Blue smoke out the exhaust but good compression?: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Feb. 6, 2012 3:43 p.m. PseudoSport Reader

I’ve been working on fixing up the engine in my 280Z and just recently got it running. I’ve installed a new head gasket, lapped the valves, leak checked them, installed new valves seals, adjusted the valves, checked compression, and replace PCV valve. All cylinders are about 190 psi hot. Problem is I’m getting blue smoke out the exhaust. When I rev up the engine there is enough blue smoke to eclipse my neighbors house. Other then that it runs great and is nice and quiet. Looking at the plugs #5 has a lot of oil on it. When I turn over the engine I actually get some oil spray out the hole and only on #5.

I cleaned up the head while it was off I noticed there was a lot of carbon build up on the exhaust valves and ports. Before putting it back on I checked the cylinder walls and they looked fine. There was no ridge or scrapes and I could still see some hone marks. I’m thinking that maybe the oil control rings are gummed up since compression is still good and everything is quiet. Before I rip it all apart again to re-ring it is there anything else I can try. Maybe soak the rings, and flush the engine?
Feb. 6, 2012 3:53 p.m. Curmudgeon SuperDork

My 280Z burned oil on #6, it had good compression all down the line too. It was bad enough to foul #6 plug once a week. Turned out that sometime in the past the engine had overheated and the rings on #6 I guess just lost tension. The cylinder walls looked good and there was an almost imperceptible ridge. Ran a flex hone through them all, reringed it with new rod bearings, problem solved.
Feb. 6, 2012 4:00 p.m. Donebrokeit Reader

Sounds like a bad oil control ring or cracked ring-land both can cause oil smoke and show good compression at the same time.

BTW: Did you have the valve guides replaced?

Paul B
Feb. 6, 2012 4:34 p.m. PseudoSport Reader

Guides were not replaced. When I was putting the head back together they were in better shape then I expected. Also I do not get a puff of blue smoke on start up. Engine fires up and its clean for about 10-20 seconds then it starts to smoke.
Feb. 6, 2012 4:59 p.m. 44Dwarf Dork

Blocked PCV or other crankcase vent?
Feb. 6, 2012 5:36 p.m. Streetwiseguy SuperDork

Is this a new problem since the head work, or is this the first time you've ever run the thing?

If never run before:

Maybe good compression rings, bad oil rings. Back in the olden days, Datsun rings were good for about 50,000-75,000 miles.

Are there any hoses from crankcase breathers or vacuum pumps or automatic transmission going to the manifold somewhere around the #5 runner?

If it was fine before:

You broke something.
Feb. 6, 2012 9:31 p.m. novaderrik Dork

is there a potential pathway from the crankcase to an intake runner on those engines? maybe the pcv system is sucking in oil and burning it..

that's the easy stuff- but it's probably toasted oil rings on one or more cylinders.
Feb. 7, 2012 8:37 a.m. iceracer SuperDork

Too much oil ?
Feb. 7, 2012 12:03 p.m. PseudoSport Reader

This is the first time I’ve had it running. Oil level is right between the low and high marks. Guess I’ll double check the PCV system.
Feb. 7, 2012 12:34 p.m. Dr. Hess SuperDork

If only 1 cylinder is smoking/dumping oil, I'm going with Major Mechanical Problem. Ring(s), piston, head gasket, head, block. I think that thing's gonna be coming apart (one way or the other).