Sudden knock without warning

OK, this is something that is bothering me. Coming home from work on tuesday, my fathers grand marquis suddenly started knocking. 87, 5.0L, the usual package. But the odd part is, that it doesnt seem to knock till the motor has been run for a few minutes. Pulling the plugs turned up a Badly fouled plug on #7 cylinder. all of the others are fine. However, the knocking is coming from the bottom of the oil pan, near the back, making it a safe bet that its a rod bearing. But what bothers me is that it would just start knocking unprovoked. the car is regularly maintained(by me), the oil was up to range, the pump puts out 40psi on idle, anyone have any ideas what caused it? or is it just a fluke?:confused:

About 130K. But ive never had one go under 160 without excessive abuse or neglect.

Yeah,,sounds like some wear on the rod bearings,,,when the oil is cold it's thicker,,taking up the slack,,,when the oil gets warm it thins out leaving the bearing easily,,,then it would knock consistently while running.
Mind you,,,diagnosis over long distance can't guarantee anything.
Probably time to do an overhaul, or rebuild on it.

Hmm, never heard of a soft crank, but i believe it. I knew there was a reason I drove Olds and AMC! Kind of figured the oil was a concern, so I'll expect a set of oversized bearings are in order for a few rods. Time to get the plastiguage and the torque wrench. Thanks for the confirmation guys:)

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Originally posted by DennyW
:) Well, I didn't mean as in soft material, I should have said weak spotas wear on the bearings. Not the material, Like chevy chevy fac cams, hahaha. They all have their draw backs, even yours, hahaha. :)

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Denny, the only thing that doesn't have their draw backs is your pretty 34 Chevy truck. I haven't even seen that thing and KNOW that. heh.