View Full Version : Smoke out of the Valve Cover

06-24-2005, 10:36 AM
i have a 86 MC and when i pull the PCV valve out of the valve cover smoke comes out not much just a little. i think it is clogging up my PCV valve because when i replace the PCV valve it runs really smooth and after about 1 tank of gas it starts to rev up and down while in idle and take a little more to start. my question is, is smoke supposed to come out of the area, and if not why is it? any info would be appreciated.

06-24-2005, 04:09 PM
The PCV is used to route blow by gases during combustion from the crankcase and rocker cover back into the intake manifold. It would be normal for some smoky gases to exit if the engine had been running. As to how much, little should occur at idle and opposite under load. Many things could cause an undesired amount but I would try a high quality one first. Just my 2cents.

06-27-2005, 09:16 AM
try a high quality PCV valve?

06-27-2005, 01:38 PM
There shouldn't really be any smoke (vapors) coming out of a "tight" engine. Smoke coming out, especially at idle, indicates worn rings/cylinders, and blowby.

A leak down test would verify this, and the severity of the issue. But any vapor coming out of there is a direct result of gasses leaking past the rings. How many hours does it have? Maintenance?

I borrowed a '77 Nautique a couple summers ago from a friend. It had about 750 hours on it, and terrible (little to no) maintenance. It had so much blow by, that it would blow smoke out the breather at WOT, WITH the PCV valve installed and working! At WOT the boat would begin to reak of oil vapor! At that point it's time to rebuild, IMO. Though the thing still ran pretty well.

06-27-2005, 02:37 PM
thanks for the info. that is a lot worse than i thought i was thinking the valve gasket needed to be replaced my boat has about 900 hrs on it. like i said the the motor runs really smooth when i replace the PCV valve. do you think thats whats happing is the blow by is clogging up the PCV valve?

06-27-2005, 03:16 PM
The smoke you see could cause build up in the PCV valve and cause it to not function correctly, but I doubt it would do it in 1 tank (about 10 hours).

PCV stands for Positive Crankcase Ventilation. The PCV valve is basically a controlled vacuum leak in the system that draws contaminated air from the crank case and burns it in the engine. If you simply ran a hose from manifold vacuum to the valve cover (or any place on the crank case area), to remove the fumes and vapors from the crank case area, then you'd have way too much air flow into the manifold, via that hose...a big vacuum leak. So they put the valve in there to simply regulate the airflow (to about 9 CFM). The manifold vacuum is still drawing the contaminated air from the crank case area, but it's doing so in a controlled manor due to the PCV valve

If it were my boat, I'd throw a PCV valve in there, and run it until it starts to run poorly. Then I'd adjust the idle mixture screws to compensate for the change in flow through the PCV. With a surging idle, it sounds like you need to go slightly richer.
Then do a leak down test ASAP.
Then start forming a plan to re-power (assuming the leakdown verifies my theory).

07-12-2005, 10:14 AM
thanks for the reply Storm 861. how would i adjust the fuel mixture? have messed with the idle screw. but not the fuel mixture setting i have a 4 bbl holley carb.

07-13-2005, 02:51 PM
The Fuel mixture screw is the what I'm reffering to. If it is surging it is on the lean side and you need to add fuel. I believe those are airadjusting screws, so to richen the A/F/ ratio, you would need to turn the screws in. It's best to do this a bit at a time and try to achieve the highest, smoothest idle you can.

Also look around for vacuum leaks, while you in there.


07-14-2005, 09:51 AM
thanks Storm

03-12-2007, 04:34 PM
habby birthday JBS