Dirty oil

[94RS]

Despite regular oil changes, every 3,000 km, my oil looks like tar. Much dirtier than it should be. I'm thinking I must have a valve problem or perhaps rings. All the carbon in my oil is indicative of poor combustion isn't it? I'm thinking of perhaps trying an engine flush to try and remove some of the carbon. Any thoughts?

[guyaverage]

Its normal for a certain amount of combustion gases to make it past the rings and valves, and the PCV valve's job is to vent these into the intake so they can be reburned instead of being vented to the atmosphere. If the PCV isnt working, the gases mix with the oil and turn it black. This could be nothing worse than a bad or gunked-up PCV valve. Do an engine flush, then change the PCV valve and your oil again. If it keeps getting very dirty quickly after doing that, then you could be looking at some surgery, either new rings or rebuilding the head.

[94RS]

Thanks for the input. I will as you suggest try an engine flush. I've just recently put a new pcv valve on. I'm hoping the problem is just some gunked up dirty valves and not bad rings. The car seems to run well otherwise, however in a recent emissions test I barely passed with 620 ppm. The pass for that particular test is 630 ppm. I beleive that test measures carbon particulates in the exhaust. I don't have any noticeable oil burning though, and the compression seems fine.

[guyaverage]

Was the new PVC valve before or after the emissions test? And was the 620 ppm for HC or NOx? What were the CO and CO2 percentages? You may have a leaky injector or a plugged catalyst.....

[Stan W.]

Had the same problem on my wife's car. There was a faulty temp sensor causing the ECU to think the engine was cold. This caused the mixture to be overly rich. This in turn causes a burn that is very "rich"(pun intended) with carbon. Even with a good PVC valve, a little "blow by" will get through. To complicate matters, a rich burn clogs up the catalytic converter which further screws up things. So, if you change oil and the PVC valve and the oil still turns "dirty" check the temp sensor.<p>Stan