Oil Pressure Going Away

[Hans]

My daily driver is a 2800CS that I've had for about 3 years. The PO told me that the engine was rebuilt in '91 -- about 100,000 miles ago. Since I bought the car, the oil pressure light has always gone out about 2 seconds after the engine starts (which, if the car's been sitting for a day or so, happens only after the engine cranks long enough to get some fuel into the Webers).

Yesterday, the oil light flickered on while the car was idling hot, stuck in traffic. As I headed home, the threshold for oil-light-on rose from 1000 rpm to as high as 2000 rpm when the temp was hot and to 1600 rpm when the temp was cooler. I checked the oil level several times and it was always OK.

Is this an oil pump problem? Relief valve? Or???

Suggestions appreciated.

Hans

 

[Honolulu]

Oil light

You should note whether there are other issues - noises - in the engine, and whether it sends any blue smoke out the tailpipe. We'd want to know, before making a recommendation, the condition of the engine. 100,000 is a lot of miles on a "rebuilt" engine, and we don't know the extent of the rebuild, general engine condition, or your driving habits.

BMW engines have notoriously high cold oil pressure, and can have pretty low hot idle oil pressure. Do you know your sender is good, i.e., have you tried a gage to check that the oil light is "telling the truth"? I suspect it is.

Is the #1 cam lobe getting oil, or is it visibly worn? M30 engines are known to have the front (of 2) banjo bolt loosen on the cam oiling pipe, and lose pressure there. This means that the front of the cam isn't lubricated, and I've seen very worn cam lobes particularly the farthest front one. Check this before going further, may have to remove the valve cover. Late type banjo bolts come with some sealer on them to lock 'em to the head. Be sure the female hole in the head is real clean before screwing the banjo in.

There are several other places to lose oil pressure: the oil pump itself (second place to check), rod and main bearings (usually not a problem, but your motor may have, what, 200k on it?)

The oil pump cover over the two gears, may wear and excessive clearance (which will show as a polished spot on the cover) will cause low hot oil pressure.

I went in after my pump a year ago, and replaced rod bearings while I had the sump off. Just beccause I could, mostly; I don't believe they needed replacing.

Sump removal requires the crankshaft be turned to just the right position, and the engine slightly lifted off the crossmember. You'll be able to see the crank when you partially drop the pan, and rotate to the needed position.

Others may comment about the potential to lose pressure at the oil filter head. I'm not familiar with that item, but I do not recall having heard of problems there.