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. The knock doesn’t sound heavy like a rod and doesn’t sound tinny like a valvetrain issue. It is definitely not an injector tick, more of a clunk/knock and it sounds like it would be on only one cylinder/piston.

Here is the best part: It disappears completely just above idle. Is this piston slap? What causes piston slap? It doesn’t smoke or use oil.

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 if this assessment makes you say,

“Wait one second there, Sanjeev! You condemned a Jeep to piston slap with the same of circumstances so ZOMG why did you contradict yourself?”

Then I shall admire and respect you for that valid (and possibly damning) observation! I’m diverging from the previous diagnosis because the Jeep 4.0’s piston-slapping tendencies are documented both via Chrysler technical service bulletin and in multiple cases mentioned on Jeep forums: I see no such evidence for any of the Chrysler LA engines.

That said, I wager this is a mild case of rod knock. While far from great news, a mild case of anything at 207,000 miles is a sign of a well-maintained motor. Perhaps it won’t be a serious concern for a long time, provided we do something about it.

As it warms, thinning oil compounds the problems of a worn engine with spinning parts that are out of spec, which could account for rod knock at idle when warm. If true, the solution is opposite of the aforementioned Jeep: run thicker viscosity oil (or add an additive like Lucas) to see if it silences the knock. I’m leaning toward going from 10W-30 to 10W-40 oil, especially since it doesn’t get very cold in Florida.

Modern engines are far less likely to tolerate a thicker oil, but it’s worth a shot here. But which viscosity oil? Will it actually solve the problem? There are many questions to ponder …

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