucking in unfiltered air, I would fix it no matter what. Especially if it's just changing out some hoses.
LTFT is just a compensation value. As long as it’s inside the range of -20 to +20 it’s fine.
I would move on to something else on your truck to obsess about.
Just going to mention it since it's easy, but you can give the MAF a cleaning if you haven't in a while. It helped my STFT and LTFT get a little closer to 0 on my other truck. Those values are within spec as mentioned above.
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Yep, cleaned old MAF w/ no change. Tried a new OEM MAF and no change. Today, she’s idling around 8% LTFT and about the same / maybe a little higher on the higway. She’s happy today.
