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Old 09-16-2020, 02:14 PM   #1
Magoo2
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Default PCV valve good idea?

I have a 31 pickup. Stock engine. Rear main seal leaking. It was suggested I install a PCV valve to help with the oil leak. Any drawbacks to installing a PCV valve?
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Old 09-16-2020, 02:43 PM   #2
Russ/40
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Default Re: PCV valve good idea?

That's an interesting question. I'm curious also. Awaiting some answers on this, although I doubt much rear main leaks are from crankcase pressure.....but maybe??
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Old 09-16-2020, 02:49 PM   #3
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Default Re: PCV valve good idea?

Maybe too much open area around the rear main to be effective.
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Old 09-16-2020, 02:56 PM   #4
1930-Pickup
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Default Re: PCV valve good idea?

A PCV valve for a rear main leak sounds like a band-aid.
I would recommend fixing the problem, rather than band-aiding it.
Besides, do you really want to build up junk on the valves faster?

By the way, you don't say how fast your leak rate is.
A slow drip from a main seal on pre-war cars is very common, even when the cars were new.
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Old 09-16-2020, 03:01 PM   #5
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Default Re: PCV valve good idea?

I have been driving Model A’s for 60 years.

I have never had one that did not mark it’s spot.

Rebuilt engine. Still left a couple of drops under the rear main each time I would park it.

Down about 1/2 quart at 500 miles, and I fill crank case to full mark on dip stick.

Enjoy.
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Old 09-16-2020, 04:07 PM   #6
Jack Shaft
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Default Re: PCV valve good idea?

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your engine does not have a traditional rear main seal.the oil flows through the bearing,into a slinger/trough,then drains back through a tube into the pan..generally,a leaking rear main means the rear main bearing clearance is too large,allowing more oil than the tube can flow back into the pan and it runs out the back.

Crankcase pressure can also force out the rear main bearing causing the condition as well.cleaning the oil fill cap mesh and making sure the cap isn't rammed down on the fill tube blocking the breather is a good check my engine has a second draft tube from the upper rear of the valve cover over the rear bearing feed...does it work? perhaps..
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Old 09-16-2020, 04:19 PM   #7
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Default Re: PCV valve good idea?

Additionally, Model As do not handle vacuum leaks very well and a PCV system is a conrolled vacuum leak designed into the system. Suspect idle quality would be impacted to some extent. Not sure where the best place would be to plumb in the valve or the fresh air inlet.
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Old 09-16-2020, 04:22 PM   #8
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Default Re: PCV valve good idea?

Newer cars use them . If you use a PCV valve you will end up burning crankcase fumes in the cylinders . I feel that the crankcase fumes will weaken the charge in the combustion chambers . I want every advantage that I can get with the model A . I once considered it but decided against the use of a PCV valve in my model A . Burning crankcase fumes can leave oily deposits on the spark plugs and weaken the spark .
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Old 09-16-2020, 04:23 PM   #9
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Default Re: PCV valve good idea?

Quote:
Originally Posted by WHN View Post
I have been driving Model Aís for 60 years.

I have never had one that did not mark itís spot.

Rebuilt engine. Still left a couple of drops under the rear main each time I would park it.

Down about 1/2 quart at 500 miles, and I fill crank case to full mark on dip stick.

Enjoy.
Great logic and good common sense. Totally agree with you.
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Old 09-17-2020, 08:04 AM   #10
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Default Re: PCV valve good idea?

Quote:
Originally Posted by WHN View Post
I have been driving Model Aís for 60 years.

I have never had one that did not mark itís spot.

Rebuilt engine. Still left a couple of drops under the rear main each time I would park it.

Down about 1/2 quart at 500 miles, and I fill crank case to full mark on dip stick.

Enjoy.
I rebuilt my engine a month or so ago and was very disappointed with the oil leaks. However, perhaps 1200 miles later, the oil leaks have subsided to about half of what they were aft the rebuild. I just changed oil yesterday (500 miles) and it was down to half way between add and full. Initially it was twice that amount.
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Old 09-17-2020, 08:33 AM   #11
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Default Re: PCV valve good idea?

The crankcase in the model a engine is very well vented with the large oil fill/crankcase vent tube. It should not be building up much back pressure in the crankcase that could cause a rear main leak- unless you might have put something like a mesh scouring pad in the oil fill cap to stop oil vapor blow-by from coating your engine compartment. But if you have excessive blow-by, then you likely have a piston ring problem. Like 1930-Pickup said, band-aid.
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Old 09-17-2020, 08:40 AM   #12
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Default Re: PCV valve good idea?

The use of modern sealants during engine assembly greatly reduces the incident of oil leaks.
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Old 09-17-2020, 08:48 AM   #13
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Default Re: PCV valve good idea?

Quote:
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The crankcase in the model a engine is very well vented with the large oil fill/crankcase vent tube. It should not be building up much back pressure in the crankcase that could cause a rear main leak- unless you might have put something like a mesh scouring pad in the oil fill cap to stop oil vapor blow-by from coating your engine compartment. But if you have excessive blow-by, then you likely have a piston ring problem. Like 1930-Pickup said, band-aid.

Ford used a larger diameter oil fill tube on the model B engines to reduce crankcase pressure .
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Old 09-17-2020, 10:32 AM   #14
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Default Re: PCV valve good idea?

I modified a reproduction Air Maze by putting a copper fitting into its throat. I also took an accessory draft tube that was on the breather and replaced its metal flex line with a rubber hose. The hose was connected to the Air Maze. No PCV was used. This setup did reduce leakage from the rear main, and it ran really well. I am demanding of my engines to produce power, and I did not notice any degradation in power. The engine had Motorcraft TT10 Spark Plugs and they ran clean.
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Old 09-17-2020, 11:07 AM   #15
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Default Re: PCV valve good idea?

A lot of replies, not many to the question you asked. My answer ... no disadvantage. I do similar to what Bob does, also with good results. A vacuum in the crankcase can also increase horsepower, that's why racers use a pump to draw a vacuum, dyno proven. Any effect of the fumes diminishing power is negligible.
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Old 09-17-2020, 11:19 AM   #16
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Default Re: PCV valve good idea?

Vendors back in the '70's were selling a hose to run from the intake vac.. fitting to the side of a valve cover bolt . That supposedly discharged oil into the intake manifold ! I can't find the info. but someone fitted a pipe that ran from the intake manifold fitting around the block to the oil filler cap with a PCV valve mounted on top of it.
The vendors now sell an enlarged oil fill pipe similar to the '32 B engine pipe which breathes better.
Chrysler flat six engines have a separate breather pipe in addition to the oil fill pipe, recognizing the need for better pos crankcase venting.

Last edited by duke36; 09-17-2020 at 11:24 AM.
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Old 09-17-2020, 12:02 PM   #17
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Default Re: PCV valve good idea?

I only put in 4 quarts when changing oil. About 1/2 quart stays in the engine between the valve galley and the dipper tray. That gives me 4 1/2 quarts in the engine. This lowers the oil level in the pan. Be sure to check the oil level on the dipstick after the oil change so you will know where the new fill level is. The 5 quart fill puts the oil level, especially when going up a hill, at the rear main causing oil to exit and drain through the bell housing. Give this method a try. It could be your answer.

Other issues could be your mains are worn and set too loose; or, you could have a cracked babbitt on the rear main; or, your rings are worn leaking compression into the pan causing pressure forcing oil out from the rear main.

Best of luck!
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