Fuel Trim

Joined
Jan 7, 2017
Messages
222
Location
Indiana
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?
 
Joined
Jan 7, 2017
Messages
222
Location
Indiana
Any exhaust leaks before the upstream O2 sensor?
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!
 
Joined
Jul 27, 2021
Messages
210
Location
Florida
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.
 
Joined
Jan 7, 2017
Messages
222
Location
Indiana
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.
Thanks. Also, I know ethanol blended fuel (which I use) plays a part in the LTFT. Maybe also the 33” tires I’m running?
 
Joined
Jan 7, 2017
Messages
222
Location
Indiana
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?
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
9AF10BBB-34CC-4D04-AFF5-FBADD1BC2816.jpeg
turns to go toward passenger side to a hard line. If that short rubber line is leaking would that cause missy idle and higher long term fuel trims?
 
Joined
Jan 7, 2017
Messages
222
Location
Indiana
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 View attachment 3078729turns to go toward passenger side to a hard line. If that short rubber line is leaking would that cause missy idle and higher long term fuel trims?
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?
 
Joined
Jan 7, 2017
Messages
222
Location
Indiana
a vacuum leak would increase your LTFT values. and the values you are reporting are in line with what I've seen.
Thanks, Landtank. So you see +10% LTFT as normal on these trucks with 33” tires and heavy aftermarket bumpers?
 

landtank

Supporting Vendor
SILVER Star
Joined
Mar 27, 2003
Messages
20,527
Location
Groveland MA
Website
landtankproducts.com
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.
 
Joined
Jan 7, 2017
Messages
222
Location
Indiana
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.
Gotcha. Thank you. Would you keep digging to try to figure out why LTFT is always around +10 or call it good?
 
Joined
Jul 27, 2021
Messages
210
Location
Florida
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.
 

bajaphile

Boojum Hugger
SILVER Star
Joined
Apr 6, 2008
Messages
2,190
Location
Solana Beach, CA
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.
 
Joined
Jan 7, 2017
Messages
222
Location
Indiana
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.
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.
 

Users who are viewing this thread

Top Bottom