smooth1990

I have a misfire on my 1992 supercharged 383 c4 corvette and I can not figure it out .The misfire only happens when the car warms up to 150* or more. It is repeatable. As the car gets warmer, it misfires more. When its cold It runs perfect.

I take the car down the road from my house and almost every time in the same place that car starts missing. It will run like crap all the way back home and get progressively worse on the way there.

I have done alot of things to try to figure it out and i would like some input from you guys. What can I be missing? What do I check next?

Here are the things I have checked.

***The car does have a LTCC system on it which is a ls coil over conversion. I have checked that and ruled it out. I think lol****

1. Fuel Pressure. 43 lbs at idle and it goes up to 48 or so under throttle. (supercharged, 63 lb injectors, and a aeromotive pump)

2. Air fuel. the car has a wideband on it and the air/fuel is 14.6 at idle and 12.3 or so under light throttle. When it starts missing the gauge goes lean! 15-17

3. O2 sensors! The Tune is set to keep the car in OPEN loop! 02s are essentially shut off. I know this is a typical problem for lt1s missing at closed loop. This is not the problem on my car.

4. Plug wires. I have tried 2 sets with the same results.

5. Spark plugs...I put a new set in. Nothing changed. I did notice that cylinders 3&7 on the drivers side a 2&8 on the passenger side are misfiring. The plugs are black on those cylinders.

6. Opti Spark! I pulled the opti out tonight. Everything looks great inside! I took pics and attached them to this post. Its dry inside. no moisture at all. cap and rotor are like new. The opti has only maybe 500 miles on it since I installed it last fall. I dont see any signs of failure at all. I dont know if I should put it back in or try a new one while Its all apart.!!!


The only thing I can think of besides what I have checked would be the ECU or a GROUND???

I dont think a ground would cause the car to misfire only when hot. I did poke around under the car and looked for grounds that may have been missed just in case., I didnt find anything.

Last year the car ran good on the same tune that is in the car now. The engine was rebuilt and put back in the car and almost all the same parts were used. The only changes made were a better camshaft and a different throttle body.

I can not figure out why it would run good last year and go back together and have a major misfire this year. Its driving me crazy!!! If you guys can think of anything That I may have missed PLEASE let me know! I am debating putting the MSD opti back in or swapping it out with a new one since I have the car apart.

Here are the pictures and a video. The video is of a data log that I did today. As you can see the last couple of minutes of the video the spark advance is really shaky.

adtbrown

Its not by chance the injectors, is it?

smooth1990

No. They are new Siemens 63 lb injectors.

tombrammer

I notice 3 things from your data log. 1) your IAC counts are way high, should be around 40-50 I believe, what throttle body did you go with? 2) Your 02 readings are funny, the one on the left stays around 200-300 while the right one is going back and forth between 900 and 4 so one side must be rich and the other it's hard to tell where it is, do you have a wide band A/F meter you can take some readings to see where your fueling actually is? 3) You are still in open loop at 3/4 of the way across the log. 4) You have a ton of knock counts but it doesn't seem to be pulling any timing??

69mako

Bad EGR vavle, very easy to over look when trouble shooting.

smooth1990

This is all with a 2 bar map tune.

I have a tpis 58 mm throttle body.

A/f is around 14.6 at idle and 12.3 normal driving

The tune is set to keep the car in open loop.

Not sure what's going on with the knock counts...

smooth1990

Egr is deleted.

Red 91

You could switch injecters around, or insulate the ltcc box, or open the box and cool it. Good luck

antfarmer2

Vacumme leak?

kcbc

What 15- 17 means on ur A/F wide band as you mentioned during the misfiring ?...ur answer maybe right under your nose!!! ..wink wink

DanielRicany

Does it misfire at idle?

QCVette

You might take a look at the ignition control module.

I had one that would act up when it got warm, but fine when cold.

Good luck.

jmgtp

^^what QCVette said. I too have had ICMs that were fine cold and full of problems warm.

bjankuski

When does it missfire, idle, cruising, mid power, WOT? If we can rule the missfire down to a certain condition we can trouble shoot the issue.

rhyno

camshaft added w/ boost? that much added air via cam might be giving you a lean condition. i'd check your tune.