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Leaking Bimmer

Q. Hi, I have a 1989 BMW 635CSi with a 3.5 liter fuel injected engine, auto transmission, ABS, P/S, A/C, and cruise with 126,100 miles on it. The oil filter is the cartridge type.

Until 1,000 miles ago there was no change in oil level between oil and filter change outs every 3,000 miles. Now after my last change 1,000 miles ago I am down ½ qt. and I have an oil leak that has spread oil on the underside of the engine compartment, transmission and exhaust.

There is no oil coming out of the filter housing, valve cover gasket, oil pressure sending unit (mounted at the drivers side rear of the valve cover), timing chain tensioner, or oil drain plug. The top of the engine is clean.

Now my question. After I clean the underside of my car and whip out the creeper and jack stands, How do I tell if it's just a loose oil pan or if I have a bad rear main oil seal? What would I need to remove in order to look up into the bell housing to inspect there?

Nothing is said about this in the Bentley 5 series manual and I appreciate your time.

Thanks,
Grant

A. Knowing BMW's as I do, I would say there is a 97.32% chance the rear main seal is leaking. They usually last until about the 130,000 mile mark before they start to leak. If you look up at the bell housing where it bolts to the engine, you will see a small notch. If you clean around the notch and let the engine run, you will see oil start to drip out of it. That's the clue the rear main is leaking.

As far as the oil pan goes, I have rarely seen them leak enough to cause a mess under the engine. They will seep and you may see this seepage, but it is not a real leak.

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