Fuel Trim

[dalesocks]

Anyone care to explain how the short trim work under normal operation(not at idle)
I'm asking because I have a problem with my car where the short trim is at -40%, so the ecu is trying to take away alot of fuel and I'm running very rich.

 

[pauleyman]

So what did you change? What mods are on the car if any?

 

[Black-Out]

Anyone care to explain how the short trim work under normal operation(not at idle)
I'm asking because I have a problem with my car where the short trim is at -40%, so the ecu is trying to take away alot of fuel and I'm running very rich.

Well you've basically answered your own question. You seem to have a grasp on understanding what the fuel trims are, I think your actual questions are what are they trying to tell you and what should they be. your short term fuel trim is the actual adjustment your computer is trying to make at that moment- long term fuel trim is what your computer has adapted to. At any given time your fuel trims both long and short should be near 0 + or - 5% at normal operation on a stock vehicle (this isn't solely towards DSM's either, this is generally for every vehicle with OBD1-2 type management systems.)

Obviously + numbers mean the computer is compensating for lean running - rich running. You could have several causes for these conditions going out of whack but one that comes to mind is the fuel pressure regulator control solenoid staying shut when it should be open (allowing vacuum). The vacuum signal, under normal operation, to the fuel pressure regulator is allowed to pass until the computer senses boost and cuts the vacuum signal to boost fuel pressure by restricting return flow to the tank- if the diaphragm in the regulator fails, or the solenoid fails, or the vacuum line fails- all of these can drive the short term trim to the maximum (or in your case minimum) range of compensation the computer can make before it simply says there's a problem it can't fix and trips your check engine light.

The main thing for you to do is think about your modifications and how they influence the fueling. If you have an AFC or anything else that changes the air flow meter signal to the ecu then a bad adjustment or a faulty computer can cause this too. The possibilities are vast......almost anything from bad ect sensor, front O/2 sensor, air flow meter, leaking injectors, larger fuel pumps without any way to control rail pressure (lack of an RRFPR) restricted air filter,partially clogged exhaust systems, to a poor spooling or bad turbo,bad air temp sensor, boost leaks, or various vacuum leaks ect.

List your mods to your car and a more specific diagnosis can be directed to you but without knowing every little detail about your car and its condition and modifications it'll be a bit of a shot in the dark as fuel trims alone only tell part of the story. Your air flow rate in grams per second is another-

William-

 

[dalesocks]

I understand my ecm was trying to take fuel away because it was running very rich. I seem to have had a very small leak right after the maf sensor, put it back together and car seem to run a bit better and not miss yet. Btw this is on an ls1 trans am not a dsm. But that is irrelevent.

Well you've basically answered your own question. You seem to have a grasp on understanding what the fuel trims are, I think your actual questions are what are they trying to tell you and what should they be. your short term fuel trim is the actual adjustment your computer is trying to make at that moment- long term fuel trim is what your computer has adapted to. At any given time your fuel trims both long and short should be near 0 + or - 5% at normal operation on a stock vehicle (this isn't solely towards DSM's either, this is generally for every vehicle with OBD1-2 type management systems.)

Obviously + numbers mean the computer is compensating for lean running - rich running. You could have several causes for these conditions going out of whack but one that comes to mind is the fuel pressure regulator control solenoid staying shut when it should be open (allowing vacuum). The vacuum signal, under normal operation, to the fuel pressure regulator is allowed to pass until the computer senses boost and cuts the vacuum signal to boost fuel pressure by restricting return flow to the tank- if the diaphragm in the regulator fails, or the solenoid fails, or the vacuum line fails- all of these can drive the short term trim to the maximum (or in your case minimum) range of compensation the computer can make before it simply says there's a problem it can't fix and trips your check engine light.

