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Engine Knocking??


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I just bought an 88 MJ 4.0, auto 2wd. The oil pan was removed cause the PO said he put new main bearings in. I have talked to him several time since i bought it, and he swears he did put in new ones. The truck is knocking pretty bad, sounds like its coming from the lower end. I didn't check anything really before i put the oil pan back on. My bad i believed someone. Any ideas on what the problem may be, or some stuff to check? Is there anything i can do before i remove the pan again?

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If it's an automatic, a cracked flex plate can make a knocking sound that's similar to a bad bearing. The test is to run it at idle. If it knocks in neutral, but it in gear with the brakes on and step on the bas just a little. If the knock disappears, there's a high probability of a cracked flex plate.

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He just got back to me and said that he changed the rod bearings. Is it possible they aren't getting oil? The valve cover and oil pan had a ton of sludge in them. Could some of the oil ports to the bearings be plugged? Or is that to easy?

If it's an automatic, a cracked flex plate can make a knocking sound that's similar to a bad bearing.
I will have to give the flex plate a try tomorrow morning.
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Yes there is a oil pump, he says he micrometer the crank and everything was in check. But with the odometer reading just under 230k i was thinking that thing could need a grinding too.

Why? My '88 XJ has 287,000 miles on the clock and doesn't knock. It is very common to just throw new bearings into a 4.0L engine to resolve oil pressure issues. They rarely need the crank turned, unless the owner waited much too long before deciding to do the bearings.

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Yeah, or he was a doofus and only replaced the main bearings.

 

If you run 20W50 oil, does the knock go away when the engine speed is over about 1500 RPM? If the vehicle has gauges, what kind of oil pressure does the gauge think you have, at idle and at 1500 or 2000 RPM?

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it has an aftermarket oil gauge, the reads 55-60psi all the time. The knock seem to get quieter the higher the rpms, but it may be because the over all motor is drowning it out.

My buddy came over and swears it sounds like a lifter, of piston slap.

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While it's running, pull the oil fill cap out and watch the rocker moving. If the sound seems to be happening once every time the rocker moves, it's a valvetrain knock. If it seems to be happening twice as fast, it'll probably be coming from the bottom end.

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I had the whole valve cover off with it running and it seems like its in the valves but now the trick is figuring out which one its coming from? They all seem tight, all the push rods are pumping oil out. My buddy wants me to just beat on it an maybe the lifter will come loose. I don't think this is a good idea.

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