Rough idle, sulfur smell, gas cap venting, "diesels" sometimes when I turn it off

97 with 120k, mechanic cleaned MAF sensor and throttle body, still has problems. After shutting off, engine continues to try to run and restart is harder

Do I assume correctly that you took your car to a mechanic with the symptoms you mention here? If yes, find a better one.

Rough idle can be caused by a number of things, including a dirty IAC. Not sure what you mean by “gas cap venting”. What’s the problem? If you think there is one, get a new gas cap for $15.

The sulfur smell comes from heated but unburned gasoline, which contains small amounts of sulfur and is exhausted through the catalytic converter. In other words, unburned gas goes through your cat and if you don’t get it fixed asap add a new cat to the repair bill. That is probably caused by the dieseling. The dieseling is likely caused by the rough idling. When you shut off the engine there is still gasoline to be burned but there is not spark. Some of the gas is “burned” by the hot interiors of the cylinders and the rest expelled through the exhaust manifold, catalytic converter and muffler.

When is the last time the car had a decent tune-up?


plugs and wires recently

constant vibration ie. growling noise coming from gas cap, that’s what I meant by venting

That’s the air sucked into the tank to relief the pressure as the gasoline is pumped out to the engine. Get a new cap.

Thanks. biggest problem is of course, no symptoms when it’s at mechanic.