KYWes

'95 Z28 Camaro - new AC Delco plugs and ICM, Delco Optispark is a few years old as are the plug wires. New knock sensor. New cam and RRs.

Symptoms - Engine runs good when first started but after approximately 15 minutes the engine has a sporadic miss. The engine will miss, try to fire, miss and so forth. It seems mainly under load.

I'm attaching a Scan 94/95 Dlog of a run. I'll be reviewing it for changes as it progresses.

DTCs reported after the run are 16,18,36,41,42,46,47,51

Did a fuel pressure diagnosis

After sitting all night I hooked up the fuel pressure gauge and turned key to the on position. Pressure read 0. Started engine and the FP went to 38psi. Shut off the engine and it dropped to 20 psi quickly. It was easy to watch the needle fall.

Turned key back on and pressure jumps to 42psi. Start engine, pressure goes to 38psi. Shut off engine and pressure goes to 30psi but more slowly.

Turn key on, pressure to 42psi, turn key off pressure holds at 40psi.

With engine running (idle) pressure is at 38psi. Remove vacuum line to regulator and pressure jumps to 46psi. Reinstall vacuum hose and pressure returns to 38psi.

Start and rev engine and pressure increases 1-2 psi

Road test - all through the road test the pressure stays in the 38-42 range. It was hard to see exactly while driving but I could tell that's where it was,

Return home, shut off engine and pressure drops to 34 and holds.

I've read several threads on fuel pressure here and I've read information on Shoebox's site. I think I have one or more intermittent fuel injectors. I have a second set that I sent off for ultra-sonic cleaning, repair and testing that I hope to get back by Tuesday.

I was surprised to see the DTCs. Some of them suggest to me problems with the Optispark.

What do you guys think is my car's problem?

KYWes

After doing some research, I've decided to clear the DTC codes and make another run. The codes point to situation where the car won't be allowed to run. I'm not having that problem. Codes also point to the PCM. I'll see what, if any, returns after clearing the DTCs and doing another run.

Injuneer

Sounds more like an electrical problem... e.g. bad ground, ignition switch problem, PCM 12V supply problem.

I'll take a look at the data log, to see if anything is obvious.

KYWes

OK, cleared DTCs and went for a drive. After sitting all night the car ran pretty good. Drove mostly interstate for about 20-30 minutes and no DTCs popped up. However, the miss showed up after 15 minutes and it is getting worse. For the first time, while sitting in traffic and idling the car stopped on me. It started right away. Stopped at AutoZone to return a part and the car was cranking but wouldn't start. Held pedal to the floor, no start. Let up on the pedal and it started after a few seconds. Driving home it tried to cut out a couple of times. In the short 5-10 minute drive home I got two DTCs. A 16 and a 51. Parking the car for the night to go to work.

Does the PCM have to see the DTC code a few times before it will post the code?

Good news is, sorta maybe, the miss is getting worse. Hopefully that will make it easier to find. I've got data logs of the drives I took but I doubt they will show anything new or related.

Chimera96

did you put heat sink paste behind the ICM when you replaced it?

confirm your wiring & connections on DS head at coil are good

check coil wire..any sign of arcing/corrosion inside coil wire connection

you replaced plugs, you should also use dialectic grease on plug boot ends

Injuneer

Depends on the code. DTC 16 sets immediately, as soon as the PCM sees 720 high resolution pulses, without seeing a low resolution pulse. That's 4 revs of the crankshaft.

DTC 16 can pop up at any time. Does not turn on the SES light. But if it is anything more than a fraction of a second, the PCM shuts down the fuel system, and the engine doesn't start or run. It isn't unusual to see DTC 16 set at startup, when the PCM just can't seem to find the pulse signal.

DTC 51 is more problematic. The shop manual description (94, I don't have 95) is very limited, and simply says if DTC 51 sets during normal operation, replace the PCM. We see this one so seldom that I really don't know much about it.

In your data log, as I recall on line 2462 of the spreadsheet, there are numerous incorrect entries for SOME of the sensors and PCM parameters. I don't know if this may be a symptom of a PCM problem, or power supply to the PCM, or just a "burp" in the data stream to the logging program, which is not unusual, and can be caused by interference on the data wire.

Otherwise, your data log does not look all that bad, except for some high negative LTFT's, particularly on the left bank, but I think I may have pointed that problem out previously.

KYWes

Thanks for all the input. I know you don't like it but I'm going to throw some parts at it tomorrow. I've got a replacement ICM, freshly cleaned, repaired, and leak and flow tested fuel injectors, a distributor wiring harness, and a new Opti coming. Gonna try one at a time and see if something makes a difference.

I've made sure wiring is secure on the ignition coil and other items in the area. I've used thermal paste on the ICM (and I've added spacers to help cooling), I relocated the head mounted grounds to the block thinking that the cast iron block would be better than aluminum heads and I've checked their connection is good.

About the only thing that will leave that isn't new is the plug wires and the PCM. If I'm not successful with any of the above I'll check the PCM connections and maybe reflash the PCM.

I'm thinking of checking with a friend who is Service Manager for a local Chevrolet dealer if they have something that can do diagnostics on my engine.

KYWes

Problem solved. It was the Optispark.

BaconHeart

Sorry I missed if it was the original LT1 motor. If so , bout bet my lefty, it is the distributor ( option spark) my 94 did same thing. And Don't buy cheap ebay ( china) replacement, spend the coin or u being pulling it out again real soon. Goid Luck.