68hemi

The 383 in my 57 has a pcv valve in the hose from the draft tube hole to the rear carb (dual quads) and a vented oil fill tube cap. When I hold my hand over the fill tube I get little to no suction. It is creating oil leaks due to too much pressure build up in the crankcase. I am thinking possibly the tomato can under the intake may be plugged? I want to hear opinions before I have the intake pulled to check it. I have no idea of what else could be causing it? Yes, the pcv valve is working well.

Avispa

Seems you should be able to blow through the draft tube hose if the tomato can is working correctly to check it. Otherwise it would have to be a bad blow by problem causing oil leaks.

68hemi

Not seeing any smoke out the tail pipes like bad new rings. The engine has about 5000 miles on it so the rings should certainly be seated.

MikeM

If you're not using a sealed dip stick, you'll lose all your vacuum there. '63/late engines used the sealed tube. See post #2 for a way to check for a plugged vapor separator. I haven't seen one of them plugged up since the old days. Very old days.

Frankie the Fink

You can’t measure PCV effectiveness at the oil fill tube.

Factoid

If you are using a pcv valve with nipples on both ends (hose on both ends) make sure it is not installed backwards.

There is not much else that can go wrong.

SJW

I don't see how you could be building up enough crankcase pressure to cause oil leaks if you have a vented cap on the fill tube, regardless of what the PCV valve is -- or isn't -- doing.

Live well,

SJW

ganshert

Not at idle (high manifold vacuum) anyway, if you could get your hand up there when the engine is loaded (low manifold vacuum) you’d feel flow.

PCV valves have low flow at high vacuum and high flow at low vacuum.

If there’s enough flow to pull a piece of paper against the fill tube your PCV is working at idle.

How do you know you have excessive crankcase pressure?

68hemi

That is what my mechanic is telling me.

GTOguy

This.^^^. If you ever look inside the 'tomato can', you would realize how tough it would be to clog it up. You'd have to have so much sludge in your engine you wouldn't be able to see the lifter bosses. Like Mike said, the very old days, of parafin-based oils. If you have the right PCV valve and an open breather, you should be just fine.

68hemi

Yes I am aware and find it hard to believe that is the problem too. Just trying to figure out the problem. The block I used in this car is a 1967 327 and I would have never thought to check the tomato can for blockage and doubt the machine shop would have either. The block WAS vatted.

Does anyone have a suggestion as to what pcv valve I should be using with a 383 11 to 1 with basically a 350 h.p. cam?

MikeM

Do you have a sealed dip stick tube and yes, your working PVC should try to draw a piece of paper to the oil fill pipe at idle because of crankcase vacuum.

Even lawnmower engines work this way.

68hemi

That is what I thought also. I assume the dipstick is a sealed unit as it fits tight over the top of the tube and you can feel it tighten up when you push it in.

GTOguy

I'm running a generic 350-400 cid SBC PCV valve in my 383 with 11:1 CR....and an unknown cam. It works perfectly. Even with 30% leakdown in every hole, no oil leaks from the engine.

DAN70

I agree....

MikeM

Where are the oil leaks?

Frankie the Fink

Uh yeah - pretty critical question...not all oil leaks are from a pressurized crankcase (which is rare to start with)...

The OP could just plug the carb PCV valve opening and drop a long rubber hose off the road draft tube block adapter down beside the bellhousing (e.g. convert BACK to a road draft system) temporarily and see if the leaks stop...

Cut the street end of the hose at an angle if needed and zip tie it in place.

I've done it before...

cardo0

I would take a vacuum gauge and measure vacuum first at the carb port, then on the other side/breather tube side of the PCV valve. You can measure these first connected and next measure them dead ended into the vacuum gauge. You could also seal up the fill tube and the dipstick tube with tape and make the vacuum checks. That should locate the vacuum source/amount and where it fails.

Good luck.

Muttley

Even a threaded pcv valve could be "backwards" if it's the wrong valve. Depending on application, the threaded end might be the inlet or the outlet.

MikeM

You are not going to have a negative crankcase pressure with a vented oil fill cap. You will have crankcase ventilation only.