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Discussion Starter #1
2008 Nissan Bluebird Sylphy.
I'm losing oil rapidly with no visible leaks and no noticeable smoke.

Curiously, the car misses on one or more cylinder when the oil is at the correct level, but runs smooth when the oil is very low, almost dry?

Could this be valve stem seals?
 

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Sup Mod keeping the peace
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Your engine is most likely burning an excessive amount of oil due to worn piston rings. When the oil level is at the correct level, there is plenty of oil being deposited on the cylinder walls to cause increased burning of oil and contamination of certain spark plugs. When the oil level is almost dry, there's a lot less oil being burned; a very dangerous situation for the engine; a good way for the engine to seize up or throw a rod through the crankcase.

To verify your situation, perform a compression test on all cylinders and if not sure, follow it up with a leak-down test on all cylinders.

A easy way to test for oil burning is to fill with oil to the correct level. Then fully warm up the engine.
Stand behind the car. Have someone rev the engine to 4,000 RPM and hold at that RPM for about 15 seconds. If you see a lot of blue smoke come out of the tailpipe, the engine is burning excessive oil; time for new rings.
 

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Discussion Starter #3
Thanks. Makes sense. I was hoping it was just the valve seals, which is a smaller job than rings. I'll do those tests you mentioned to verify.
 

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Sup Mod keeping the peace
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One sure-fire way to tell if you have worn valve stem seals is to perform a cold engine test. When your vehicle has been sitting overnight or for a longer period of time, the top of the head of the valve cover area will have some oil left over from the last time you drove. When you start the engine, the oil ends up getting sucked down through the worn seal(s) into the combustion area, producing a blueish smoke out of the tailpipe. This will generally last for few moments and then clear up as you start to drive.

Another way to test for worn valve stem seals is to be aware of what happens while your vehicle is idling. When your vehicle is stopped for a significant amount of time, high vacuum levels will cause the oil to build up around the valve stems while the valve is closed. In a faulty valve seal situation, when you begin to accelerate again, this oil can end up getting sucked past the seal into the combustion area. This causes more of this blueish smoke, due to the burning of oil, to come out the tailpipe. Again, this will generally last for few moments and then clear up as you start to drive.
 

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Discussion Starter #5
Didn't (Couldn't) do compression or leak tests but the cold engine and after-idle tests suggest valve stem seals. I'll go for that challenge tomorrow. Thanks again.
 
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