usaf1965

Only under accelration

x_doug_x

I can't edit, I mean if it does it by just reving it up when it's not in gear it could be rod knock..... New to this forum.

Z28KLR

It's either an exhaust leak or detonation. Since the car is already running 89 fuel on an 87 tune then my money's on exhaust leak.

usaf1965

I meant 87

Z28KLR

Just for fun, try 91 for the next couple tanks. Will only cost ya another $5 per tank. It could be detonation but I doubt it, worth a try though.

Otherwise we're back to exhaust leak.

usaf1965

So if i use 91 for a bit and the noise continues its most likely an exhaust leak?

Z28KLR

Correct.

Just to clarify, there are really only a couple reasons why your car would be experiencing detonation using 87 octane fuel. (1) Aftermarket tune; spark timing is advanced, and (2) higher compression ratio than stock. #2 happened to a friend of mine that had a stock '98 GT after the head gaskets blew. The shop that did the repairs ended up milling the heads a little bit to ensure a perfect mating surface between the cylinder heads and engine block, which gave a slight increase in compression. Afterward when running the same 87 fuel as before it pinged like crazy under load. Switching to higher octane fuel cleared that up.

Like I said earlier though, given that your car is running the stock (87) tune and we have no reason to believe your compression ratio is anything other than stock I have doubts that it is detonation you are experiencing. But it's easy enough and inexpensive enough to try a couple tanks of higher octane fuel in the attempt to rule that possibility out.

If no change after switching fuels, then we're back to exhaust leak.