Why did my air filter fill up with engine oil?

-
cruiser

cruiser

Well-Known Member
Joined
Feb 26, 2016
Messages
799
Reaction score
890
Location
Bloomington, MN
The slant six in my 1974 Duster (completely stock engine) was running poorly, so I checked my air filter. Normally its dry and pretty clean. This time the upper and lower rubber gaskets of the filter were coated in engine oil, and there was engine oil in the filter element itself. What would cause this? Ideas, anyone?
 
I just
abodyjoe said:
is the breather clogged?

maybe the motor is tired and experiencing blow-by?
Click to expand...
thoroughly cleaned out the breather with gasoline, then washed it in detergent and dried it with a hair dryer. I then filled it with 30W motor oil and drained the excess oil, as the FSM recommended. The engine has good compression on all six cylinders. Very puzzling.
 
I recently rebuilt a 455 Pontiac engine with the same problem. The top rings on 7 out of 8 cyls were broken, in pieces. The ring pieces were carboned up, obviously been running this way for a long time. Engine ran reasonably well, just had this oil problem.
 
Don’t over think it. Replace the air filter and drive it. See if it repeats. Also not all pcv are the same. The spring inside is different especially between manufactures.
Try either a new unit or test your old one.
Slants are a really dependable engine and simple to diagnose.
 
Sounds like a evacuation or blowby issue, east to check take the oil cap off when it is running and see if the "fumes" come out. If so either one it can be. Then troubleshoot it. Prob the PVC I hope.
 
Last edited:
Yea, replace the PCV and Oil Breather. They are cheap enough. See what happens from there. Maybe Slant Six Dan will chime in..
 
Make sure the PCV is actually "sucking". The hose could be collapsed, or more likely the port on the carb plugged up. To clean the port, remove the carb, and using a drill bit by hand, go up through the bottom hole, then go through the nipple for the hose. Also, if you have had the carb off, it might have been reinstalled with the small hole in the gasket in the wrong position.
 
When was the last time you changed this?

smoke.JPG
 
cruiser said:
The slant six in my 1974 Duster (completely stock engine) was running poorly, so I checked my air filter. Normally its dry and pretty clean. This time the upper and lower rubber gaskets of the filter were coated in engine oil, and there was engine oil in the filter element itself. What would cause this? Ideas, anyone?
Click to expand...
Seems like your PCV valve isn't functioning. The blowby is getting into your air filter housing through the breather.
 
Valleyant said:
Seems like your PCV valve isn't functioning. The blowby is getting into your air filter housing through the breather.
Click to expand...
This is likely exactly what's happening. Normally, the hose attaching the valve cover breather is the inlet for the PCV system. The PCV valve puts a vacuum on the crankcase. That vacuum causes a low pressure area where the breather hose attaches to the air cleaner and pulls air into the valve cover through the breather. Without the vacuum applied by the PCV valve, the suction of the carburetor will pull oil and vapor in reverse through the valve cover breather, the hose and right into the air cleaner. I would check all of the already outlined and make SURE the PCV hose itself isn't collapsing under vacuum and closing off.
 
RustyRatRod said:
This is likely exactly what's happening. Normally, the hose attaching the valve cover breather is the inlet for the PCV system. The PCV valve puts a vacuum on the crankcase. That vacuum causes a low pressure area where the breather hose attaches to the air cleaner and pulls air into the valve cover through the breather. Without the vacuum applied by the PCV valve, the suction of the carburetor will pull oil and vapor in reverse through the valve cover breather, the hose and right into the air cleaner. I would check all of the already outlined and make SURE the PCV hose itself isn't collapsing under vacuum and closing off.
Click to expand...

abodyjoe said:
i mentioned that in the first reply and he blew it off.
Click to expand...
Exactly. Replace the PCV valve, check the hoses and passage in the carb. The process for all the PCV system maintenance is in the factory service manual.
Screen Shot 2022-09-29 at 11.30.03 PM.png
 
1) You dropped a valve and are pressurizing the crankcase (not probable with the good compression readings).
2) Pressurizing the crankcase with worn rings (again, not probable with the good compression readings).
3) Pressurizing the crankcase with a defective PCV.

I'd go with Door #3.
 
cruiser said:
After replacing the air filter, I ran a bunch of carb cleaner through the PCV valve. The PCV valve would rattle when I shook it, before I cleaned it
Click to expand...

The PCV valve is supposed to rattle when you shake it. If it does not, there's something wrong. And even if it does, it also has to be the correct valve; see here.
 
PCV holds closed on high idle vacuum, opens on the crack of the throttle and closes again on low manifold vacuum WOT. The shuttle valve you hear rattling around is suspended in mid stroke by these 2 springs. It can still be bad even if it rattles. You can extend the PCV line up to the air cleaner through a oil catch can, just a can with the in and out hoses up top like a shop vac. The oil is heavier than air so it'll settle at the bottom. Drain at your neede interval but it should not catch much oil at all if the $4 PCV valve is good.
 
pishta said:
PCV holds closed on high idle vacuum, opens on the crack of the throttle and closes again on low manifold vacuum WOT. The shuttle valve you hear rattling around is suspended in mid stroke by these 2 springs. It can still be bad even if it rattles. You can extend the PCV line up to the air cleaner through a oil catch can, just a can with the in and out hoses up top like a shop vac. The oil is heavier than air so it'll settle at the bottom. Drain at your neede interval but it should not catch much oil at all if the $4 PCV valve is good.
Click to expand...
Have solved the problem. You were correct about the PCV valve. Cleaned it out thoroughly and the problem went away. Thanks all for your help and ideas!
 
You must log in or register to reply here.
-

Back
Top