piston slap?? please explain

[blacksunshine]

Can anyone please explain piston slap to me. I have always had a vibration at idle and always thought it was my exhaust, but now I am reading about the "piston slap" thing. Please explain. Thanks, aaron

[jsmiddleton4]

Either the cylinder wears and its now larger than the rings on your pistons can expand to maintain tight fit OR your rings have worn and are not keeping the pistion snug in the cylinder.

I think you can also have piston slap if the pin that holds the piston to the arm that attaches the piston to the crank is worn. I'm not sure if you'd hear piston slap if the main bearing side of the piston rod was worn.

Its been awhile since I rebuilt any engines but I'm pretty sure that if you are hearing piston slap, you should be looking for rebuilding.

You sure its not valves that need adjusting.

What's it sound like again?

[blacksunshine]

The sound / vibration is only at idle. Its like something is vibrating against the frame, like a loose exhaust pipe or something. I can hear it in the jeep, But when I get under the jeep I can't pinpoint the sound. Once I accelerate(even in neutral), it goes away. I've read about piston slap being a common jeep problem and that most of the time it doesn't cause any significant problems. I've had this noise for at least a year with no engine problems. The jeep runs very strong and sounds good while driving. Its very strange. -aaron

[jsmiddleton4]

That doesn't sound like piston slap to me.

Valves out of adjustment? Somethiing else that is going like water pump that would change sounds with RPM increase? Exhaust leak?

Jim

[MagicRat]

These 4L engines make a symphony of noises when they get a bit older, including slap and bearing knock due to low oil pressure at idle.
IMHO unless its something extrenal and obvious just continue to drive it, since the best remedy for piston slap or low oil pressure (scored bearings etc) is an engine rebuild.

[tblack]

That doesn't sound like "piston slap" If it is loud it's probably a damper on the exhaust near the header that is loose. This is to help the engine warm up by using heat off the exhaust pipe. My '94 had that problem and it was an easy and inexpensive fix. Piston slap is something that all Cherokees have after the first 15,000 miles and it sounds like sticky lifters but it is pretty normal and causes no problems in performance. The question is is it a dry knock or does it sound like sticky lifters. If the latter, don't worry too much. If it is the former, take it to a repair shop and it should be done before you drink a cup of coffee.
tb

[JDPascal]

piston slap occures when there excessive clearance between the piston skirt and the cylinder wall.

Sometimes this is because of wear and sometime because the piston no longer expands in the normal directions when the engine warms to operating temps (collapsed piston).

Like others have mentioned, there are other possible cause for the noise besides the "piston slap". If it is a collapsed piston that no longer expands properly at operating temps, the tapping that you hear when cold will remain when the engine is warmed up.

Valves, rod bearings, cracked flexplates, cracked v-belts, broken crankshafts, excess crank and camshaft end play are among the sources that produce the annoying and baffleing tapping sounds that everyone LOVES to search for.

JD

[Saudade]

Chasing down noises or vibrations can be a very frustrating experience. My '88 makes all kinds of noises but run well so I'm not too concerned.

Put your foot on the tailpipe to hold it steady. If the noie/vibrations ceases, it would indicate a loose connection somewhere past the cat. On mine the flange on the muffler is loose so it squeaks like the mice need feeding.

Get a "mechanic's stethoscope" and listen around the engine and exhaust when cold and when hot to see if you can isolate the location of any noises.