Rod Bearings Going Out?

So far Im pretty sure Im experience a rod bearing failure.

Signs of such a thing:
-Nice smooth knocking sound at high RPMs
-Drop in Idle/Run oil pressure after warm-up(0-10/13-16)

Im also experience the knocking sound in the morning when I drive down the street in first gear to warmup the engine.

Its not the sound of the rockers either, I can hear those babies working at any speed.

Im almost possitive its a rod bearing failure, because Ive spun rod bearings on my Cressida before and have burned the sound of rod knock into my memory.

Also will plastigauge tell me if I need oversized or undersized since I dont know the full history of the engine or jeep?

It sounds like your rod bearings are definitely on the way out, though the main and cam bearings could also be toast. Your oil pressure's so low that you could very soon spin a rod bearing or throw a rod out of the block, so don't run the engine unless you really have to.
Since you don't know the history of the engine, I suggest you tear it apart and do a full rebuild, especially if you're going to keep the Jeep for a few more years and you want the engine to last.
At the very least you're going to need new rod, main, and cam bearings. To that list I would also add a new oil pump, timing set, piston rings, lifters, freeze plugs, gaskets and seals. Why lifters? Since the head's going to come off anyway so you might as well replace them while you're there. You could also freshen up the head by cleaning up the combustion chambers and having the valves/valve seats reground.
You might wanna have the cylinder bores checked out to see if they need either nothing more than a minor rehone or a complete rebore with new oversize pistons/rings.

Well then I guess I wont spend the extra $50 on rod bearings, and just dump all the money into the stroker and get that beast in. If I can find someone to trade my cressida for a YJ or XJ Ill be rebuilding the old 4.0 for it so the sister can have a vehicle through highschool. But not worth the extra cash rebuild a motor that going to be replaced with a stroker.

As long as these engines last I would pick up a good used engine at the junk yard (low mileage body damage situation) before doing all that work.

Have you checked for an exhaust manifold leak or crack first?

What is the mileage and what oil are you running, brand, type and viscosity?

You might also change the oil sending unit to be sure it is working properly before making the plunge!:shiver: The oil pressure sensors on jeeps are notorious for going bad. I would be especially suspicious of the overall low pressure reading on a cold start up if it is at or under 25 psi at 2000 rpm, cold.

I would replace the oil pressure sensor (sendor) and oil filter first, retest the oil pressure, if still low, then change the oil to 20W 50 with a quart of marvel mystery oil and retest it. A clogged oil filter can cause low pressure readings too! Also check for a cracked exhaust manifold and or exhaust manifold gasket leak. Use a stethoscope or piece of 1/4" (apx) hose like a stethoscope to try and isolate the source of the sound. If one of the above items fixes your oil pressure problem try the Marvel mystery oil and or Lucas oil additives to see if quites the engine noise next.

I have put 30,000 miles on a 245,000 mile 4.0 that sounded, sounds like it had bad rod bearings by switching oil viscosities to getting the pressure back up at hot idle and using additives to free up sticking parts. The noise can only be heard now for a few minutes on a very cold sart up.

If you are already sure it is not an exhaust leak noise or cracked manifold noise, you are already running 20W-50 oil, or something like straight Lucas oil, and have already replaced the pressure sensor (twice!... they can be bad right out of the box I hear), then you may have a bearing problem and oil pump problem.

I already have to many engines in the garage. I have a AMC 4.2L, and AMC 4.0L, the 4.0L is being stroked you can read everything Ive learned in the Street & Pro Forums. I also have a 7M-GTE, 5M-GE and 6M-GE Yeah Im A Supra Fan...

Specs: '88 Renix with 216,5XXmiles, good ole peice of shit Fram, SuperTech 10w30, and Lucas Oil Additive.

Oil pressure is at 45 on cold start and will hold for awhile until up to temp. I do have a oil change comming up which Im going mopar oil filter, and 10w40. Really this motor needs a rebuild, last time I had the valve cover off the amount of buildup was bad. This motor was not taken care of by the previous owner of my XJ. So really not worth a few dimes for something that will only be ran for three more months then retired and sold to someone else looking to do a stroker.

Understandable.

In your case I would go ahead and go with 20W50 right off at oil change, a non-Fram filter for sure and a quart of Marvel mystery oil in the start up mix. That should buy you some time. Does not sound like the OP sender is bad.

The 20W50 should get you close to 20 psi at hot idle, based on my results and a few others here with similar recent problems. I have looked at the oil filter topic in depth here recently and I am going to test the Mobil 1 and K&N in mine next, with out an oil change.

I currently have a quaker state (PL Fram:shiver filter (not my choice :gonnablow) that is about to get trashed before its EOL (End Of Life, LOL) but I am getting 58 PSI peak on cold starts (about 40 F) and 22 PSI at hot extended idles after an extended run on the freeway with the 20W50. The FSM says 13 PSI to 65 psi is OK, but 13 PSI with any gauge error scares the hell out of me.

I am also getting ready to change the oil filter adapter out (I have the 87 renix 4.0, 1/2quart filter cap.) to the newer 4.0 engine oil adapter (found one in the private for sale area last week...1.0 quart filter capacity) so I can use the double size oil filter capacity filters (metric to 3/4 inch thread conversion...). My thinking is to reduce the oil filter back pressure on the oil pump, reducing the delta P pressure loss through the filter in order to get more oil flow to the bearings during that hot idle stage! I already have a new high flow oil pump installed to help with that task.

In my case I have an engine that was apparently partly rebuilt before I bought it at about 220,000 miles. Does not burn oil, plugs look new after 25,000 miles! The lower rod bearings were OK and I replaced the CS mains (with the high flow oil pump replacement) as they were nearing there wear limit, but they were not gone yet. Only parts I could not check and service (with out pulling the engine) that might still be a problem are the Camshaft and its bearings in mine.

Oil filter upgrade see:

http://www.yuccaman.com/jeep/filter.html
and
http://www.madxj.com/MADXJ/technical/technicalfiles/ARoilFilterConversion/OilFilterConversion.htm

Anyway it sounds like you need a few more engines around that garage.

Good luck, and please let us know how it turns out. The good, the bad and the ugly, LOL