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Oil Seeping out of Breather Caps

5293 Views 28 Replies 14 Participants Last post by  dmaaero
Hi Guys,
Need some guidance and help here. Bought my first C3 last month and have been slowly fixing it up. My problem I have now is the oil breather caps are getting saturated with oil when I drive at highway speeds which in turn leaks out onto the headers which cause then causes a lot of smoking.
The L48 engine has an Edelbrock manifold, Holley carb, and a plain after-market air filter, with oil breather caps on both Mickey Thompson valve covers. There is no PCV line running and I was wondering if this would be causing the oil blow out problem? I thought I would simply hook up a PCV line to the manifold but I can’t figure out where the hose would connect on the manifold.
I also tried getting baffled grommets for the oil breather caps, but there didn’t seem to be enough clearance and they stuck too far into the valve covers.
Is there a relatively easy fix for this problem? I have some chrome goodies and new ignition wires for it but I don’t want to do any of it until I can get this oil leaking problem resolved. How could I find out where or if I can run a PCV line?
Thanks in advance for any help or suggestions!
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If you're getting a lot of oil blowing out of the breathers, it may be from the valve guides on the exhaust side pressurizing the top of the head. Have you noticed any fouling on the plugs to indicate leaking guides (on the intake side)?
If you have no true crankcase "breather", and only breather caps that may be your problem. The crankcase needs to breath and is what most of the PCV system was for. The PCV stands for Positive Crankcase Ventilation.

In the "old" days there was an attachment of a tube from the valve cover that ran down to pan level to vent the crankcase pressure. If you just have breathers, I would suspect that is your problem. It can also be caused by to much blowby past the rings, thus "pressurizing" the crankcase. I would look into creating a better crankcase breathing system, even if you do have worn rings, you will need it to breath better after an overhaul.

Hope it helps! :thumbsup:

Here is an example of the old school breather tube that was used ...

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Also inside the valve covers you need deflector shields. You can not simplely drill a hole in the valve cover, install a rubber grommet and press in a breather. Oil flung off the rockers come out that breather. You NEED a small deflector shield bolted to the inside of the cover to prevent oil from flinging out the breather.
as requested, here's pictures of engine...oil fills up the fins directly below the breather caps and drips (or pours) onto the headers.

left side:

right side:
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How old/many miles is/on this engine? I have the exact same covers on my SB coupe. I run a PCV valve and have done fine. If your engine is getting tired, you will get oil because of the lack of breathing. Go through the normal diagnostics (plugs, compression, pressure tests, etc) to make sure it is not your engine causing it. Those breathers are not enough crankcase breathing capacity anyway in my humble opinion.

Good luck!:thumbsup:
I have the same valve covers on my 6,000 mile SBC. The driver side breather is in the rear hole, with a vac line from the charcoal cannister to a tee on the breather cap to the PCV fitting of my OEM Q-jet. The passenger side breather is in the front hole, with a 90*, 7/16" fitting open to atmosphere because I never ran it to the air cleaner. They're both metal mesh filled, with no baffles or deflectors in the covers on either. I get misting from both, enough to be a bummer, but not enough to make it as high on my fix-it list as other things. (My covers are wrinkle blacked, with only fin tops polished, so it doesn't show much!)

For whatever that's worth.


