‘96 Corvette Negative Fuel Trims. Help!

[johnny_b]

Background on the car it’s a 96 Corvette A4. When I bought it ran like it had a miss. Since then I’ve found and corrected a number of problems. Removed a jet piggyback tuner pos. Ran great. Still blew raw fuel out one side of the exhaust ( pipes are divorced). #4 injector leaking changed along with spark plug, all good no more fuel out the exhaust. Fuel pump shit the bed, replaced with a new one. Oil change, previous oil was fuel contaminated from injector constantly dripping. Frankenstein 2pc coil wire? Yank. Proper coil wire in. Pulled fuel regulator vac line? Gas pours out, fpr replaced. Now the car fires right up forcefully every time.

now I’m doing datalogs and it just goes max negative fuel trim as soon as closed loop hits. I tuned it with a maf delete cause it’s de-screened. No change.

someone give me an idea what to check next....

I’ll post logs later.

[NomakeWan]

Apologies if this is obvious but I didn’t see it mentioned. Did you replace all the O2 sensors after correcting your massive overrich condition? If not, they are probably shot and causing your huge fuel trim issues.

That is, assuming that the car is not running pig rich in open loop, as well.

[johnny_b]

Apologies if this is obvious but I didn’t see it mentioned. Did you replace all the O2 sensors after correcting your massive overrich condition? If not, they are probably shot and causing your huge fuel trim issues.

That is, assuming that the car is not running pig rich in open loop, as well.

I was always under the impression that when O2s fail they fail lean? Causing the pcm to add fuel (blm above 128 for safety). They are switching but they have been moved I think so may need to tune for o2 delay and such.

either way it’s at 108; -16%, soon as CL kicks in.

i believe it’s false because throttle response become immediate trash but if I force open loop and clear blms the throttle response is snappy.

[johnny_b]

Forgot to mention car has been converted to obd1 witch proper knock sensors.

[NomakeWan]

I was always under the impression that when O2s fail they fail lean? Causing the pcm to add fuel (blm above 128 for safety). They are switching but they have been moved I think so may need to tune for o2 delay and such.

either way it’s at 108; -16%, soon as CL kicks in.

i believe it’s false because throttle response become immediate trash but if I force open loop and clear blms the throttle response is snappy.

If the O2 circuit fails, then that may be how the PCM compensates; but if the circuit is good then the PCM will trust whatever the O2 sensor is telling it, including if the O2 sensor falsely reports a rich condition. I've seen it happen.

If the O2 sensors are not in the stock locations, then yes, it's also possible that you'll need to change the appropriate sections in the BIN. That being said, was this something it always did since the swap (and/or the moving of the O2 sensors) or did this just suddenly happen for no apparent reason? That should help narrow things down.

[johnny_b]

The o2s have been like that since I bought It. I will adjust the tune for that. Kinda doubt that’s causing max negative fuel trims but It can’t hurt.

im trying to avoid band-aiding the problem through tuning

[NomakeWan]

If they've been like that the whole time, but the trims only started acting up now, then there's no reason to suspect the location of the sensors to be the culprit.

[johnny_b]

If they've been like that the whole time, but the trims only started acting up now, then there's no reason to suspect the location of the sensors to be the culprit.

as far as I know the trims may have always been messed up. That was the reason I converted to obd1 as I’ve tuned several obd1 lt1’s. I’ve been using the data logs to point me to things like the leaking injector and the regulator.

[Rocko350]

Background on the car it’s a 96 Corvette A4. When I bought it ran like it had a miss. Since then I’ve found and corrected a number of problems. Removed a jet piggyback tuner pos. Ran great. Still blew raw fuel out one side of the exhaust ( pipes are divorced). #4 injector leaking changed along with spark plug, all good no more fuel out the exhaust. Fuel pump shit the bed, replaced with a new one. Oil change, previous oil was fuel contaminated from injector constantly dripping. Frankenstein 2pc coil wire? Yank. Proper coil wire in. Pulled fuel regulator vac line? Gas pours out, fpr replaced. Now the car fires right up forcefully every time.

now I’m doing datalogs and it just goes max negative fuel trim as soon as closed loop hits. I tuned it with a maf delete cause it’s de-screened. No change.

someone give me an idea what to check next....

I’ll post logs later.

Descreening the MAF does not mean you need to turn it off. The car most likely is not in full speed density mode even though you set the speed density flag. You still need to set the MAF to fail at 1 mghtz and turn the codes off and unplug the sensor. I have a few b bodies that will still try to use the MAF even though its been "switched off" until i unplug them. Then they "act right". If the 02s were moved, was this due to headers being installed?

Chris

[johnny_b]

Descreening the MAF does not mean you need to turn it off. The car most likely is not in full speed density mode even though you set the speed density flag. You still need to set the MAF to fail at 1 mghtz and turn the codes off and unplug the sensor. I have a few b bodies that will still try to use the MAF even though its been "switched off" until i unplug them. Then they "act right". If the 02s were moved, was this due to headers being installed?

Chris

you know you’re right about it still trying to read the maf, I’ve had that happen on another car, I will mess with this and report back.

The maf was already descreened and I did the SD flash just to try to eliminate it as a possibility. O2s were moved because previous owner cut out the cats and put straight pipes in place

[steveo]

I have a few b bodies that will still try to use the MAF even though its been "switched off" until i unplug them.

if the speed density flag is set in EE or throws a maf code it absolutely does ignore the MAF 100%

there's no such thing as 'not full speed density mode' in EE either. it's either using speed density or the MAF

[johnny_b]

Drove the car around for a little bit yesterday still need to post the logs, i always get on from my phone.

Anyways the negative trims persist. Fairly certain the rich condition is false because it seems it’s running lean as it’s running kinda hot. It touched 230 before I forced the fans on.

[NomakeWan]

If you're sure they're fake that's good enough for me. The O2s were "like that" since you bought the car. So, have you tried replacing them? They're not that expensive and it's a very easy thing to do. I just really want to make sure that we're not chasing ghosts here.

[johnny_b]

New O2 sensors will be here Wednesday. Will report back

[johnny_b]

Update:

new o2s seems to have made a difference in the trims.

Now I need to resolve a code 43. I had the code turned off in the tune so I could focus on other issues the engine might have running wise.

Brand new ‘95 knock sensors were installed when I changed the O2s. Suspect it might be a wire issue. Plan on getting the meter out and going through it.

Does anyone have any experience or advice on resolving code 43? I’d rather not put resistors in place of the knock sensors like some people do