Hi. I have a very intermittent issue. Car runs and drives great. Rarely when I start the car it runs very rough, sounds like running on 4 cylinders of timing off. If I shut the car off and restart, everything is fine. It seems to be happening more often. No check engine light on. Car has a lot of miles on it. Vented Optispark with about 200k on it.
thank you.
94 z28.

 

Does the rough after start occur when the engine is still hot from driving? When you turn the key to “START” how quickly does the engine start?

 

The engine normally starts in 2 or 3 seconds. Problem can occur warm or cold. Thank you.

 

Try scanning the PCM. DTC 16 for loss of the low resolution Opti pulse signal can only set when cranking the engine, only if there is a high resolution signal to tell the PCM the engine is cranking (takes several seconds of cranking to make this determination), and DTC 16 does not turn on the SES light.

 

Found Dtc 18? I guess it’s time to drag out the shop manual. Do you have any other suggestions?
thank you so much!

 

Found Dtc 18? I guess it’s time to drag out the shop manual. Do you have any other suggestions?
thank you so much!

 

DTC 18 is for injector electrical faults. Unfortunately it doesn’t tell you which injector(s).

Key on, connector off injector, verify ~12 volts at the pink wire in each injector connector. Connector still off, measure the resistance of each injector, should be in the range of (roughly) 12-16 ohms. Verify exact range in factory service manual. Then reconnect injectors, start engine, and using a ‘noid light verify the PCM is supplying the ground to fire the injector. Going to be hit or miss if the problem only shows up intermittently.

 

I’ll look at it tomorrow. It’s so intermittent, going to be a challenge. When it happened it’s sounds like it lost a bank of cylinders. Car runs perfect the rest of the time and only happens on startup.
thank you for your help.

 

Your comment about "sounds like it lost a bank of cylinders" makes me think you might be losing power from one of the injector fuses. There are two that each supplies half of the injectors. Might check them to be clean and tight. Or maybe one of the power supply wires from one of the fuses is compromised.

 

Thank you! On my list for tomorrow.

 

The injectors are supplied by two fuses in the under hood box. Fuse #9 feeds #2, #3, #5, #8. Fuse #10 feeds the other four cylinders.

 

Hi,
lt looked like we just needed to crank a bit longer and it went away for awhile. Happened again to and repeated twice today . No codes logged. Stumped😢.
Recently replaced the cap, rotor and plugs. Found the Opti bearing a bit noisy. . just ordered a Petrie Opti and new wires.
Thank you,
Walter