crankcase ventilation question, 71 GMC 350 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

rickf · 03-02-2007, 10:48 PM

I have an engine from a 1971 GMC C20 Camper Special, it happens to be in a 64 Chevy C10 now. As far as I know, the engine is stock, except for 600CFM Holley on factory 4-barrel manifold, tube headers, and a Pertronix ignition.

My question has to do with proper crankcase ventilation plumbing.

The right side valve cover has a PCV and hose going to carb base (I assume PCV, maybe just a flame arrestor).

The left valve cover has two holes, one has oil fill cap, not vented.
The other has a mushroom style breather.

This strikes me as wrong. Having the vent on the left side effectively disables any kind of crankcase vacuum that is supposed to be provided by the stuff on the right side. And worse, it actually makes a flow-thru airway that is going to cause lean running, especially at low throttle setting.

What is your wisdom on all this?

**WorkinLonghorn** · 03-02-2007, 11:22 PM

It's supposed to have a PCV valve installed in a gromet on the drivers side.This valve nearly closes all the way during high vacuum such as idle,and low throttle.When the throttle is open and vacuum low,the valve opens. On the other valve cover there should be a hose going to either the spacer under the filter or into the filter housing.

Longhorn Man · 03-03-2007, 12:38 AM

they flow through is EXACTLY what you want. It does nothing for the fuel mixture, but it sucks all the acidic gasses and contaminants that are a normal byproduct of combustion, and feeds it to the intake charge through the carb. This is one emissions control product, that not only cost you zero horses, but cleans your engine, helps with the gasket sealing and helps your engine last long.

rickf · 03-03-2007, 01:09 AM

It has a non-original air filter, about a 12" dia chrome thing. It has no place to put a hose that would come from anywhere. So, from what you have just told me, it looks like the prev owner put on that filter and had no place for the hose, so just stuck a mushroom vent in the valve cover hole. Sounds plausible.

So, does that mean it is okay, performance wise?
(That is, disregarding the pollution issue, at least the engine will run right?)

Or should it still not be allowed plain atmospheric pressure, and ought to have a hose to the intake?

Thanks for sharing your knowledge.

**WorkinLonghorn** · 03-03-2007, 02:41 AM

If I understand correctly,your set-up is OK. The idea is to have filtered air going in one side and sucked out ( along with all the contaminants) through the PCV valve on the other side and into the intake stream.Just make sure the PCV valve hoses do not leak and that the valve itself is operational.Usually you can shake one and if you hear it rattle it's good.If in doubt,they are a few bucks at any auto parts place.

rickf · 03-03-2007, 04:03 AM

Cool. The grommets are hard, and the PCV fits loosely, clearly not sealed by evidence of oil drool on the valve cover. Don't know of the PCV condition. Since it sounds like the system is is fundementally appropriate, I'll replace the questionable components and move on to the next problem. Thanks for helping me decide what to do with this.

03-02-2007, 10:48 PM	#1
rickf Registered User Join Date: Dec 2005 Location: Lake Oswego, OR Posts: 318	crankcase ventilation question, 71 GMC 350 I have an engine from a 1971 GMC C20 Camper Special, it happens to be in a 64 Chevy C10 now. As far as I know, the engine is stock, except for 600CFM Holley on factory 4-barrel manifold, tube headers, and a Pertronix ignition. My question has to do with proper crankcase ventilation plumbing. The right side valve cover has a PCV and hose going to carb base (I assume PCV, maybe just a flame arrestor). The left valve cover has two holes, one has oil fill cap, not vented. The other has a mushroom style breather. This strikes me as wrong. Having the vent on the left side effectively disables any kind of crankcase vacuum that is supposed to be provided by the stuff on the right side. And worse, it actually makes a flow-thru airway that is going to cause lean running, especially at low throttle setting. What is your wisdom on all this?

03-02-2007, 11:22 PM	#2
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,861	Re: crankcase ventilation question, 71 GMC 350 It's supposed to have a PCV valve installed in a gromet on the drivers side.This valve nearly closes all the way during high vacuum such as idle,and low throttle.When the throttle is open and vacuum low,the valve opens. On the other valve cover there should be a hose going to either the spacer under the filter or into the filter housing. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, 2nd owner, Work-Truck supreme. Best $300 I ever spent.

03-03-2007, 12:38 AM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: crankcase ventilation question, 71 GMC 350 they flow through is EXACTLY what you want. It does nothing for the fuel mixture, but it sucks all the acidic gasses and contaminants that are a normal byproduct of combustion, and feeds it to the intake charge through the carb. This is one emissions control product, that not only cost you zero horses, but cleans your engine, helps with the gasket sealing and helps your engine last long.

03-03-2007, 01:09 AM	#4
rickf Registered User Join Date: Dec 2005 Location: Lake Oswego, OR Posts: 318	Re: crankcase ventilation question, 71 GMC 350 It has a non-original air filter, about a 12" dia chrome thing. It has no place to put a hose that would come from anywhere. So, from what you have just told me, it looks like the prev owner put on that filter and had no place for the hose, so just stuck a mushroom vent in the valve cover hole. Sounds plausible. So, does that mean it is okay, performance wise? (That is, disregarding the pollution issue, at least the engine will run right?) Or should it still not be allowed plain atmospheric pressure, and ought to have a hose to the intake? Thanks for sharing your knowledge.

03-03-2007, 02:41 AM	#5
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,861	Re: crankcase ventilation question, 71 GMC 350 If I understand correctly,your set-up is OK. The idea is to have filtered air going in one side and sucked out ( along with all the contaminants) through the PCV valve on the other side and into the intake stream.Just make sure the PCV valve hoses do not leak and that the valve itself is operational.Usually you can shake one and if you hear it rattle it's good.If in doubt,they are a few bucks at any auto parts place. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, 2nd owner, Work-Truck supreme. Best $300 I ever spent.

03-03-2007, 04:03 AM	#6
rickf Registered User Join Date: Dec 2005 Location: Lake Oswego, OR Posts: 318	Re: crankcase ventilation question, 71 GMC 350 Cool. The grommets are hard, and the PCV fits loosely, clearly not sealed by evidence of oil drool on the valve cover. Don't know of the PCV condition. Since it sounds like the system is is fundementally appropriate, I'll replace the questionable components and move on to the next problem. Thanks for helping me decide what to do with this.



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