Vortec 350 PCV positive crankcase ventilation question.

Schurkey said:

Yes. The PCV is vacuum-operated where vacuum is opposed by spring tension. High vacuum pulls the metering valve into position that restricts--but does not eliminate--air flow.

Under positive pressure (backfire) the metering valve inside the PCV should block flow--no flames in the crankcase.

Sounds like you had a backfire AND a failed PCV valve, which ignited the fumes in the crankcase.

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It backfired through the passenger side vent - which is why I isolated it from where is could draw LPG and yes, with a filter.

Schurkey said:

Yes. You're blocking the vent that allows fresh air into the crankcase. The fresh air replaces the crankcase fumes drawn out by the PCV. Thus there is continual air FLOW through the crankcase. With no fresh air inlet, you'll develop a vacuum in the crankcase which will be less effective in removing moisture and fumes.

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True - but not to a degree that is problematic as far as I can see.
It can never drop below manifold pressure and that would be lowest at at idle As soon as the throttle is opened and the PCV opens with it, the crankcase pressure will equalise to the now higher manifold pressure. If that occurs, it will be beneficial to my aim as it will increase the signal to the mixer and richen the mixture quicker than the actuator can open to make the compensation - which is what this is all about. EGR is disconnected for the same reason.

Schurkey said:

And during times of high blowby-low vacuum, you'll pressurize the crankcase with blowby leading to oil leakage from oil-flooded seals, pushed-out valve cover gaskets, and similar undesired venting.

You MUST retain the PCV suction side, AND the PCV venting side of the system.

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I will be (retaining the suction side - with the original PCV valve in place on the driver side valve cover - and any excess pressure can vent back through the passenger side as per GM's arrangement - as I propse to fit an identical PCV valve there which will flow in the outward but not inward direction (under snap throttle openings as it will be pulled into its backfire prevention closed position). Thereafter, slight inward flow can be compensated for by the actuator as it will not have to react so quickly.
Outward flow will be unimpeded but I'm not expecting much as the suction side seems capable of handling all blow-by - judging by the lack of any signs of flow back through the vent as it currently is, open to atmo.

Schurkey said:

What you would not have to do, is connect the PCV vent to the inlet of the air-intake system for the throttle body/fuel mixer. You'll have to tune the fuel mixer a little richer to compensate.

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That's my problem - I can't. Because when I do it it also richens it at idle and the disparity of actuator settings screws up my transition between idle and on throttle.

Schurkey said:

Mind you, not having the vent side connected to the air stream for the throttle body/fuel mixer will (illegally) increase the hydrocarbon emissions of the engine via the fumes escaping from the PCV vent into the atmosphere under high-blowby/low vacuum conditions.

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Initially I'll leave it vented to engine bay (as it currently is) and if there's excess emissions (none noticeable so far) I'll route it into the filter box. As it's end is as it is on the drivers side, it will be immune to backfire.
Theoretically I could route it downstream from the mixer but the valve only completely closes when subjected to a rapid variation in pressure. Otherwise it could be open to the same ingestion of LPG into the crankcase as before.

Schurkey said:

Be sure to put a filter of some sort on the vent side of the PCV system, when you disconnect it from the air intake side of the throttle body/fuel mixer tubing.

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Either just downstream from the filter (pre MAF) or in the filter box pre-filter but with its own filter - though the air drawn though it will be minimal - but zero on snap throttle opening - the desired aim.

Schurkey said:

As said, if you can connect the PCV vent upstream of the fuel mixer, but downstream of the MAF, you keep all the benefits with none of the drawbacks.

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See above.

Schurkey said:

PCV marketing stinks. There's no chart or guide to flow rates, spring tensions, vacuum needed to pull the metering valve, etc.

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Rule of thumb! I've a fair idea what's going on just by placing my thumb over its end when disconnected from the valve cover. The variations (0n/off throttle) are in the direction I need and could be seen on the lap top readings of actuator response.

Schurkey said:

But PCV flow rates are in general very low. I'm surprised that you're having problems with it--PCV is only about 5 or 7 cfm.

