Why does my SBC 350 run like crap with a PCV valve?

Discussion in '40- & 55-Series Tech' started by nat, Mar 10, 2007.

  1. nat

    nat

    Messages:
    1,997
    Location:
    Los Gatos, California
    I determined that with the PCV valve hooked up, my SBC 350 will not idle right, it just fluctautes up and down. With the line removed and capped off it sounds great, no RPM fluctuations.

    My setup is the PCV in the passenger valve cover and a K&N oil breather/filter in the driver's side valve cover.

    Could I be drawing fresh air in from the oil breather? Is there that much vacuum pumping going on? Could I just get another breather/filter and get rid of the PCV?

    Thanks for any advice :cheers:
  2. Everybody correct me if I'm wrong but shouldn't the PVC go in the driver side valve cover???? Or does it matter? Every SBC i've ever owned the valve was in the driver's side.
  3. Do you got any pics of it installed. Could be hooked up to the wrong inlet on the carb??

    I got my PCV on the passenger side and it works fine, I don't think it matters which side.
  4. 67azcruizr

    67azcruizr SILVER Star

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    which valve cover the pcv valve comes out of shouldn't matter. Positive crankcase ventilation is a result of blowby mostly HC's which increase as RPM's go up. They are usually fed back into the manifold and/or carburator via vacuum port to be burned up. Usually vacuum is higher (at least it should be) at idle, thus the need for the PCV valve to help regulate the blowby flow. Not sure why when you plug up your PCV valve your idle improves but I would start by unlugging the PVC valve to obtain the rough idle, and take some carb cleaner and spray it around the outside of your carburator while the engine is running. If the idle improves or increases then you have a vacuum leak. You also might want to replace the PCV valve since it only costs $5 or less to see if it wasn't plugged up or something. sorry for the run-on sentences,
    Sean
  5. nat

    nat

    Messages:
    1,997
    Location:
    Los Gatos, California
    Pic of my driver's side valve cover breather:

    [​IMG]

    Here is a pic of the PCV hook up:

    [​IMG]




    I have a TBI unit and it definitely has a vacuum leak, from the PCV valve. The valve is brand new, so I am wondering if maybe the grommet sucks or I am pulling frwsh air from the breather.
  6. FL cruiser

    FL cruiser

    Messages:
    1,361
    Location:
    West coast Florida
    I had cheap valve covers once that didn't have a baffle under the PCV valve. Also the PCV valve hole was also located dirrectly over a racker arm. Oil from the pushrod hole was squirting directly at the PCV valve. Car ran bad when PCV valve hooked up and kept fouling #8 spark plug.
    If you think the PCV valves flow rate is too much causing vacuum loss, you can get different rated PCV valves. A PCV valve from a 4cyl will flow less than a big block one. Maybe your PCV valve is faulty too causing a vacuum loss.
  7. Plowboy

    Plowboy SILVER Star

    a K&N oil breather/filter in the driver's side valve cover.

    The filter could be the problem. The pcv is a closed system. On my sbc the oil filler hole in the valve cover locks down with a gasket, no air intake.
  8. workingdog

    workingdog

    Messages:
    598
    Location:
    Santa Rosa, CA
    It won't leak from the grommet, the valve is one way. It can only leak if the valve is not functioning.

    Peter
  9. The PCV valve is designed to be opened by engine vacuum during idle. It also can be opened by blowby pressure in the crankcase, especially when vacuum is low (heavy load).

    Fuel injection airflow sensors are more sensitive to PCV operation than carbed applications. It could be that your setup is allowing too much aurflow through the PCV at idle, giving a result that acts like an intermittent vacuum leak.

    To get around this, the PCV airflow is "metered". Metering means that the amount of air getting to the fuel injection system is restricted in such a way that it won't upset idle.

    My guess is that your PCV valve is either defective, or more likely you are using an earlier PCV valve that has no metering. See about getting one from a late application 350.

    In the meantime, you *MIGHT* be able to *temporarily* restrict the PCV airflow by inserting another vaccum hose into the existing hose, reducing the internal diameter. It might take two hoses to do this. Just make sure whatever you do, the restrictor doesn't get sucked into the fuel injection system port.
  10. grant5127

    grant5127

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    Location:
    Meridian,TEXAS
    Your theory sucks no fresh air.. My Man!
  11. There can be closed system PCVs. They will not evacuate the air during idle. They are uncommon. Some race engines use them.

    An open system will have some path to allow fresh air into the engine. This can be a breather on a valve cover, or an oil filler cap attached to the intake manifold that's breathable, or simply a breather on the intake manifold (the latter goes into the valve lifter gallery, not the intake runners).

    Interestingly, military vehicles, which use an open PCV system, can shut off (temporarily) both loops of the PCV. This traps blowby in the engine, creating a positive pressure even at low RPM, which keeps water out. Some even tap into this to provide pressure to the axles, gas tank, etc. This is why they can idle submerged under water, with only the intake and exhaust stacks showing, for hours. This dates back to the late 1940's, long before PCV was mandated for passenger vehicles.

  12. nat

    nat

    Messages:
    1,997
    Location:
    Los Gatos, California

    WOW!!! that was it. I bought a PCV for a 1990 Corvette with a FI 350, now it runs the same as when I had the PCV out of the system. Thanks!! :cheers:

    Now my only issue is the fuel pressure, which is 11psi, I think it defintely needs to be a bit higher, like 13-15psi. My fuel pump is super noisy as well, so I am considering an in tank model, or locating it externally back by the tank.
  13. Plowboy

    Plowboy SILVER Star

    Not thinking outside the box. My 40 is setup water tight. Uses a closed system with the vacuum set to secondary vacuum not manifold. No fresh air intake. Uses engine backpressure to open valves.
  14. grant5127

    grant5127

    Messages:
    5,022
    Location:
    Meridian,TEXAS
    On long term usage....I see internal engine sludge in your future.......
  15. Great! Glad to help.

    As for your fuel pump, check out what Edelbrock has to offer from about halfway down the page at this link. They have regular electric pumps that are aadjustable, plus EFI pumps all the way up to 45 PSI.

    I don't know about the other pumps, but my Quiet-Flo is indeed quiet. You can't hear it once the engine starts. No annoying buzzing.

    http://www.edelbrock.com/automotive_new/mc/fuel_pumps/fuel_pumps_main.shtml
  16. nat

    nat

    Messages:
    1,997
    Location:
    Los Gatos, California
    Those look like nice pumps. Which model are you using? What PSI are you getting? How long has yours been on your rig?

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