3100 cold start engine knock

[Talontwoswords]

97 Lumina, 3.1L engine.


Sorry about another "knock" post, but I tried the search function, and it wasn't very useful.

Cranked the car up this morning for work, and heard a loud knocking noise from engine. It warmed up for a minute, then suddenly stopped. Engine was smooth and quiet as usual afterwards all day when in use.

I was thinking it was a sticky valve problem/hydraulic cylinder.

Read some online engine additive information sites, and some skeptical sites said don't add anything to oil, including the engine flush (Miracle oil, Sea foam,etc), so I'm not going to do that.

Read some sites about piston "slap" and GM doesn't think it is a problem. Is piston slap a problem in the 3100?

Anyone here have/had this problem, and how did you fix it, or do you just ignore it? Thanks.

[tractorman]

I know piston slap is a common problem on 3100's, and GM says is isn't a problem. There's a Malibu over here in the college fleet that has been knocking when cold for the past 90,000 miles (has about 160K on it).

I have no way of knowing if what you're hearing IS piston slap... but I'd keep my eye on the oil level. If I was reasonably sure the noise was piston slap, and the engine didn't use oil, then I wouldn't worry about it much.

[Juglenaut]

care to enlighten me on how piston slap develops??

[Juglenaut]

I would suspect a valve train component (s) before piston slap.

As the piston slap is caused by both the combustion and the clearance.

Another result is excessive piston-to-cylinder wall clearance which causes piston slap and excessive blowby.

Quote: Knowledgeable source

Piston slap. This is the sound that the piston makes on its power stroke when clearances between the piston and the cylinder are somewhat excessive. It’s a dull-thud clunk that can be heard every two rotations of the crankshaft. Piston slap is most commonly heard when the engine is warming up, before the piston to bore clearances have decreased due to the pistons expanding.

There are a whole host of noises associated with problems such as rod knock, noisy valves, broken rings, chain tensioner failure, detonation, and broken or pulled head studs, worn lifters. Unfortunately, it’s nearly impossible to accurately describe these noises in writing so that someone can diagnose them. An engine can be loud and noisy, and if you haven’t listened to a whole lot of them, your imagination can get the best of you. Of course, listening to other finely tuned engines in cars owned by your friends will help you with an idea of what a normal engine should be sounding like.

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