1994 Corvette automatic, will automaticlly idle up, and down, and start missing when stepped on

1994 corvette auto trans, when car is parked or in drive with foot on the brake will start to idle up, the back down and like lunge forward if in drive and foot on the brake. Then if you drive it, and step on it, it will miss fire. My mechanis put it on his scanner but it showed no problems. The opti- spark, wires, plugs were replaced 2 years ago and 12,000 miles ago. Any ideas??? Thanks

help please!!

Slow down man. Its not an automated Q&A computer system - just a discussion board.

What happens if you just sit at idle without your foot on the brake?

How old are the air/fuel filters?

Did anyone check the fuel pressure? Under load?

Is “opti-spark” what this car’s ignition system is called? Or is it some supposedly hot aftermarket ignition stuff?

Anything else on the car not stock? Even something like an oiled-type air filter?

Sorry. Question 1- It does the same thing you hear it idle up then down. Also to note it does not do this all the time. Most of the time it runs great. @- air filter is almost new, gas filter??? 3-No fuel pressure check yet. 4- opti spark is were the distributer is under the water pump, (Water pump is fairley new). Its all stock.

I would just be looking for vacuum leaks. If its at all related to brake use then the first thing you look at is the brake booster. A vacuum leak may or may not create any misfire. Vacuum leaks tend to cause trouble at idle & low rpms. If its missing with heavy throttle then I might be wondering about two problems.

Get a can of MAF sensor cleaner and clean your MAF.

Get a new fuel filter on it - right around when the fuel pressure is checked.

I assume that what you mean about your mechanic not finding any problems with a scanner is that there were no error codes present. Do you know whether scanning for codes is all he did? Or did he check out live data on stuff like MAF, TPS, fuel trims, etc? If you don’t know, just ask him.

He checked codes and live data. I will check the fuel filter, and put a vacuum gauge on it. If you can think of anything else let me know. Thanks, ANYBODY ELSE OUT THERE WITH HELP???/

Report on the live data. E.g. what were the fuel trims like?

I can think of about 100 “anything elses” - uneven idle & some missing on accel are very generic things. Unless you start providing more specific info on the problem itself and the diagnostics and the basic background state of maintenance, all anyone can do is guess.

If it’s idling fine when in park and your foot is NOT on the brake and the brake application is making the idle go crazy, than I’d bet lunch that you have a vacuum leak…probably in the line to the brake booster, the booster itself, or perhaps the check valve.

Clamp the booster vacuum line off and see if the problem goes away. Post back.

By the way, am I correct in assuming that when he tested it with a vacuum gage it was in park with no brakes applied?

No, it will act up in park or in drive with or without your foot on the brake. Then if you’re driving it and step on it, it cuts out and starts missing. Then there are times when it’s fine with no problems. Damn cars!! I will try clamping the vacuum line at the booster when it’s idling and see. I need to get a vacuum gauge and check that too. I might as well check the fuel pressure. It just seems to be something that is sporadic with it.