Here is an interesting one for you all. We have a 1994 1LE Camaro race car. We have replaced the fuel pump, lines (6AN steel braided) adj fuel press regulator, optispark with a new MSD unit. When the car warms up on the track it just noses over then developes a misfire, from the outside it just belches black smoke. Fuel pressure is good etc. If you watch video attached at the 3.20 mark you can actually hear it! This was the 1st time out int he car. Scanned the computer an no codes.......this only happens once the car is warmed up and under load.
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Any ideas????

 

What is the fuel pressure under WOT/max RPM? What brand AFPR are you running? Is it in the stock location at the end of the driver side rail? Is it tuned open loop or closed loop (O2 sensor feedback)?

 

The fuel pressure is just under 50 psi at full throttle. We had low fuel pressure issue (Under 40psi) before updating the regulator. It had the exact same problem with the higher pressure. I cant tell you about the tuning as it was done before we got the car. From what I am told the o2 sensors don't do anything at wide open throttle on OBD1? This car is pretty much wide open or off throttle, very little time spent at neutral pedal position. I don't know what you are referring to AFPR? Pretty much every sensor other than TPS, MAP, AIC, Water Temp, o2 and Mass air are gone.

 

He's talking about the fuel pressure regulator. Most of (all of?) the rail mounted adjustable regulators are junk.

Did you pull the plugs after it started misfiring?

 

If you raised the fuel pressure with the AFPR (Adjustable Fuel Pressure Regulator) you have to adjust the injector flow rate in the tune, or it's going to run rich. Your change in pressure raised the fuel flow by about 12%. My point in asking the brand of the AFPR is the fact that there is an Aeromotive unit that is often used, and it is highly unreliable. It's the one that goes in the end of the driver side fuel rail.

If you have no idea who tuned it or how it's tuned - appears to be running open loop, speed-density, etc - it's going to be very difficult for someone over the internet to diagnose the problem. I listened to the video at the 3:20 mark and it sounded and looked like you let off for an upcoming turn. Is that the specific problem you are referencing?

An even more basic question - are you using the factory computer or an aftermarket engine management unit?

 

The fuel pressure regulator is an Accel rail mounted unit. The car has the OEM computer that was supposedly "tuned" for the mods on the engine. I didnt let off for the corner at that point in the video, the car just nosed over and then kept stumbling around track. WIth the fuel pressure at either 40 or 50psi (in car mechanical gauge) the car did the exact same thing after warmup. We didnt pull the plugs right away because it felt like it just lost ignition and we were searching for a loose wire etc. I only posted here wondering if anyone had experienced a simular symptom. The only big change we made was to replace the optispark after we got the original one soaked on initial cleanup, being a Ford/Mazda racer before you could soak the distributors in either car and they ran fine.

 

Was the car running fine before the change? What fuel pump was put in it? Does that regulator have vacuum compensation? Are the plug wires new? If so, brand used? How high are you twisting the engine?

In the early days, the MSD units were not good. Not sure if they have resolve the issues or not. I would love to upgrade my opti in some kind of way, but nothing has beat the OEM units for street reliability. Of course, racing could change that reliability. I drag race my Z quite a bit at the local 1/8mile and have had no issues with the Delco opti twisting it close to 6500. Again, much different than sustained road track use.

I have also found that some aftermarket wires are no where near as good as AC Delco units.

 

Unlike these smarter guys up there, I'm just gonna throw out a blanket diagnosis from my experiences. Any time I've had an issue that occurred after it warmed up it was always ignition related. Any time it started stumbling and studdering it was usually a burned or cut spark plug wire. I also had an issue where car would randomly die while driving that turned out to be the Crane ignition box I was using went bad.

Fuel, air, spark make an engine run smooth. Very unlikely it's air unless you have a bad sensor. If your fuel pressure is not drastically dropping when it noses over then it's probably not that either. That leaves ignition, and the black smoke leads me to believe that as well since that is a sign of a rich condition. I'd start with the old run the car in the dark and look for any arching of wires and see where that leads you. If nothing there, try the plugs, and if still nothing start diagnosing opti, coil, and any aftermarket ignition pieces. Just my 2 cents.

 

Problem solved! Swapped out the ECM with one out of another LT1 race car that swapped to a LS engine. Interesting the car we swapped out of was a 1996 that was "tuned" and "converted back" to OBD1. The car idles amazing and run great now! New problem emerged finding the oil pressure dropped massively on right hand corners, so we have to find out which door inside the Canton oil pan is stuck lol.
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last run of the day just messing around with it. We had already realized the oil pressure issue so no hard right hand corners lol

 

It aint a problem with the oil pan, the pickup is over to the right side of the pan?! Sigh fix one thing then its another lol

 

How much oil are you running in the motor? I usually had 6 1/2 to 7 quarts in my 242t pan plus remote filter and blower.