Going to be diagnosing a small oil leak for my 2000 4.7L Durango (blue smoke out the tail pipe) and I'm going to start with the PCV system. I've only changed the PCV valve (#2) once or twice in 148,000 miles I've owned it. But I never checked the crankcase vent valves (#5.) They should only be able to flow air one way correct? At what mileage should I have replaced them? Starting with this & hoping it's not valve seals. Other than a puff of blue smoke it runs great though.

 

yes, valves work out only, but they are supposed to, the idea is to let pressure out of crankcase......now that said, you say you have a puff of smoke. If you get a puff and it stops on start up, probably valve seals (easy fix), maybe guides, not so easy. If it smokes on deceleration, again, guides or seals. If you get smoke under acceleration, probably rings. Now....if you are getting enough oil through the pcv or crank vent valves that it is showing as smoke, you still have a ring problem, a tight engine should not have that much blow by from rings. Now there are other reasons....none good, but not necessarily worth tearing it apart for either. It is possible, (just possible) you could be pulling in oil from the passage that oils rockers, not sure on 4.7 but on some engines that passage is very close to a cylinder and a head gasket that is not as young and perky as it once was could let oil seep there and get pulled into cylinder....not common, possible.
My guess, clean or replace the two vent valves, they get gummed up, check for cracked rubber elbows when you take it apart, a bad seal won't cause smoke but can be an intake leak.

 

I think the vent valves should allow flow INTO the crankcase. PCV valve is a metered vacuum leak. Also, aftermarket PCV valves tend to be VERY inaccurate in the their flow rates..... get one from the dealer, they are less than ten bucks.

 

Has the truck been run low on oil? I'd start with a leakdown test.

 

No. It get's low after a month or after a long drive but I always keep up on it.

So if the vents don't help what are the next steps?

1. Check intake manifold
2. Pull plugs check for oil
3. Leakdown test

 

Try a dry/wet compression test. Compare the numbers. See what ya get.

 

...and when exactly are you seeing the smoke? Startup only or while running? Accelleration/decelleration?

 

Startup mostly. And occasionally in stop and go traffic.

I'm guessing it's the dreaded valve seals. From day one I've always fed this beast synthetic.