94 Z28 Weird engine stumble

Do you know about the "Opti-spark" distributor? Always the first thing to suspect when you have weird ignition problems with this engine. I had similar symptoms when it started going bad on my '94 Caprice.

I'd put a timing light on it and see what the spark looks like at all the wires. We're not checking the timing here, just seeing if there is a nice even spark without misfires. If its the Opti-spark going out, you'll see misfires on some or all of the spark plug wires. If its all the wires that could also mean the coil of course, but that's less likely and pretty easy to test. Could also be something besides the ignition, but I'm going to bet its the Opti-spark.

Replacement involves removing most of the accessories from the front of the engine including the waterpump. If you haven't replaced the waterpump I'd do that at the same time. If you put the old one back on there it'll leak next week and you'll have to do 2/3rds of the work all over again (been there, done that). The opti-spark itself is a relatively expensive item, and while rebuild kits are available, I'd recommend replacing the entire unit as they are a bitch to get at and you do not want to do it twice.

You can probaby dig more information up at

Good luck, Harry

Thanks for the reply. I actually thought this was the problem originally, since the water pump was leaking. I have since replaced the water pump (so much fun), and also installed the Del-Teq Opti-spark replacement system (essentially deletes the opti-spark distributor from the system) but the same problem remained. I did all these things on the advice of someone who works at the GM tech center, and was formerly a mechanic at a local Chevy dealer. So I am stumped and my wife (love her!) doesn't want me spending a bunch of money on the car...

"Planted Koala" wrote: 94 Z28 Engine Stumble

The car runs fine until it gets fully warm, normally about

20 minutes. Then it will have a bit of a rough idle and a definite engine miss, but only below 2200 RPM. Above 2200 RPM, the car behaves normally. If I park the car, and let it sit for just a couple of minutes, the car will run horrendously. If I let it cool off further, it will once again run normally. It appears that something is getting heat-soaked, but what? _______________________________________________

Perhaps it is an intermittently sticking Idle Air Control valve, or a dirty throttle plate, or a defective throttle position sensor. Any of these can cause rough idling at low RPMs even though the engine runs smoother when the throttle is opened wider.

It is possible that the Engine Coolant Temperature sensor is malfunctioning, because the problem seems to be associated with increasing temperatures - the rise when the engine heats up and the temporary rise when the engine is shut off.

Good luck.

Rodan.

Ah, so you're way ahead of me here. What about the EGR valve? They are known to cause stumbling just off idle when going bad. You could try first just unplugging the electrical connector (I wanna say its controlled by an electric-operated vacuum solenoid, but its been a while) to disable it and see if that makes any difference. Or maybe block off the ports under the valve since it could be leaking by internally. A piece of aluminum cut from a beer can should be enough just to see if disabling it takes care of your symptoms. The valves are fairly expensive ($70-80 bucks) which I why I can't recommend just throwing a new one on there. People also say you can simply get rid of them (there are block-off plates and whatnot available), but I also hear not to unless you've replaced the cam with one with more overlap (which gives sort of a natural EGR effect).

Good luck, Harry

Thanks for the help...I pulled the throttle body off and it was incredibly filthy on both sides of the throttle blades, so I cleaned that up, and stuck a new coolant temp sensor in just in case. Waiting on a new TB gasket, will report on running condition after all this. Just wondering though, if the throttle body was this dirty, what's the likely condition of the rest of the intake tract? What about that Amsoil ($7) can of intake cleanser you spray through as the car is running?

Thanks again, Mike