An engine that is running LEAN will detonate "ping". An engine that is misfiring (ignition; optispark) does not.  Vacuum leaks are not intermittent. The problem does not go away for periods of time like Laurie's has if caused by a vacuum leak. EGR open too far, or at light throttle (wrong time), will cause lean mixture, rough running, and detonation.

I also suspect the EGR valve. These valves have several modes of operation, but as the passages become clogged with soot during normal service, the valve's tendency to remain closed by pressure differential (the way its designed to) becomes impaired.

With the engine running, put your fingers under the body of the valve. There's a neoprene diaphragm. Push it inwards, engine should run rough or stall. The higher you lift it, the quicker and more severely the engine should react. If you can lift the diaphragm all the way up, and engine doesn't react much, the passages are clogged, and could be contributing to your rough running issue.

Laurie: You show a keen interest in understanding how your Corvettes work and function. I highly suggest you obtain the Helm's manuals for your cars. They aren't inexpensive, and the one for the 85 may be out of print (can be found on ebay), but these manuals are a treasure trove of information. Just reading a few random pages with coffee in the morning will teach you a ton about your cars.

I learned one the other day during casual reading. Try this in your 95:

Roll down the drivers window (So you CAN'T lock the keys in it).
Put the key in the ignition.
Lock doors with power door locks.
Close both doors.
Doors should unlock. The idea is that the keys can't be locked in the car, if left in the ignition.

This is pretty neat. I don't have a 95 or 96 to try it in, though.
Regards,
DaveP







|UPDATED|7/10/2013 8:49:03 AM (AZT)|/UPDATED|

Hi Dave,  we'll investigate the EGR valve.  One thing, though, is that this hasn't been an intermittent issue.  This came on severely and suddenly.  Monday morning, the car drove in its usual great, smooth manner.  That afternoon, I started it up, and it was very rough - car shaking rough, along with some pinging.  It has not done either of these things before, and the problem has remained, so we're not driving it. 

I just spoke with my husband, who's at home with the car.  He did the test on the EGR valve, and the car got noticeably rougher and was about to stall.  He let up on the diaphragm at that point, and it went back to its current level of functioning.  So, I guess it's not that.  Too bad.  I think I could have replaced that myself.  He did notice a knock, though, that made him think a problem with one of the injectors may very well be an issue.  A stuck injector has been suggested to us.  But, we will have to have a mechanic test the injectors, since we don't have anything to do that.  He's going to have it looked at tomorrow to see about the injectors.  We already looked at the injector fuses (along with a number of other fuses) last night, and they're both fine.  

The car is at the shop and it turned out to be the Optispark.  At just under 100,000 miles, we decided to be prudent and change the water pump at the same time as the distributor.  Thanks to everyone for all the helpful advice.   You guys are great.

Back in the old days:

Steve, the good old days have something to be said for them.  These modern cars have a lot more than can go wrong LOL!  

Sounds like you are dealing with some up standing individuals here. Glad to hear good stories about repairs. Too often we only hear of the not so good experiences.

Thanks, Brian. The car is now ready, so we'll pick her up after work this evening.  Looking forward to having her back home, and was very happy with the reasonable rate we were charged for the repair.

I also want to add that my FSM set arrived today, so I'll be checking that out tonight.  And, now that the 95 will be back home, I'm going to try Dave's door lock trick.