Ok so I've been chasing this one for a while. I have a '90 B250 with a 5.9 with about 155,000 miles on it. I just took it on a 5,000 mile trip. During the trip I noticed that the engine was making a metallic knock sound whenever it was started after it had cooled off. It's loudest in the morning. It lasts for a few minutes and gets gradually quieter until it reaches OT, then it completely disappears.

Since I've been home, I've tried to isolate the sound. Took belts off, listened to various parts with hose/screwdriver to no avail. Oil pressure is good and the engine otherwise runs and drives perfectly fine. The knock does not affect drivability; it just makes a noise.

This doesn't sound like typical knocks because it is not exactly rhythmic - it sort of "clatters," but it does not sound like valvetrain noise (at least not to me).. it's more of a hammer on the block sound. A mechanic buddy of mine says it's probably just piston slap since it goes away when warm. It sounds like it could possibly be coming from the harmonic balancer but the rubber gasket looks ok, and I can't see how the noise would disappear if it is in fact the HB.

I could live with the piston slap theory, but after searching on the net I can't find anything specific on dodge 5.9/360 piston slap.

Also, I should probably note that the trans (A518/46RH) was rebuilt right before I went on the trip. My first thought when I heard the knock was loose TC bolts.. until the knock went away. Loose bolts would always be knocking, no?

Any ideas? Thanks in advance.

 

Timing chain most likely.

 

Slack in the chain? What would cause it to diminish with temp?

 

Lubrication via oil flow.