351m/400 knock help

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Dec 4, 2004 | 03:35 PM

jd4430

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351m/400 knock help

i have a 351 m egine that has a knock in the warmer it gets the louder the knock almost sounds like a small diesel engine when at an idle. i have pulled plug wires one at a time but it does not take the knock away . someone told me it could be the lifters. i put in mechanical gauges the oil presser reads 60 pounds when i first start it up the wamer the engine gets the oil pressure drops to about 10 or 15 punds when warm if you rev the engine the oil pressure goes up . it runs about 25 pounds at 55 mph. can anybody help narrow this problem down thanks for all your help . great forum by the way

Dec 4, 2004 | 04:13 PM

79_250RangerLariat

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miles on the engine? any repair work? probably a rod or main bearing going out

Dec 4, 2004 | 05:19 PM

jd4430

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the motor has 94,000 original miles
if it was a rod bearing wouln't quit knocking when i pulled the plug wires off one at a time?

Dec 4, 2004 | 06:26 PM

EdwrdEdmnds

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From: MAYBEE MI

sounds like timing issue, type of fuel used. does it diesel on after shutting off?

Dec 4, 2004 | 06:38 PM

jd4430

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no it shuts down fine. timing checked out ok motor still has plenty of power no hesitation

Dec 5, 2004 | 11:41 AM

Bill_Beyer

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From: PacNW

The knock wouldn't go away necessarily by shorting out the plug on that cylinder but it should change somewhat. Same thing applies to a wrist pin knock but they tend to disappear almost completely when you short out the correct cylinder.

A rod or main knock is generally a deeper sound than a lifter rattle which sounds more like diesel clatter to me. Engine noises are really hard to diagnose even when you're hearing them first hand, trying to diagnose one over the net is really tough.

If the noise doesn't change at all by shorting out a plug then you may have a collapsed lifter that's causing your noise. Even with good oil pressure a bad lifter won't pump up and will continue to make noise. Try using a piece of hose as a stethescope on the valve cover to see if you can isolate the noise to a single lifter.