69cvette

Okay so I’ve got a bit different car than the forum applies to but the overall drivetrain is basically the same on a B body as an F body or C4 LT1. After a month at the shop with this 43k mile 1996 caprice 9c1 I’ve been chasing this intermittent misfire and stumble off idle. P0300 will come on and it will flash SES occasionally. I would also get the P0172 code.

I have had the following done,
new triple platinum plugs, wires and whole new OEM remanufactured gm optispark assembly with cap and rotor.
checked all grounds and wires
coolant temp sensor is good
ICM is new
EGR was sticking open, all new now, no difference..
vaccuum is in spec no leaks
replaced injector on cylinder 2
new Delphi fuel pump, and new fuel filter
fuel pressure is excellent
compression is good on all cylinders
all new 02’s
checked the exhaust for clogs, all clear.
cleaned and even tried a new MAF

they are in contact with a gm technician of 30 years to pick his brain as the last thing I haven’t touched was the PCM.

This car sat for years obviously and I think it was due to this issue, first year OBD2 I feel that I’m at my wits end with this car.

the car will max out fuel trims when in closed loop and it dumps so much fuel and therefore misfire
it is primarily on cylinder 2 but will misfire on cylinder 1, it’s NOT a dead miss..

the car runs absolutely great in OPEN LOOP Only. The check engine light even went out on its own one time for 15 minutes..

i have owned a few LT1 f body’s both were 1996 models and I’ve never even so much as had a hiccup with them, but I am bound and determined to save this car.

I’ve fired the parts cannon at it and don’t know what to do. I’ve searched the forums all over and although there is B body specific forums I felt that I’d get more feedback here in the C4 tech section.


I have told them that if they can’t get it done then I told them to reset the pcm, tune out the 02’s and run it that way. I may rip the LT1 out and replace it at some point.

THE 383 admiral

Sorry, to hear about a intermittent mis- fire chase.
Members need to catch up to your current status.


96 caprice with 43k original mileage.

Do you have the original opti.?
Very hard to believe the opti failed with such low mileage. Most survived 150+k

DTC code P300 generic misfire
*The reman opti. Did they use genuine components.

*So far this reman is suspect. Very common even brand new aftermarket Opti's have a high fail rate.

ICM typically just straight up fails.
*Plug wires status
*Coil status
The 96 PCM does not have a common fail rate.
Set back to factory.
This indcates upgrades. Requiring a tune?
What are the upgrades.?
You removed the EGR. Turned off in the constant tables. Correct.

Testing with a known good is suggested. To rule out PCM failure. The current DTCs. rule out as bad.

DTC code P0172 extremely rich. PCM cannot adjust.
Have you examined/ validated the fuel regulator diaphragm.
You replaced the 02/s
New FP obvious plenty of fuel.

Ripping the great LT1/4 out is extreme.
You could simply convert to a 411 PCM/ LS coil packs. eliminate the opti for good.
Replacing with another platform is your choice.

bjankuski

Do you have any exhaust leaks that could be letting in air that will cause the O2's to add fuel since they see air and think it is lean? This could be an actual leak or a failed check valves on the air injection system.

A95Y

Just by reading your first paragraph you have typical symptoms of a failing optispark. The O2 DTC is most likely from the misfire. Did you solve this?

69cvette

it’s been replaced. Same issue

update though I forgot I made this post lol

took the car for the day because I needed it. The shop got in touch with a retired 30 year GM tech and they went over a laundry list of items to check again. Decided it’s likely mechanical relate more than anything. I’m not a mechanic so what I’m going to explain will be roughly what I was told.

they replaced all the plugs again, did a hot compression test which came back fine, then checked all the injectors voltage after finding that cylinder 1 plug was wet again. They traced the grounds to the injector and found the grounds were fine. When the car gets hot the OHMS To the injector would drop considerably, so the way he described it was that something in the PCM was failing as the car would run and warm up causing the faulty firing injector, they were swapped and without change so the injector itself wasn’t the issue. I had mentioned 2-3 weeks prior after dumping time into diagnostics that never made a difference that it HAS to be the PCM, first year for OBD2, late model year run for a final production year, GM gets faulty parts right out of the box, or premature failing parts which is why I believe the car had been parked at 43k miles, it was troubleshot before you could tell they tried to find the issue to no avail, even thought the plugs and wires and optispark were original yet, it was likely determined they weren’t faulty years ago, so that’s why I changed them out.

the big day is coming, they ordered a new reprogrammed oem replacement ECU, and I’m banking on this being it, it’s the last part we didn’t change.

C409

..... Im not sure how sophisticated the 96 PCM is but I had a similar P0300 issue on my 2001 Suburban 6.0L ... I was browsing the PCM with a SnapOn scanner and discovered a crankshaft position sensor re-learn test . The test involves the scanner/PCM running the engine up to about 5600 rpm to a rev limiter ... after the test/re-learn , no more P0300's ... apparently this test is used when a crank or cam position sensor has been replaced .....

93TALT4A4

Excuse this comment if it was mentioned, but how long did it sit before you bought it and was that tank of gas used/dumped?

Jaguarxjr@1968

Again NO updates on these P0300's lol WWHHYYYY I am having the SAME issue reman PCM and BCM, new plugs/wires, opti from a running LT4 with 36k miles that was parted out, new fuel pump, new regulator. I have th WET #1 plug even managed to get a code 301 for that and then drove it again went to P0300, what is with these cars man I am ready to sell as is.

ex-x-fire

Those 90s GMs were known to have injector problem. Do a fuel pressure test, does it hold pressure? Maybe one off the injectors is stuck open.