Pinpointing cause of oil consumption-lower end or valve guides?
 

Sorry, this is a non-MB question but the concepts should be universally applicable. I have an 87 Saab 900 with a 2L 16V engine with 211K miles. Over the last 20-30K, oil consumption has suddenly increased from zero added betweeen 3000 mi changes (at MIN at change, down about a quart) to nearly a quart per 1000 mi. There are no leaks of significance. I know the oil is going out the exhaust as I can smell it and others have told me so though is isn't nearly bad enough to see in the rear-view mirror. This engine uses a Teflon/ steel-spring valve stem seal that no one ever routinely replaces (even though they come with the head gasket set) because they just don't fail very often. This engine is also known for never having lower end issues unless subjected to lack of lubrication or severe overheating. I have no reason to think either ever occurred with this car. I bought it from the original owner who had every service receipt from day one and clearly took good care of it. My compression is perfect and identical to that seen shortly after purchase 60K miles ago. The plugs are uniformly covered with a tan ash-like deposits. I'm told it smokes under load and on overrun. I've never had someone drive it while I observe to more definitively determine when it smokes but I'm going to.

So, is there any definitive way to rule in valve stem seals or is there just the process of elimination, fairly definitively ruling out everything else? It's not that big a deal to pull the head and replace them and do a valve job but I'd be very disappointed to still have the problem.

Thanks in advance.

Have a leak down test done by your mechanic
 

Greetings,

To make it easy on yourself I would have a reputable mechanic do a cylinder leak down test on your vehicle. This will eliminate false ideas as to where the oil is going and pin point where the leakage is most likely taking place. Your car doesn't happen to have an engine driven vacuum pump that operates door locks etc. does it?

Charles

I am my mechanic..
 

and your suggestion is a good one that I plan to persue next. My thinking though, is that oil leakage through valve guides will not be reflected at all on a leakdown test. But it can rule out ring leakage. No vacuum accesories except the brake booster on this car.

Thanks for the suggestion!

Unfortunately, there is no good way to test. If rings were the problem leak down testing will not test the oil rings. It will test the compression rings though and if you have a uniform carbon condition on all plugs (not caused by V/G seals), it will be because oil is being pushed into the intake through an overwhelmed PCV system. Definitely check your PCV system. Definitely check that the flow is in the proper direction. There is usually a hose from the airfilter (on most cars) to the valve cover. Air flow in this hose is from the filter to the engine (NOT THE OTHER WAY). If it is the other direction then you have either too much blow by or not enough scavanging (PCV).

I don't know enough about Saabs to state statistics but on MBs the answer to your question would be to replace the V/G seals as the test. On most engines it is easier than any realiable form of testing and the seals are cheap.

For a quick indicator, ring blow-by is suspect if you see
pessure/oil deposits coming from the dip stick tube and the oil filler cap when running [ specially when hot]
That is , of course , after you are sure that the PVC system is in order.