‘96 Corvette Negative Fuel Trims. Help!

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.

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.

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Originally Posted by 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.

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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.

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

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Originally Posted by johnny_b

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.

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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.

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

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.

Quote:

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Originally Posted by 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.

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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.

Quote:

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Originally Posted by 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.

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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

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Originally Posted by Rocko350

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

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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

Quote:

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Originally Posted by Rocko350

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

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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

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.

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.

New O2 sensors will be here Wednesday. Will report back

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

Quote:

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Originally Posted by 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

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Glad the new O2 sensors are doing their thing. It's an incredibly common issue.

Speaking of common issues, so is the Code 43. I have that same code in my '95, and disabled it in the tune because it's irrelevant. The problem stems not from an actual problem with the knock sensor elements (that is, the ability for the knock sensors to do their job), but rather from the resistor GM added to them as a means to detect whether they were there or not. These resistors go bad or can be broken by rough handling, and when they do, they throw that code despite the sensors themselves continuing to work as normal. So my suggestion would just be to leave the code disabled and continue on. However, if you're concerned because the knock sensors you installed were new, you could always just test the resistance of your sensors to see if they're within spec. If they are, then yes, you have a wiring issue somewhere. If they aren't, then there's your problem, and disabling the code is all you really need to do.

[QUOTE=johnny_b;83690]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[/QUOT

Hi
My '95 z28 Knock sensor wires were burnt by the headers & chafing on the body, grounding out. Code would come & go so visually check the wire, a meter won't necessarily find the fault.... Pull the pcm out & test: no fault. Put it back Code 43...pull it out Test ok. put it back code 43.

I tuned initially with the LT4 knock module. Current PCM has the LT1 knock module. No real difference once the false noise was eliminated.
The resistor mod... inconclusive.
I tested inputs trimmed with a Variable resistor.
Ah found my notes:
Made a Desensitizing circuit based on Mike Chaney's "Knock Knocker" :email mchaney@charm.net
I used a 1K resistor and 50K Pot so I can play with the sensitivity a little and see what happens.
connects to PCM @ D_22 (Blue Connector) - Dk Blue Wire
could be D_21 (Blue Connector) - Lt Blue Wire
I can't find the data i collected though i seem to remember it just ignored the quieter taps & scored the same with heavier noises.

This might be useful:
2017-05-23
Just spoke with Scott Pfautsch re Knock Module

I'm now running 97 PCM
see http://lt1swap.com/lt1_bcc.htm
Here you will find listed all the BCC's I've come across programming the lt1 pcm's.*
The LT1 computers can be programmed for whatever application you have, iron head, aluminum head, 4.3L or 5.7L.*
Where using these codes might come in handy, is if you have a 95 iron headed motor, they will have two knock sensor on the block.*
So if you need a PCM, you should find one orignaly from an iron headed motor, so you have the correct knock sensor module.*
The knock sensor module is located inside the pcm, under the cover with two screws.*
This module MUST match the knock sensors you are using.

KNOCK SENSORS: Installed in engine block

1993 to 1997 Camaro and Firebird used 1 knock sensor.* 1993 to 1995 sensors different then 1996-97.

1994-1995 B-Body (Caprice, Impala, Roadmaster, Fleetwood) used 2 knock sensors. 1994-95 sensors different then 1996.

* KNOCK SENSOR MODULE: Located inside the PCM, under the cover with two screws.* 8 digit # on top of module.


Knock sensor module 16177700 1994-1995 F-Body 5.7L
16214661 1996-1997 F-Body 5.7L
16214681 1996 LT4 Corvette


SO I GOT 97 PCM with 97 LT1 Knock Module & 95 Knock Sensors ????????????????
I think I bricked that PCM.


DTC Codes
43 Knock sensor Output D22 : PCM puts out 5V, sensor pulls it 2.5V DTC SET = >4.1V Open <0.78V sensor Grounded



Hope that's some use
Mitch

Man I wish I had known it was more common to delete code 43 before I ordered new sensors. So this resistor you speak of is it inside the computer?

this specific pcm was borrowed from my other corvette in which it never threw the 43. So I hope it’s related to the wires.

Quote:

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Originally Posted by johnny_b

Man I wish I had known it was more common to delete code 43 before I ordered new sensors. So this resistor you speak of is it inside the computer?

this specific pcm was borrowed from my other corvette in which it never threw the 43. So I hope it’s related to the wires.

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The resistor is integrated into the knock sensor. This is why the part number is unique compared to previous year Corvettes. 96-97 PCMs revert to the C3-style knock sensors that have no resistors in them because the OBDII PCMs detect the presence of knock sensors by actually checking their output and doing a sanity check rather than relying on silly resistance values like the 94-95 do.