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rod and main bearing replacement?

Erik the Red

NAXJA Forum User
Hi all,

11,000 miles ago I replaced the rear main seal in my '88 Comanche (4.0L, with 193,000 miles). I removed and reinstalled the main bearing cap without a hitch and torqued it to the proper spec. Both the bearing and the crankshaft appeared fine.

About 4000 miles ago I began hearing a knocking sound from down below. The knock is loudest at idle when the engine is cold although I can still hear it slightly after everything is warmed up. My oil pressure reads approx. 40 at high revs and 17 at idle when warmed up, but can drop to maybe 10-15 during a warm day of city driving (but still at 40 at high revs). I thought initially I had an oil filter with a faulty check valve that was letting dirty oil roll back into the engine when off, but I've since changed the filter and the noise is still there.

So...does all this describe a crankshaft/rod bearing problem? Perhaps a oil pump problem? My feeling is that 193,000 miles is a little early for bottom end problems but maybe my engine was built on a Monday morning ;) or I somehow goobered up the rear main cap reinstallation :(. If I replace the rear main bearings, should I replace the rear main seal again even though it's only 11,000 miles old?

Factory spec for oil pressure is 13 psi at 600 RPM, 37 to 75 psi above 1600 RPM. However, most seem to idle at 25 to 35 psi.

The knock you hear could be connecting rods, or it could be valve lifters. Before pushing the panic button, do some careful listening to confirm if the noise comes from the lower part of the engine, or from the top.

The 4.0L engine seems to wear the rod bearings before the mains. I've seen 4.0Ls that needed rod bearings at less than 100k if the oil wasn't changed at the proper intervals. If you confirm the noise is from the lower block, drop the pan and replace the rod bearings.
Hi Eagle,

Whew, well at least my oil pressure values are within the acceptable limits. I went out and listened to the engine using my trusty screwdriver stethoscope, and the knock seems to correspond to the rpm of my timing light strobe. Also, the noise is more noticeable from the rear of the oilpan than from the top of the valve cover. No appreciable knock emanating from the distributor shaft, either. I'll go back and listen when the engine is cold and when the knock is more pronounced.

Is there any way I could have screwed up the rear main bearing when I changed the seal? Geez, I guess if I bungled up the bearing then I suppose I deserve the job of removing that oilpan again.

A bad main bearing doesn't make a knock. That's rod bearings. Drop the pan and pop in a set of rod bearings. I'll bet when you pull the old ones they'll be worn right down to the copper shell.
my 88 MJ oil pressure is pegged around 80 or so .. I rebuilt at 130 ... ... cause i could .. .... I changed oil pumps . during the change .. I havce also replaced the .. sending unit and insterment cluster .. .. all that is left is to put a mechanical guage on it. BTY that oil pan is much easer to but on while the eng is out .

Yeah, the oil pan is a drag to remove from underneath. My Comanche is a 4wd so that means I have to monkey around with dropping the front axle, etc. I've done it before so I know I can do it again, but laying on my back with a face full of gasket scrapings isn't my idea of fun.

I suppose I should replace the oil pump while I'm in there. I hate the thought of spending more money on what appears--from my pressure gauge at least--to be a pump functioning within acceptable limits, but an ounce of prevention is worth a pound of...gasket scrapings in my face. And yet, I read somewhere that the vast majority of low oil pressure problems are caused by bearings and not by a failing oil pump.