LOW OIL PRESSURE 360 AMC

 I am unfortunately dealing with the same thing and have a similar post from last week. My 86 GW dropped oil pressure, probably not coincidentally, when I added Rislone to the crankcase.

This is what I found in my folks original owner 74 Matador X, 360, 2V.  My father (pushing 80 years old) has been complaining that the oil pressure light seemed to take longer to go out when he started the engine (158K miles).  I really wanted him to drive to Kenosha show, but he didn't want to with the "perceived" to be low oil pressure.  I checked the oil pressure with a direct reading gauge, found to be low at idle and with the throttle cracked open it came up, but would not go above 30 psi.  To satisfy myself and most importantly my father, I pulled the engine.  The oil pump and gears were like new (my father changed the oil every 2500-3000 miles), my father also told me that lately, it was making a noise in the bottom end only on start, but like a thump 2 or 3 times.  At 158K, I was prepared for a major event!  As I pulled the heads, mint clean engine, timing cover perfect, oil by-pass valve-clean, inside pan...well here is the surprise..lots of small flakes of silver about 1/8" plus in chunks, lots of small translucent plastic parts.  I spun off the oil pick-up-very clugged with parts similar to what was in the pan.  I pulled all the pistons-like new, hone marks in cylinder walls (at 158K...WOW), rings were worn, rod bearings still silver, camshaft worn on lobes, mains on cam weird wear marks.  Pulled crankshaft, the rear main was worn to the copper, next one forward worn but not as bad, the rest all were nice for the mileage.  Knocked all the cam bearings out....the last three had no liner (babbit) left, that is the source of the weird cam wear marks.  The cam bearings were the silver in the oil pan.  The small plastic parts were all valve stem seal parts that had broken off over the years.  I tried cleaning the debris from the pick-up for days...it never came clean to my satisfaction.  I reused the original piston, just bead honed the cylinders, had only the mains only ground, the rods all measured to new specs, same with the cylinders, new std size rod bearings, cam bearings, cam and lifters, gaskets and back together.  I took the block to the shop to get it cleaned from the debris, did a valve job and but new valve seals in.  I assembled the engine with limited new parts, plus a new pickup.  The engine now carries 25 pounds at idle and 60 at 1800 rpm, has not skipped a beat (10w30 Oil).  I ran this engine on my run stand and broke the cam in on the stand, and got all the pressure reading off the accuracy of the digital gauges.  Maybe some of this will help you trouble shoot your engine oil pressure issues.  Bottom line.....I have found many engines with flaking cam bearings, dirty/clogged oil pick up...I blame the main bearing being worn from the lack of oil from the cam bearings oil flow through to the main.

madmax wrote: later model amc engines hot at idle are around 6-12 psi, it should jump up right off idle to 20-30 psi, does this happen? the reason they dont have 40-50 psi at idle is because it would wear the dist drive and driven gears prematurely as the oil pump is driven by the dist. of course if there is wear in the cover assembly this would also reduce pressure. i have also read that some oil filters seem to reduce pressure as well. hope this helps.   rick

My 1974 AMC Technical Service Manual has the V8 normal operating pressure as 13psi @ 600rpm; 37 to 75psi @ 1600rpm +

Edited by OzJavelin2 - Apr/18/2012 at 7:20pm

The 1982 Jeep TSM (last V8 in cars was 1979) says the same as the 74 -- 13 psi @ idle (600 rpm).

Check the end clearance on the oil pump first (corrected with a thinner gasket). If that's good you might need to pull the pain and check the mains. The rear main and rods are the last to get oil, so they will be the ones to check. If they are okay then it's not crank/rod bearings.

There are no date code numbers on an AMC V8. That's the only thing AMC goofed on in my opinion! The date code is on the valve cover tag. Should be front of the right VC, but if no tag check the rear of the left one. They interchange. Assuming the VCs are original you will have a date code consisting of two numbers, a letter, and three more numbers. The only other numbers that ID the block are the casting number and the displacement, which is cast into the side under the motor mount plates. Neither give a precise year, just a wide range of years it could be.