Edit, 9 hours later, sheesh! I had passenger and driver side mixed up. Fixed now. Sorry.
I have the same problem, but I DO have a PCV hose running to the carb, so that may not be your problem. I also have a deflector underneath the breather, so its not just being flung of the rocker arms.
Do you have the little plates under the breather. Pull one and check. You should not see the valve train. You should just see metal, nothing else
Thanks everyone. Just to answer some of the questions...
-Engine supposedly has 78000 miles on it. I just bought the 'vette in April.
-There is no baffle plate on the underside of the valve covers. When the oil breather cap is removed from the grommet you can see right to the valve train.
-Plugs didn't look fouled or anything.
-Unfortunately I don't have the tools for compression checking, etc., so I guess I will just breakdown and run it back over to my 'vette guy'. I'm sure he will have a happy face :D when he sees me, since I've already had a few thousand $$ of work done just to get it running.
I just wanted to make sure there wasn't a 'fast and easy' fix that I had overlooked.
Thanks again everybody, this is a great forum!!
There are a couple of things you can do for a breather. Personally, I am not a big fan of PCV. Why suck oily fumes/vapors back into your engine? An easy fix is to make your own crankcase breather tube. Just take out one of the breather caps and get a metal tube that's in the shape of a J that fits the grommet hole. Presto!!!! Instant breather tube. Even better are the setups sold by Jegs, Summit, and others. Not sure of their name, but they plumb a pipe from the valve cover to the header pipe. This actually SUCKS those nasty vapors right out of the engine and burns them off inside the header. And no, no smoke appears at the tailpipe. With 78,000 miles you probably do have blowby. The ultimate fix is an engine rebuild. But you do have to route some kind of tube out of the valve covers to get rid of the oily stuff on the breathers.
BTW...the engine is nothing more than a big air pump. So vapors puffing out of the breather holes is actually normal. The breather tubes used on old cars were more that adequate to get the job done. The tree huggers just didn't like the pollution it caused :D

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I believe the correct term is "road draft tube."
I believe the correct term is "road draft tube."'re wrong. Here's proof positive from a source that is NEVER incorrect:'re wrong. Here's proof positive from a source that is NEVER incorrect:
I must be getting old. if the world uses E-Bay as a reference source.:rolling:
It just might be a fast and easy fix, go get you a breather cap with a vacume line hook up on it. A 2 foot piece of rubber tube. Now find a place on the intake to hook up a vacume line ( look under the rear of the carb!) Hook all that up, just might cure your problem. If it dont i just about guarentee it will slow it down. I have no baffles under my breather caps i get about 2 drops of oil every 3000 miles!

What you think your "vette guy's" face will look like when you tell him you fixed your car for less than 10 bucks? Spend your money on parts to fix up your new corvette not on labor that"s what were here for!
I must be getting old. if the world uses E-Bay as a reference source.:rolling:
If I had a kid I would home school him by making him read E-Bay 8 hours a day. I learnt myslef all mi gud English from E-bay. :laughing:

OK guys last night I took a look at the Holley carb and found a vacuum intake at the front and one on the passenger side of the carb above the idle mixture screw. The side intake (identified as the 'Timed Spark Vacuum Source' in the manual) had a plug over it and the vacuum hose from the distributor spark advance is connected to the vacuum intake on the front, NOT on the side intake.
I then moved the distributor hose to the side intake which is 'supposed' to be the distributor vacuum intake on the carb. I capped the front intake and then engine would not idle, it kept dying. As soon as I put the distributor hose back to the front intake, the engine idled smoothly as before.
So do I run the PCV hose from the valve cover to the front intake using a 'tee' connection sharing with the distributor vacuum? Can the distributor vacuum and the PCV share the same intake? Or should the distributor vacuum in fact be moved and connected to the distributor intake and then the idle speed and mixture adjusted?
I can take some pictures later tonight if that would help.
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I just skimed over the posts so forgive me if I repeat what someone has already stated.
I do think we need the PCV system for our cars. If it is correctly installed it will help prevent pressure in the crank case. Its also been proven that a vacuum in the crank case will aide in making HP and help greatly with oil leaks. I do not run vents in the valve covers, that way there is a vacuum in the crankcase and I am burning a minumum amount of crankcase vapors by not drawing in air form the vent. The tube from the vavle cover to the top side of the carb is for high speed runs were there is little to no vacuum to evacuate the crankcase in the intake. It is the nature of oil to vaporise at high temps so there will be vapors in the crankcase most of the time, this is were the oil mist is coming from. Even with a functiong PCV system there will be vapors if you do not run the vent above the carb (to the filter housing). There should be a large vacuum port on the front lower part of the carb base. The PCV system is plumbed so that the vapors will mix with the A/F mixture and help diolute the crankcase vapors (plumbed to the primary side of the carb).

Neal :thumbsup:
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Those are 1/8 inch vacume lines your talking about, you need a 3/8 vacume source like the one that goes to your power brakes. Should be one way up under your carb on the base plate. also should find a source on the intake manafold either one of those that arent being used would be fine. I"ll try and enclose a picture to help you out.

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