2003 Corolla High LTFT At High RPM

Good afternoon,

I have a 5-speed 2003 Corolla (1ZZ-FE 1.8-liter engine) with 126K miles. The check engine light came on two weeks ago when I was on the highway. The code was P0171. See the attached freeze frame. I did not notice any problems with drivability. There was no rough start or rough idle. There was no acceleration hesitation. A few days before the code appeared, I had an oil change at the Toyota dealership and they recommended front brake replacement and fuel system cleaning. I had the front brake pads/rotors replaced at another garage and did not do fuel system cleaning. I drove ~500 miles in a few days and then the check engine light came on so maybe the problem was not related to brake replacement or oil change. After the code appeared, the highway gas mileage was still high at 39-40 mpg. The check engine light has not come back after it was cleared. However, the LTFT is still high at high RPM. The MAF sensor was not dirty but I cleaned it using MAF cleaner and replaced the engine air filter. I removed the engine cover and sprayed brake cleaner over intake manifold gasket and all hoses and did not see changes with RPM or fuel trim. I applied Techron fuel system cleaner, drove a few hundred miles and it did not help. The MAF readings were consistent with normal values for Corolla cited in a forum. I understand that solutions may include MAF sensor cleaning/replacement, O2 sensor replacement, intake manifold gasket replacement (with orange one), fuel injector cleaning/replacement and other ways to fix leak or fuel delivery.

Here is the summary:

1. I did not notice any rough idle, rough start or acceleration hesitation.
2. When idle at 700 rpm, STFT was normal (+-10% or less). LTFT was normal (+-5%).
3. At 2400 RPM (60 miles/hr), STFT was +-10% and LTFT was around +20%.
4. At 2900 RPM (70 miles/hr), STFT was +-10% and LTFT was around +24%.
5. The city gas mileage was about 23 mpg. The highway gas mileage is still excellent (39-40 mpg).

Since STFT and LTFT were normal at idle and LTFT increased with RPM, this does not look like a vacuum leak. Also acceleration was fine so the fuel pump is probably okay.

For P0171, it is common to see high LTFT at idle but reduced LTFT at high RPM, indicating vacuum leak, or high LTFT at both idle and high RPM, indicating fuel delivery problem.

For this car, LTFT is normal at idle and returns to normal quickly when the car slows down but increases when the car speeds up. What may cause this problem? Please see the attached scan results. Thanks!

Please see the attached images. O2 was lean when the throttle position was 80%. The calculated engine load was 60%. LTFT actually dropped when throttle opened.

Monde wrote:

O2 sensor should read rich at WOT. Is the o2 on the rich side at WOT? What is the calculated load value at WOT?

Looking at your trims, I'm suspecting a weak pump.

For a quick test...Get some propane small can, add long length of hose, go for a ride, and when those trims start topping turn on the propane into the intake airstream.

Use an assistant, and be safe!!!!!!

Those LTFT's start to drop you can suspect not enough fuel from the pump, or a restriction somewhere in the fuel system.

Thanks for the information! I do not have the resources. Is there an easier way?

I tried again using the 3rd gear and could only reach 80% throttle. Please see the attached images.

Paul P. wrote:

Looking at your trims, I'm suspecting a weak pump.

For a quick test...Get some propane small can, add long length of hose, go for a ride, and when those trims start topping turn on the propane into the intake airstream.

Use an assistant, and be safe!!!!!!

Those LTFT's start to drop you can suspect not enough fuel from the pump, or a restriction somewhere in the fuel system.

The MAF reading looks low and irregular at times when it should be matching RPMS/throttle.
If already cleaned try another sensor.

This last batch of test data, you let off the throttle too early!!!!!

If you can't get your Calc Load past 60% at wide open throttle in only 1st or 2nd gear, then I would start suspecting an air problem. You should be hitting 80% and better.

Lean O2's>>>>>>Low Calc Load or Abs Load>>>>>>>Air issue
Lean O2's>>>>>>High Calc Load or Abs Load>>>>>>Fuel issue

Whenever I see fuel trim +10 Total trim and greater, the very first thing I want to determine is if this is an Air OR Fuel Issue.

