My '89 Mercedes 190E 2.6 liter (178,000 mi) is using 1 qt of oil every 500 miles now. Is there a way to determine if it’s from the oil control rings or from the valve guide seals? I’m tempted to replace the valve guide seals but don’t know if it’s a waste of time.
It’s a waste of time. Chances are that you need a complete rebuild.
Have a compression test "wet/dry) done on the engine as well as an overall assessment (vacuum gage etc.). The car is 22 years old now. Time to face facts.
If it was my car, I would do a compression test and if ok, would replace the valve stem seals myself, risking only about 100 bucks or less for parts. That is the easiest thing to try that does not require major engine disassembly unless I underappreciate the complexities of a Mercedes chain driven overhead cam.
Before cat converters it was a little easier to diagnose an oil burning problem. Continuous smoke under power was rings, a blue puff on acceleration after coasting was stem seals.
I was able to fix an oil burning problem a long time ago on a 1972 and a 1974 model (two other brands) with new stem seals when stem seals were made of neoprene which was not up to the task. One had an OHC but also rocker arms so the cam did not need removal.
It looks like I can buy a valve stem seal kit for $18. They don’t look too difficult to replace either, so I suppose I should give it a try.
If you’re going to do this with the cylinder head in place then you will also need the special valve spring compressor tool and a means of keeping the valves from dropping into the cylinders. This means an air compressor and spark plug adapter or by using the old stuff the rope into the cylinder trick.
It seems to me you would be better off weeding out a ring problem before even spending 18 bucks on a valve seal kit that may possibly accomplish nothing.