98 Knock Knock

[BobM46128]

Originally Posted by BobM46128

If it is on the pasenger side, it may be a vapor canister valve making the noise. at least the Express vans have this problem. There is a service bulletin about them I believe. The symptom is that it sounds just like a spark knock.

It is actually called the Evaporator Purge Solenoid according to my Service Information. The suggested remedy is to replace the valve. Apparently the old valve makes this noise when the valve seats.

[Arrowshooter]

What is a vapor canister? And where is it?

[acewelderman]

Mine started out as a knock on acceleration and ended up being a blown head gasket and two cracked heads at 105,000 miles. Could also be carbon build up quieting as the engine gets hot. Pull your egr valve and check for carbon build up. I'm leaning towards cracked heads, you will need to do compression check to verify. Bear in mind, I am a TIG welder/laser operator but these are problems that I just had on my truck.
Thanks, acewelderman

[Chevy-SS]

This is piston slap, IMO. As piston expands, knock typically gets quieter, or goes away completely. If it was a bad rod bearing, it would knock all the time. A little psiton slap is no big deal. My 98 Silverado has some. Hell, my fresh 468 race motor has some too, although forged pistons are more susceptible to the slap than cast, as bores need to be bigger to allow for extra expansion of forger pistons. Cast pistons expand less, so bores can be tighter, but still piston slap is a fairly common thing.

[Fireplug]

Pistom slap will not be there after the engine is up to operational temps. GM piston slap is a very old problem hell the 2.2, 3.1 just to name a few had that problem and it was always on a cold (first)start up then go away as the engine warmed up.
This might sound dumb but there was some talk about bad pcv valves cause this problem and we did replace a lot under a very quite recall