I load up 1 PID Calc Load OR ABS Load, put the vehicle in 1st or second and hammer the throttle to achieve the maximum load possible.

The data set you present has lean O2's at higher RPM.

What isn't apparent is, was the 60% the highest load possible?
- if it was, there is a restriction in airflow or the MAF is incorrectly measuring airflow.

If you can hit 80% or better and still have lean O2's you have got an insufficient fueling issue.

What is the Highest CALC Load you can achieve? I do see an 80%, and that probably is pretty good, but the O2's are lean!!! NOT ENOUGH FUEL coupled with positive trims.

I did not try the first gear or the second gear because the RPM would be in the red zone. I depressed the gas pedal fully in the third gear until the car speed reached 60 or 70 miles/hour. The highest calculated engine load I could achieve was around 65%. Also I was only able to achieve 80% throttle.

I cleaned the MAF sensor twice and there was no help. I just ordered a Denso MAF sensor from Amazon and it will arrive in a week.

What may cause restriction in airflow?

Paul P. wrote:

This last batch of test data, you let off the throttle too early!!!!!

If you can't get your Calc Load past 60% at wide open throttle in only 1st or 2nd gear, then I would start suspecting an air problem. You should be hitting 80% and better.

Lean O2's>>>>>>Low Calc Load or Abs Load>>>>>>>Air issue
Lean O2's>>>>>>High Calc Load or Abs Load>>>>>>Fuel issue

Whenever I see fuel trim +10 Total trim and greater, the very first thing I want to determine is if this is an Air OR Fuel Issue.

I load up 1 PID Calc Load OR ABS Load, put the vehicle in 1st or second and hammer the throttle to achieve the maximum load possible.

The data set you present has lean O2's at higher RPM.

What isn't apparent is, was the 60% the highest load possible?
- if it was, there is a restriction in airflow or the MAF is incorrectly measuring airflow.

If you can hit 80% or better and still have lean O2's you have got an insufficient fueling issue.

What is the Highest CALC Load you can achieve? I do see an 80%, and that probably is pretty good, but the O2's are lean!!! NOT ENOUGH FUEL coupled with positive trims.

I wasn't kidding about hammering the throttle. Were you over 5000 RPM when you achieved 65% Load?

That's not really good, but's it close to being on the fence for there to be 2 issues going on.

1. Bad MAF (Air not measured correctly, only pumped 65 % of air at that rpm as compared to dyno testing from factory)
2. Weak fuel PUMP (lean O2's)

Your Trims aren't indicative of an air restriction, ie, plugged cats will tend to cause Negative trims.

...and did you perform the propane test? to see if the trims improved at high rpm?

I would expect a 1.8 Liter motor to achieve 70 to 85 g/s at 5500RPM from the MAF.

Update (10/5/2022)

I installed a new Denso MAF sensor (Made in Vietnam) and LTFT dropped from 25% to 7% @ 70 miles/hour. It seems the peak calculated engine load also improved a little. See the attached screenshots.

According to the blog here ( www.vehicleservicepros.com/service-repai...-value-of-fuel-trims ): "A failing or contaminated MAF sensor will often cause negative fuel trims at idle and progressively more positive fuel trims as engine speed and/or load increase.". This roughly matched the symptoms of my car.

Do I need to be concerned about the 7% LTFT? I did not notice any drivability problems.

Thank you all for the suggestions!!!

Paul P. wrote:

I wasn't kidding about hammering the throttle. Were you over 5000 RPM when you achieved 65% Load?

That's not really good, but's it close to being on the fence for there to be 2 issues going on.

1. Bad MAF (Air not measured correctly, only pumped 65 % of air at that rpm as compared to dyno testing from factory)
2. Weak fuel PUMP (lean O2's)

Your Trims aren't indicative of an air restriction, ie, plugged cats will tend to cause Negative trims.

...and did you perform the propane test? to see if the trims improved at high rpm?

I would expect a 1.8 Liter motor to achieve 70 to 85 g/s at 5500RPM from the MAF.

The calculated engine load was just over 80% with 75% throttle. At 60-70 miles/hour, the load was 40%-50%.