Piston Wrist Pin Question

[joebeammer]

Is it possible for the piston wrist pin to get bad and lead to a knocking sound, and if so, what are the causes and remedies. My Grand cherokee has a noise in the engine on idle and three mechanics out of five say its due to bad piston wrist pins. Any suggestions?

[curtis73]

Its very difficult to narrow down a knocking noise. Main bearings, rod bearings, piston slap, and pin knock all sound VERY similar. The mechanics are probably responding to the fact that pin wear is very common on Jeeps, especially the inline 6s.

[joebeammer]

Thanks for the info, i guess i'll just proceed with a careful elimination tactics in resolving the problem. I have already had the main bearings and the push rods checked and they checked out fine, i guess the next thing to check on is the tappers and the wrist pins. Strangely the sound disappears on increasing the RPM and is louder on idle.

[maxwedge]

Try and narrow it down by shorting out one cylinder at a time, see what changes.

[534BC]

I can usually tell a pin knock very accurately , but many times it is the piston that's broke/cracked. A good engine shop can tell by listening and they are also armed with data such as what curtis posted about jeep pins because that is the business they are in.

possible causes

clip came out/broke
press fit became loose
worn rod/piston
broken piston
seized pin/lack of lube

can't really think of many more off the top of my head.

Remedies are dependent upon cause.

replace clip
oversize pin
new piston
rebuild holes
re-do lube top pin

I can't remember ever seeing a pin break under normal use.

[MagicRat]

Originally Posted by curtis73

Its very difficult to narrow down a knocking noise. Main bearings, rod bearings, piston slap, and pin knock all sound VERY similar. The mechanics are probably responding to the fact that pin wear is very common on Jeeps, especially the inline 6s.

More specifically, the inline 6 is notorious for making a variety of knocking noises after quite a few miles have been accumulated. Often the noises did not get worse and there are many engines that lasted for 100k miles after the knock started.

This does not mean you should neglect the noises...... it might be something serious...... then again, it may not.

As I recall Chrysler updated the piston design, block and main bearings in 1996, which indicates (to me anyways) problems with piston slap and/or bearings were causing the noises. However, as I recall, noises did persist afterwards.

[curtis73]

Good point. My car has had piston slap since 20k miles. I now have 130k on it and the noise hasn't changed. Depends on what the noise is.