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General Tech Question re PCV Valve

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KVH

KVH

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What happens to a Triumph engine with no draft pipe, but only a PCV valve, when the diaphragm of the PCV valve is torn open? For example, does it just run rough? Drink oil? Miss and skip? What should one expect? Thanks all—
 
Triumph Valves have a diaphragm? All the ones I have seen are just a plunger with a spring. When the intake has a lot of vacuum ( at idle or overun), then the vacuum overcomes the spring and closes the valve. There is a small built in leak when the valve is closed. When the engine has low vacuum under acceleration, the spring opens the valve wide open so all blowby goes to the intake without restriction. A valve like this would stick, either open...which would make a fast idle due to the vacuum leak, or stick closed and the engine would pressurize and blow oil.

If there is a diaphragm, then the failure would have to be wide open...resulting in a vacuum leak and fast idle. If you tried to reset the idle, then the mixture would not be right, no matter how hard you tried. Then you would have a rough, rolling idle.
 
Yes, definitely a rubber or neoprene diaphragm in there. That's what creates the seal to allow "something" to happen. I just don't know what. I guess there's a connection between the diaphragm and that little hole on top--whatever. My mechanic says the amount of oil in the PCV tells him I'm getting too much blowby, and something's wrong. He suggests removing the PCV and adding a draft tube--or venting the valve cover with a tube curved from the valve cover down under the engine as others have suggested. To add a standard TR4 type draft tube I'd need to raise the engine to pop out that freeze plug. Not enough room to work down there, as there is only 4.5 inches to the side firewall.
 
CJD said:
Triumph Valves have a diaphragm? All the ones I have seen are just a plunger with a spring. When the intake has a lot of vacuum ( at idle or overun), then the vacuum overcomes the spring and closes the valve. There is a small built in leak when the valve is closed. When the engine has low vacuum under acceleration, the spring opens the valve wide open so all blowby goes to the intake without restriction. A valve like this would stick, either open...which would make a fast idle due to the vacuum leak, or stick closed and the engine would pressurize and blow oil.

If there is a diaphragm, then the failure would have to be wide open...resulting in a vacuum leak and fast idle. If you tried to reset the idle, then the mixture would not be right, no matter how hard you tried. Then you would have a rough, rolling idle.
Click to expand...
This is the type of PCV valve that was on the TR250 and the 69 TR6 engines
The internal diaphragm is Moss part # 360-635
TR250 69TR6 parts 001.JPG
 
Then, as I said, a bad diaphragm will act as a vacuum leak, raising the idle speed. You will not be able to slow the idle, or correct the idle mixture. The purpose of the diaphragm is to limit the blowby to the intake at idle and low speeds. Without the diaphragm integrity, the valve will fully open, venting too much blowby into the intake and acting as a vacuum leak.
 
What would happen if one were to remove the PCV, plug the intake port, and vent the valve cover with an aluminum pipe to the ground, somewhat like the original TR4 draft tube (except all of the venting would be from the top of the engine/valve cover)?
 
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It would work. However your engine will accumulate bad blow by waste much faster, You should try to get airflow into and through the crankcase to purge blow by. The original road draft allowed air to enter the valve cover under fan pressure, flow through the crankcase, and then get sucked out the draft tube.
 
Just to close this out, after much research and analysis, it appears my oil loss, which was substantial over time, was caused by my faulty PCV valve. Surprising to me, but not to others apparently. Small easily overlooked item, but a big potential problem. I'd prefer the old draft tube, or just venting the valve cover outlet to the ground and plugging the intake manifold draw, but one way or another a defective PCV is nothing to ignore.
 
At idle the engine has a high vacuum, so a bad PCV valve will allow it to suck too much blowby (and oil with it) into the intake. The PCV system is much better than the road draft tube...I'd keep it!
 
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