Setting Fuel Trim

[desertspyder]

I read the post on setting low throttle fuel trims with my datalogger but I can't seem to get my fuel trims to change with my AFC. They're stuck at 12 for long term fuel trim and 5 for short term. Tried adding and subtracting but they won't change. Any advice would be greatly appreciated.

 

[desertspyder]

Also, Can having messed up fuel trims cause my car to bog down and make a wierd noise when I first press the pedal or could it be that my TPS is set wrong?

 

[desertspyder]

Are you telling me there's NOONE out there that has encountered the same problems?

 

[iiiwildfireiii]

Uhm...reset your ECU. Check your o2's and make sure you're not down an o2 sensor. Adjust from there.

 

[iiiwildfireiii]

Also, Can having messed up fuel trims cause my car to bog down and make a wierd noise when I first press the pedal or could it be that my TPS is set wrong?

Yes, having messed up fuel trims can bog it down. It’s called running rich. If you're pegging out at +12 on the LTFT and +5 on the STFT, then you're running at least 17% rich. My car did the exact same thing when I lost the front O2 sensor. Replaced it for 50 bux, reset the ECU and was golden. Running rich is very bad for your engine. The reason that your fuel trims aren't moving is because the data is accumulated over a long period of time and is averaged in. Just reset the ECU and check to see if your O2's are cycling like normal.

 

[desertspyder]

I reset my ECU... Which was probably a bad move because I will be getting smogged soon... and now my fuel trims are at 5 for short term and 0 for long term and the only time I can get it to change is when it drops to - 4 after I rev and let off the gas. I'm not bogging anymore though.

 

[desertspyder]

How are my o2's supposed to cycle?

 

[erebusknight]

Okay Ill start slow...

Oxygen sensors moniter the Oxygen coming out of the spent combustion chamber. Almost all typical sensors will vary in Voltage based on what they are. O2 Sensors read from 100mV to 1,000mV (1V).

When a STFT or LTFT reads positive (like +12%) then the computer senses a LEAN conditon (more Air then Fuel) which is why it is adding a little bit longer time to hold the injectors open. Vice Versa if the nubers were negative.

So, you have a pre-CAT and post-CAT O2 Sensor. The pre-CAT fluctuates within 450mV at a good idle. Remember: HIGH VOLTAGE means RICH so fuel trim will be NEGATIVE and vice versa for LOW VOLTAGE.

now the CAT in there (if you still have it in the system) to clean up the byproducts of combustion so we can all go to the park and be happy. So now the pre and post-CAT do different things

The Pre-CAT = STFT(short term fuel trim)
The Post-CAT = LTFT(long term)

the Pre is getting all the heat and nothing much in front of it so it has an immediate effect on the injector pulse width(time open). this will be a quick sin wave on a logger. The Post O2 effects LTFT in the sense that if the Pre O2 cannot keep up with the Trim desired then the LTFT will increase or decrease injector on time from a set table in the computer.

Example: In your case of having such high (+) STFT/LTFT readings then I will assume there is a large air leak in the system. say your hose has a crack anywhere between the MAF and the manifold. The Airflow sensor did not pick up this air coming in. when these 'parts' of air enter they enter the engine and the computer says: I only need to piss in this much fuel. As soon as it exits the O2 sees that there was a lot of air (low voltage) then it tells the computer: WTF? Give me more fuel in the next pulse to keep it closer to 14.7 to 1. when the computer does this it may tke the STFT from 0-2% to something like 5% or more.

Example for LTFT: so if that air just coming in like middle school girls in a Backstreet Boys concert and the bouncer (Mr. O2 Sensor) couldnt tell security (Fuel Injectors) to get more backup (+trim really high) then the Post-CAT O2 takes over. This sensor will change slowly due to the CAT eating up all the spent junk from the motor. This is why there will only be little changes to it. It will adjust the fuel on a grand scale to help the Pre-CAT O2 come to a happy stable state again.

After the Education: Basically you could have a bad O2. Check for stored codes in the computer (if any). Also, having the injectors pulled back means there is serious air being bled in from after the Airflow Sensor. check all vacuum hoses like the brake booster hose or any other hose in the intake system. A good way to watch O2 and STFT/LTFT is to dump propane in to the intake or pull a manifold hose to suck in outside air. You will see things happen on a logger or you will hear the car stumble in different ways. Anyway, Im trying to be a Grease monkey for a living so Im trying my best to get things very clearly across.

Good luck with it...PM me for any other things you want to know or to tell me to keep it simple stupid

 

[dub2gsx]

Make sure your sensors are set for Karman on your S-AFC II because if they aren't then then the car will run funky.

 

[desertspyder]

They are set for karman.

 

[iiiwildfireiii]

Erebusknight...

Did you just use a Backstreet Boys analogy to explain DSM's?? I call on the moderators for their judgment as to whether or not that is blasphemy!

On Topic, there are literally TONS of sources on here about O2's and what to do with 'em. Seriously. Tons.

 

[erebusknight]

It was late and I was tired (Ricer excuse), but anyway I couldnt really think of any other thing to compare it to at the time. It just came to mind (PS - I dont like boy bands )...

but yeah Im sure you'll find plenty of things with the search function