Seafoam.
I have been having an engine knock problem with my 1994 GMC (350 V8). I use my local Chevrolet dealer for service because I have found the service manager to be a reliable, honest guy who has saved me some $$$ on repairs in the past. Anyway, he said that before we tried doing anything costly about the problem that I should try running an "engine top cleaner" through it in case the knock was being caused by carbon buildup on the pistons. I had no idea what he was talking about, but looked around and ended up going in a parts store (I think it was AutoZone?) and they recommended Seafoam, so I bought a couple of cans for $5 each, and put one in my truck, and one in my 1992 Jimmy (V6).
Well, it didn't fix the knock, but I would swear the both trucks seemed to run smoother as a result. Of course, that may be subjective -- sometimes a wash and wax seems to have the same result... /forums/images/graemlins/wink.gif
I admit to being skeptical about the stuff at first -- anything that can be used in gas, diesel, oil, poured directly into the intake, etc., etc. sounds like snake oil to me, but it seemed to help smooth things out. I will probably run another can through sometime in the future.
Once I told him the Seafoam didn't work, he said he suspected the knock sensor, ran a diagnostic to confirm, and replaced it. No more knock!
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