Try a different part number PCV valve. Perhaps one intended for a four- or six-cylinder?

And as I said earlier, I think the PCV you have is defective, there shouldn't be any flames in the crankcase to begin with. Which is why the manufacturer of the fuel mixer allows a combustible mixture in there, when liability lawyers would eat them up in court for damages if it was actually dangerous.

"I" would be studying OEM propane-powered engines--forklifts, for example--to see what solutions they've come up with.

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As earlier, the PCV didn't permit the backfire into the crankcase - the unchecked vent on the passenger side did.

I've ordered two new PCV valves so that's what I'll be installing. I'm confident that this will work and rectify my fuelling transition problem and without the crankcase explosion risk that frankly, I am astounded people are prepared to accept. Having had it happen, I know the risk is more than theoretical.

Also, it should be compatible with running on gasoline (as back-up) but if not it's a simple case of swapping the newly installed PCV valve on the passenger side for the original tube and reconnecting it to the currently blocked connection (elbow) on the inlet tract where it normally connects.

PCV valve systems were the first emission control device. They are nice to have but you can eliminate it by removing vent tube and PCV valve and replacing both with push in aftermarket breathers. Plug holes on intake and air duct. Of course this is not legal and will increase hydrocarbon emissions. I would try another solution.

I would NEVER do what you're proposing. I blew my rear main seal out of my dually flat against the flex plate because the factory breather hose was woefully inadequate. It had been pissing out of it for years and created a gunky mess. You do that and I guarantee you'll start smelling oil as it gets pushed from every seal onto hot parts that burn it off and make it smell like a POS. I would find another way to fix your problem, the crankcase NEEDS to breathe. My Camaro picked up around 20whp by beefing up the crank ventilation. It's kind of a big deal.

Supercharged111 said:

I would NEVER do what you're proposing. I blew my rear main seal out of my dually flat against the flex plate because the factory breather hose was woefully inadequate. It had been pissing out of it for years and created a gunky mess. You do that and I guarantee you'll start smelling oil as it gets pushed from every seal onto hot parts that burn it off and make it smell like a POS. I would find another way to fix your problem, the crankcase NEEDS to breathe. My Camaro picked up around 20whp by beefing up the crank ventilation. It's kind of a big deal.

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That's where I am already though - with the passenger side tube open to atmo (with a filter on it) and no smell. All I'm proposing that is different to that is a one way check valve to inhibit air entering the crankcase via that tube on snap throttle opening. It will still be able to breathe outwards. And inwards under more moderate vacuum with open throttle.

It's a GM stock PCV valve I intend using and it flows outwards under moderate pressure and only snaps shut under a sudden pressure rise - the equivalent of the sudden vacuum it sees on throttle closing when installed on the driver side. I can't see how the crankcase pressure could rise so quickly not least as under that situation (high blow-by) the driver side PCV valve is open to the inlet manifold. There's been no sign of that so far with the pipe open. The filter on it is bone dry, no sign of oil vapour or anything untoward at all.

Nothing pressurises a crankcase like an explosion in there does. The further I am from that risk, the better.

Try a push in breather on passenger side.
Leaving PCV intact...Many go this route including myself.

El Tigre said:

Try a push in breather on passenger side.
Leaving PCV intact...Many go this route including myself.

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I have. It's that that is screwing up my fuelling.

Did someone mention over-pressuring a crankcase?.....



Miraculously, it didn't blow any seals. It did have a stab at firing the dipstick to the moon though.

Pinger said:

I have. It's that that is screwing up my fuelling.

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Really,sure doesn't affect mine that way,or I'd do something else.

https://www.flammergmbh.com/englisch/products/deflagrations_flame_arresters.htm

Something like this installed on the fresh air draw tube should prevent a flashback into the crankcase. Similar devices are installed on the inlet ports of industrial natural gas engines to prevent flashbacks into the inlet manifold.

I would use the aluminum Holley front cover. Had no luck with aftermarket plastic ones sealing because of flex distorting front seal after bolting on oil pan. Twice the price but works great.