Vortec 350 PCV positive crankcase ventilation question.

Discussion in 'Engine Performance + Maintenance' started by Pinger, Jul 31, 2020 at 6:12 AM.

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  1. Pinger

    Pinger I'm Awesome

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    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.
     
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  2. b454rat

    b454rat I'm Awesome

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    Usually how they are, filtered air intake on one side, PVC on the other.
     
  3. 454cid

    454cid Sooper Pooper

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    What is your good reason? Closing off one side is going to function about as well as removing the ground on an electrical circuit.
     
  4. Pinger

    Pinger I'm Awesome

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    As set up, the air has passed through the MAF so has been measured for how much gasoline is required to burn it once it gets to the cylinders.

    For my LPG system I have had to disconnect the passengers side tube (and vent it to atmo) as when in its original position it is downstream from the LPG mixer and thus the crankcase becomes primed with an explosive mixture which, in the event of an inlet backfire will explode. This has happened (and blew the timing cover apart).

    However, as it is, free to air, air is entering the engine with out having passed through the mixer so on opening the throttle the mixture (when the PCV valve opens) is momentarily weak until compensated for with the actuator but richer when idling as the PCV is closed.

    Or should be closed at idle.......
    Mine isn't entirely closed as there is a slight vacuum at the passenger side vent tube even when idling. So, closing it off isn't making the big difference expected. But, if my PCV is leaking at idle when it shouldn't be (does anyone know if they are supposed to close completely?) and I replace it the disparate signals to the mixer at idle and throttle on will inevitably mean I have to blank it off as it will magnify the mixture response problem.

    So, the two questions are - does the PCV allow a certain amount past at idle?

    And, are there any negative consequences to blanking off the passenger's side one if I have to?

    So far - with my thumb over the end of the tube there's nothing but vacuum. Driving with some blow-by past the rings should theoretically pressurise the crankcase so no difference there. Hard to see anything really going wrong (as on over-run the PCV valve should close) but there's always that nagging doubt when deviating from what GM intended.
    Add to that, the slight oil film around my filler cap tube must surely suggest there is pressurisation more than the PCV valve is passing.

    Thoughts???
     
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  5. Pinger

    Pinger I'm Awesome

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    It does.



    Or rather than blank it, insert a check valve (similar to the PCV valve) that can prevent air entering the crankcase but breathe outwards to prevent over-pressurisation from blow-by that the PCV valve may not be capable of accommodating (as standard, over-pressurisation can (if necessary) vent back through the passenger side hose into the inlet tract between MAF sensor and throttle plate).
     
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  6. 454cid

    454cid Sooper Pooper

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    I have no idea how to answer the answer the question, but rather than disconnect a basic emmisions system, and possibly create an over pressure situation in the crank case and blow out seals, I'd suggest finding out how this is normally handled on conversions. I know nothing about propane systems, but this can't be a unique situation. Maybe join a propane conversion group?
     
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  7. arrg

    arrg I'm Awesome

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    Connect the tube to a portion of your inlet duct that is downstream of the MAF and upstream of where the LPG goes in? Does it even use the MAF when running LPG? If the LPG mixer is the only source of air metering on the system, I don't think you can have a functioning PCV system without circulating an explosive mixture through your crankcase or having the problems you're currently having. And just for clarification, are you right hand drive on that thing?
     
    Last edited: Aug 1, 2020 at 12:44 PM
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  8. Pinger

    Pinger I'm Awesome

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    LHD so referring to same sides as you know them in USA.

    I need to stop the flow through the crankcase - not just remove the possibility of LPG being in the crankcase as (as you surmised) the mixer is the only source of air metering and and it being bypassed more on open throttle (because of the way the PCV valve functions) than at idle is creating a disparity in my fuelling which the actuator has to compensate for. As it is responding to an O2 sensor there is an inherent lag. The momentary weakness of mixture going on throttle from idle (as the PCV opens) has to be compensated by the actuator but if it can't open fast enough there's a risk of backfire.

    Where I'm trying to get to is the other way around where the actuator is more open at idle (weaker mixture there) so that on throttle opening the actuator either stays put or has to close - a lag in closing isn't a problem.In actual fact it has to be done the other way around where the mixture is inherently richer at open throttle than at idle thus when the throttle is opened the actuator is forced into closing.

    As it is it idles with the actuator open 100 'steps' but when I open the throttle the air from the PCV means the actuator has to move to 150-160 'steps' open. When I block the PCV that reduces to 130 steps so the actuator has less to open and the lag is reduced. Re-adjusting the reducer (two of mechanical adjustments) could improve on that further.

    See next post for proposed solution.
     
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  9. Pinger

    Pinger I'm Awesome

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    It is 'handled' by allowing the LPG to circulate in the crankcase - a risk I will not tolerate. One explosion there was enough and that was when the truck was stationary. If it happens at speed it's oil all over the road and the tyres.

    The stock GM system allows for over-pressurisation in the event of the PCV valve being overwhelmed by permitting the passenger side tube to reverse its flow back into the inlet tract (like a system from the 1960s!). So long as I preserve that it cannot over-pressurise. A check valve there that allows flow in that direction but not into the crankcase achieves that and it can be routed back into the inlet side (eg filter box) so as not to become an external 'emission'.

    Another PCV valve fitted there will do that (and plugs right in). On snap throttle opening the PCV closes (as set to do in that direction as it in essence simulates a back fire as seen in it's usual location so the crankcase is sealed from air ingress - exactly what I need to preserve the signal to the mixer and avoid the actuator having to compensate in a time period it can't quite manage. It can however flow in the opposite direction when the crankcase is experiencing positive pressure.

    All I have to consider now is any negative effects of higher vacuum in the crankcase during normal running - but not at idle or over-run as the PCV valve on the driver side closes under those conditions.
    At large throttle openings where blow by could occur it can be drawn into the manifold from either side but that won't matter because by the time the pressure has built, the actuator can respond. And can respond to its composition - ie hydrocarbons burned or unburned (mild EGR in the case of the former).

    And I really can't see an negative consequence of crankcase vacuum - not least as it is exactly what happens with a dry sump system with scavenge pumps.
     
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  10. Schurkey

    Schurkey I'm Awesome

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    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.

    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. 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.

    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.

    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.

    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.

    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.

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

    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.
     
    Last edited: Aug 2, 2020 at 2:34 PM
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