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Piston Slap: If this van’s a-knockin’ …

Joe writes:

I have a 2003 Dodge Ram van with a 5.2 with 207,000 miles. I bought it four years ago with 195,000 miles and use it to help me remodel houses in Florida. The Carfax and records indicate regular maintenance since new while it was owned by a local HVAC company before I purchased it. The engine starts almost instantly and does not knock when cold. After coming up to normal operating temp, I get a “soft” regular knock knock knock at idle ...

 

Sajeev answers:

Thank you for your detailed explanation of the problem. This is probably not piston slap because it only happens when warm: A cold engine has wider engine tolerances (in the piston rings to the cylinder bore, to be exact) while warmer ones are tighter and therefore less likely to slap. But ...

 

Read the full article on Hagerty.com:

https://www.hagerty.com/media/advice/piston-slap/piston-slap-if-this-vans-a-knockin/

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Does the knock volume increase when the vehicle is under load? If it does, that would indeed indicate a possible rod bearing issue. If it stays consistent, I'd wager that the crankshaft is walking a bit, and the thrust bearing faces are getting thin. A backyard diagnosis would involve disconnecting one plug wire at a time to see if the noise changes. If it does, then that would be a hammered rod bearing. If not, then the crank thrust bearing would be suspect.

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