Vortec 350 PCV positive crankcase ventilation question.

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

Supercharged111

Truly Awesome
Joined
Aug 20, 2015
Messages
12,612
Reaction score
15,235
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.

Oh, derr. I'll have to reread the problem again. It just seems to me that there's a better way to crack this nut. Crankcase vapors need to be purged and this won't allow it. You can't be the only one who's had this issue.
 

El Tigre

I'm Awesome
Joined
Sep 20, 2014
Messages
304
Reaction score
281
Location
Dayton,Ohio
Consider installing a LS style "constant orifice" PCV as I installed on my '96 Impala w/LT1.... Fluctuating idle went away as did all PCV variables since it has zero moving parts.
 

Supercharged111

Truly Awesome
Joined
Aug 20, 2015
Messages
12,612
Reaction score
15,235
Consider installing a LS style "constant orifice" PCV as I installed on my '96 Impala w/LT1.... Fluctuating idle went away as did all PCV variables since it has zero moving parts.

Go on. I reverted to an L31 truck valve cover on my driver side LT1 to aid in crankcase ventilation. That's what got me the 20whp which dictated that I go down from a 38mm to a 35mm restrictor.
 

1989GMCSIERRA

I'm Awesome
Joined
May 2, 2014
Messages
547
Reaction score
599
Has any one closed off the PCV tube on the passenger side valve cover (while leaving the driver's side one as is)?
I have good reason to do this and just want to know of any possible problems that may ensue.

I would love to know the reason for this.
 

Pinger

I'm Awesome
Joined
Mar 10, 2020
Messages
3,013
Reaction score
5,935
Location
Scotland.
Really,sure doesn't affect mine that way,or I'd do something else.

On mine it does. The mixture is naturally rich at idle which pushes the actuator to a closed position and on throttle opening the actuator has to move very quickly to compensate for the then lean mixture. The delay in it getting the info from the O2 sensor can create a momentary lean mixture. Think of it like a carb with an accelerator pump working in reverse - pulling fuel out when you want it going in. That's what's currently happening.


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.

Worth considering but ideally I'm aiming for zero LPG in the crankcase.
LPG, although in vapour form is still a hydrocarbon fuel. I'd rather not have it and my oil in close proximity - though that is secondary to the explosion risk.

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.

Plastic is better - it behaved like a fuse. If it hadn't blown, who knows what would have. I had a few weeks while the repair was being done not knowing if the rear seal had popped or the oil control rings were damaged. Thankfully neither occurred.
 

Pinger

I'm Awesome
Joined
Mar 10, 2020
Messages
3,013
Reaction score
5,935
Location
Scotland.
Oh, derr. I'll have to reread the problem again. It just seems to me that there's a better way to crack this nut. Crankcase vapors need to be purged and this won't allow it. You can't be the only one who's had this issue.

I think it will.
Try this. Remove your PCV valve and think if it as if mounted on the passenger side valve cover, fat end in the grommet, thin end pointing away from it.
Blow through the thin end and it flows in a manner that will purge air. Now blow quickly which invokes its backfire mode (as GM intended) which is what a snap throttle opening will induce and it closes.

If that prevails in service then I have both purging at constant throttle and momentary closing on snap throttle (from idle when the driver side valve cover PCV opens) which will cure my throttle transition issue. Thereafter, any blow-by will balance that pressure and permit to to open again. If there's no blow-by then there isn't an excess of vapour requiring purging.
It can also breath outwards (as it does when its on the driver side valve cover) to prevent over-pressurisation though but what I've seen so far the driver side valve cover PCV copes with this adequately anyway.

My fuelling issue is confined solely to the transition between closed and open throttle when the driver side PCV valve transitions from very nearly closed to open - and deprives the mixer of signal which the O2 driven actuator cannot respond quickly enough to.


Out of interest - when you gained 20-30hp, was that by reducing crankcase pressure?

edit PS.
I agree that a simple one way check valve (as used on turbocharged engine) would prevent purging - it is the ability of the GM PCV valve to be open or closed depending on 'suddenness' of the pressure differential being applied that makes both momentary closing and purging possible.

Appreciate the points made and the debate - you guys are keeping me honest here!
 
Last edited:

El Tigre

I'm Awesome
Joined
Sep 20, 2014
Messages
304
Reaction score
281
Location
Dayton,Ohio
Your "theories" on how the PCV system operates are merely opinions without any sources that support them. You've received many suggestions,yet cling to the ridiculous idea of closing of one end of the PCV system rendering the entire system totally useless.
 

Pinger

I'm Awesome
Joined
Mar 10, 2020
Messages
3,013
Reaction score
5,935
Location
Scotland.
Your "theories" on how the PCV system operates are merely opinions without any sources that support them. You've received many suggestions,yet cling to the ridiculous idea of closing of one end of the PCV system rendering the entire system totally useless.

I have the description in the Haynes manual that describes the three modes of operation of the PCV valve.
I have the PCV valve removed from the truck and have physically confirmed that the information in the Haynes manual is correct.
I do not intend 'closing of one end of the PCV system' completely - only momentarily during the condition that is at the root of my fuelling issue - as discussed and agreed with the company who supplied me with the new reducer/vaporiser and have been giving me advice from their extensive experience with LPG systems.
 

El Tigre

I'm Awesome
Joined
Sep 20, 2014
Messages
304
Reaction score
281
Location
Dayton,Ohio
Haynes? You need GM service manuals if you intend to work on this platform yourself. Haynes/Chilton's are better than nothing,but not by much...
 
Top