LIM replacement - low oil pressure

[kipwrite]

Had the LIM/UIM replaced (engine flooded with coolant) and now we have low oil pressure. On cold start-up, and once warm above 2000 RPM, I get reading of about 40 PSI. When at a stop sign, engine warmed, with foot on the brake - about 1000 RPM - oil pressure drops into the red zone and I get the 'check gages' warning and dinger. Have disabled the dinger as it was driving me crazy.

Car runs beautifully, no issues other than some oil use. Running 10-30. Have tried a heavier oil - no real difference. Replaced sending unit and checked with manual guage - unit is reporting correctly.

If this is a main bearing, is engine death imminent? Any suggestions as to how to handle this short of a new engine or rebuild, which isn't going to happen?

Car has 133,000 miles.

[Bing]

I'm thinking your rod bearings are worn out/out of tolerance. So yes engine death is probably in the future. This is my best theory and there might be another explanation or cause.

[MattStrike]

Low oil pressure will kill the engine. Driving down the road it might make pressure and be fine however if there isn't enough oil pressure at idle then the oil film on the bearings cannot be maintained and the bearings will wear out and the crank will get damaged and the engine will die. 20psi is minimum at low idle (500rpm).

There are two main causes for low oil pressure. Bad oil pump or improper bearing clearances (rod, cam, main). Given that the LIM/UIM failure caused the engine to flood with coolant, it sounds like coolant must have gotten into the oil. This is bad, it can pit out the bearings and cause enough damage to prevent the bearings from maintaining the proper clearance, causing low oil pressure when the engine is idling. Stepping up oil weight is a band-aid to get you home.

Start the car and put your head by the oil pan. Does it make any knocking noises?
If the engine has not started to knock yet, I think you should investigate the cause of the low oil pressure and fix it before any serious damage occurs. At only 133k, there should be plenty of life left in the vehicle to justify fixing it (hopefully you have a garage and tools). The engine doesn't need to be removed to replace the oil pump, crank or rod/main bearings (IIRC the rear main seal is in the rear main bearing cap, unlike the series 2). Rod bearings are the first to go in a low pressure situation, they bear direct force from the combustion and are the farthest away from the oil pump. If a main is pitted (thereby leaking oil), the rods it feeds get starved even if the rod bearings are fine. The cam bearings require engine removal to replace, but I think that can be avoided.


More info for you to consider...
A reman crank with bearings is about $250, the bearings are about $150.
Fastest way to check the severity is to hold the oil on the dipstick up to a bright light and check for sparkles. The sparkles are a mixture of the babbit from the bearings and hard metal from the crank. There shouldn't be any. Coolant doesn't play well with the bearings,especially if it was left sitting in it, but that won't leave much babbit in the oil unless one is starting to wear out as a result.
Oil pump is in the timing chain cover, it is a gerotor pump that rarely fails and can be serviced on car after removing things that are in the way.
Pull the oil pan and look for metal chunks in the bottom, check the oil pickup screen, push and pull on each rod cap in a direction perpendicular to the crank. Rods will have some lateral play, but should have zero movement perpendicular to the crank - it's a quick and dirty check for the rod bearings.
Remove and visually inspect the bearings for pitting or wear, because if they are pitted enough they won't hold oil pressure. You should use plastigauge to check the bearing clearances for all rods and mains if you don't find anything; this will take 4+ hours to do properly, but it will tell you if the bearings are worn. You also need a proper torque wrench for the rod & main bolts, and I'm sure somebody has a '95 CSM to give you the torque sequence for the rod & main bolts. Make sure there is a mirror finish on the crankshaft, absolutely no gouging or grooves. It might be safe to assume that if one bearing is pitted then they all are, and just replace the rod & main bearings. If that is enough to get the oil pressure back to safe at idle then the cam bearings shouldn't get any worse if they are also pitted because you will have enough pressure to maintain the critical oil film on the bearings.
You may find that just one rod bearing is bad and worn grooves in the crank, meaning the cam is going to be fine, but you will have to replace the crank and bearings, but if the rod is damaged as well then you start getting into rebuilt territory. If the engine hasn't started knocking yet, then the crank and rods haven't reached this point...all the more reason to fix it now!

But, without getting a little dirty then it's impossible to know for sure what is the problem or if it requires a rebuild.

[wjo165]

though this is old, it is an excellent response mattstrike. u just taught me a lot