The main thing for you to do is think about your modifications and how they influence the fueling. If you have an AFC or anything else that changes the air flow meter signal to the ecu then a bad adjustment or a faulty computer can cause this too. The possibilities are vast......almost anything from bad ect sensor, front O/2 sensor, air flow meter, leaking injectors, larger fuel pumps without any way to control rail pressure (lack of an RRFPR) restricted air filter,partially clogged exhaust systems, to a poor spooling or bad turbo,bad air temp sensor, boost leaks, or various vacuum leaks ect.

List your mods to your car and a more specific diagnosis can be directed to you but without knowing every little detail about your car and its condition and modifications it'll be a bit of a shot in the dark as fuel trims alone only tell part of the story. Your air flow rate in grams per second is another-

William-

And Also how does the ecu know the car is running lean or rich?

 

[@TDC]

Maybe this will help out! Its a bit long but it should serve as some good knowledge!

[ame=http://www.youtube.com/watch?v=oRX2V6_a3dc&list=WLA0D7729464C5E2BF]Understanding Short Term and Long Term Fuel Trim - YouTube[/ame]

 

[Black-Out]

I understand my ecm was trying to take fuel away because it was running very rich. I seem to have had a very small leak right after the maf sensor, put it back together and car seem to run a bit better and not miss yet. Btw this is on an ls1 trans am not a dsm. But that is irrelevent.



And Also how does the ecu know the car is running lean or rich?

By monitoring the front O/2 sensors against the calculated air flow into the engine (via mass air flow) you computer can determine the amount of oxegen left in the exhaust gasses against air flow rate to figure a proper afr- if for any reason these sensors start picking up a mixture issue consistently outside of it's ideal parameters the computer adapts the fuel to try and correct the condition until it can no longer adjust for the condition the computer sees from the reading off the primary O/2 sensors. once here they trip codes and turn on the c.e.l. to make the driver aware that there's an issue.
By the way the leaking air coupling to the throttle body is pretty common on those things, the plastic flex pipe can get pretty shotty after a 5-10yrs of service- they have silicone replacements that work pretty good you may want to look into them....

My pops has a 2000' Pontiac Firehawk Trans-Am.....I love my TA's been a fan of F-boats for ages!

William-

 

[dalesocks]

By monitoring the front O/2 sensors against the calculated air flow into the engine (via mass air flow) you computer can determine the amount of oxegen left in the exhaust gasses against air flow rate to figure a proper afr- if for any reason these sensors start picking up a mixture issue consistently outside of it's ideal parameters the computer adapts the fuel to try and correct the condition until it can no longer adjust for the condition the computer sees from the reading off the primary O/2 sensors. once here they trip codes and turn on the c.e.l. to make the driver aware that there's an issue.
By the way the leaking air coupling to the throttle body is pretty common on those things, the plastic flex pipe can get pretty shotty after a 5-10yrs of service- they have silicone replacements that work pretty good you may want to look into them....

My pops has a 2000' Pontiac Firehawk Trans-Am.....I love my TA's been a fan of F-boats for ages!

William-

Thanks for the info, what I didn't understand was that the fuel trims are the adjustments the ecu is trying to make.

By monitoring the front O/2 sensors against the calculated air flow into the engine (via mass air flow) you computer can determine the amount of oxegen left in the exhaust gasses against air flow rate to figure a proper afr- if for any reason these sensors start picking up a mixture issue consistently outside of it's ideal parameters the computer adapts the fuel to try and correct the condition until it can no longer adjust for the condition the computer sees from the reading off the primary O/2 sensors. once here they trip codes and turn on the c.e.l. to make the driver aware that there's an issue.
By the way the leaking air coupling to the throttle body is pretty common on those things, the plastic flex pipe can get pretty shotty after a 5-10yrs of service- they have silicone replacements that work pretty good you may want to look into them....

My pops has a 2000' Pontiac Firehawk Trans-Am.....I love my TA's been a fan of F-boats for ages!

William-

I got it fixed already, the coupler to the throttle body was not hooked properly and a tiny bit of un monitor air was getting in.