smoking at idle

[Paulz]

My 1980 720 pickup, which has the L20, was puffing blue smoke if left idling.  I disconnected the PCV valve, smoking stopped.  The pipe from the manifold runs underneath, where does it pull crankcase air from?  Anyone had this issue before?  

[datzenmike]

Intake vacuum pulls combustion gasses and oil as well as water vapors from the crankcase. As these vapors are removed make up air is pulled in to replace it from the air filter through a hose to the top of the valve cover. On a normal engine some blow-by from the combustion chambers is normal and at most speeds it is removed by the PCV valve but at times as in under full throttle when intake vacuum is lower it can overpower the PCV. When this happens it will reverse out the valve cover and through the hose to inside the air filter where it is sucked down into the carburetor and burned. A properly functioning PCV system will remove vapors that would otherwise condense and pollute the oil thus extending the oil change period and keeping the engine insides cleaner. The PCV has zero effect o the running of the engine.

 

Smoking has to be oil vapors or mist of fine droplets pushed out by an excess of blow-by. The PCV won't be the cause.

 

Is your engine high mileage? If so it may just be worn out.

Does the oil smell of gasoline? A ruptured fuel pump diaphragm would allow gas into the engine and dilute the oil.

 

 

 

[Paulz]

I don't know the mileage or history but it appears to have a fairly fresh head.  Doesn't smoke under acceleration, deceleration or cruising, just idle, and not all the time.  Like flipping a switch, one minute it's fine, the next puffing badly.  But since I disconnected the pcv that has stopped too.  No blowby out the valve cover breather, air cleaner is dry.  Very confusing.  It's like oil is getting slung up to the area where the pcv tube enters the crankcase, which is why I asked where that is.  

[Crashtd420]

Have you tried the obvious and replaced the pcv valve?

[Paulz]

Ha, no but that's an idea.  Or clean it.  The valves I'm used to fooling with are open at idle but I believe there are some that have restrictions at various vacuums.  I'll have a look, thanks.

[datzenmike]

At idle, you are likely sitting still. Anything else you are driving and it's hard to see the smoke when it's being mixed with passing air. I'm saying it's easier and more noticeable to see at idle. Have someone follow you on the highway with the PCV hose connected and try it. Have them watch the end of the tailpipe. Pull your plugs and take a look at them. If it's smoking all the time, like I suspect, they will be black.

 

The valve can stick open or closed or in between but it can't cause the smoke. Pulling the pipe off stops it so it has to be oil. (mist)  On an engine with lots of blow by it's most pronounced on acceleration and not at idle, worn rings with poor oil control on deceleration, bad valve guide seals tend to blow a blue cloud on restarts.

 

Is the PCV hose from the engine oily? Tie a rag over the end and go for a drive, then check the rag for oil.