Excess Crankcase Pressure

Slant 6 Engines

  1. bob_o

    bob_o FABO Gold Member FABO Gold Member

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    I recently upgraded the 225/904 combo in my '66 Valiant with a Super Six intake and 350cfm Holley 2bbl. During initial setup and tuning, I noticed that it was smoking quite a bit, which it hadn't at all before the swap. I chalked it up to some oil or water possibly having leaked during the swap and took it for its first test run. The smoking stopped completely about 2 minutes into the drive and the car ran pretty good but clearly still needed some minor tweaking.

    When I got home, I popped the hood and saw that it had sprayed oil out of the dipstick tube and I believe the distributor hole. I know the distributor o-ring needs replacing because I recently upgraded the ignition and noticed it was hard as a rock, so I put it on my list of things to buy. Upon further investigation I realized that the dipstick seemed to be missing an o-ring all together.

    Regardless, obviously the engine is now experiencing excess crankcase pressure. When I did the swap, I tried to replicate the original PCV setup with the front valve cover breather attached to the air cleaner and a pcv breather at the rear of the valve cover connected to the factory location at the intake. I noticed the new PCV had a relatively stiff spring in it and since it was kind of a cheap POS, I went ahead and bought a better pcv breather. After installing it and firing it up, the car was smoking pretty bad again but stopped after going for a short test run. I got home to see it had puked oil out of the same 2 places again.

    I don't have a good grommet to install the car's original pcv, so while I wait for that to show up with a new o-ring for the distributor, is there something else I should be looking at? Is it creating so much pressure at idle that it's pushing oil past the rings? Am I missing something? Any help or insight would be greatly appreciated. Thanks in advance!

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  2. BobW

    BobW Curmudgeon At Large FABO Gold Member

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    Worn or broken ring(s) allowing blowby?
    The only way the crankcase is pressurized to that extent would be blowby from compression.
    Probably coincidence it happened with the intake change, unless you dislodged something into the intake tract and it scored the bore or damaged a ring.
    Compression test & leakdown test. Go from there.
     
    Last edited: Nov 23, 2020
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    • Charrlie_S

      Charrlie_S Well-Known Member

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      I can't see where the PCV valve is hooked up. If it is connected to the port on the #6 intake runner, that is wrong. It should connect to the port on the base of the carb. Also I am concerned about the rear breather cap. Are you saying that is the pcv valve?
       
      Last edited: Nov 23, 2020
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      • 512Stroker

        512Stroker We are all here because we are not all there.

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        You have somehow screwed up the PCV system, should be hooked up to full manifold vacuum at the base of the carb and vent thru the vave cover.
        What goes in must come out.
         
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        • RustyRatRod

          RustyRatRod Lemmie see your b00bs. FABO Gold Member

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          Without the PCV valve introducing vacuum to the crankcase, the rings will not seal quite as good. They need that added vacuum on the oil pan side of the pistons to aid in complete ring sealing. This is why there were road draft tubes before the PCV systems were introduced. There was so much crankcase vapor, that it had to "exit" the engine compartment, or it would appear the engine was on fire.
           
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          • RedFish

            RedFish Well-Known Member

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            It looks like you have both caps attached to vacuum ports. I dont know how that would cause excessive crankcase pressure. I know one cap should be a breather and take in fresh air. Vacuum and pcv valve at the other.
             
          • RustyRatRod

            RustyRatRod Lemmie see your b00bs. FABO Gold Member

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            No, the front one is attached to the air cleaner base. I don't see the rear one attached to anything. I see a vacuum line going to #6 runner but I cannot see the "other end".
             
          • bob_o

            bob_o FABO Gold Member FABO Gold Member

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            Here's a better picture. The rear one is a PCV breather, while the front is just a breather. The carb I bought has the vacuum port in its base plugged, so it sounds like I'll need to find a fitting for it to redirect the PCV to.

            [​IMG]
            [​IMG]
             
          • BobW

            BobW Curmudgeon At Large FABO Gold Member

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            Seems I learn something new almost every day here! :thankyou:
             
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            • 67Dart273

              67Dart273 FABO Gold Member FABO Gold Member

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              FIRST thing I'd do is make sure the PCV is actually "sucking" and make sure nothing is plugged. Take a scrap clean hose so you can blow through the complete breather hose.

              When idling, remove the breather. Observe what is going on, IE fumes exiting or not from the filler port. Now remove the PCV from the rear breather and see what difference it makes. You can place/ remove your finger over the PCV and should immediately be able to tell if it is opening and closing and has "suction" going through it
               
            • bob_o

              bob_o FABO Gold Member FABO Gold Member

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              I'll try this when I get home from work, thanks!
               
            • bob_o

              bob_o FABO Gold Member FABO Gold Member

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              This is the plug that was in the base of my Holley 7448. Does anyone know what size/thread-form fitting I should be looking for? Or an online source to buy the right part from? Thanks again.

              dDPXzXO.jpg
              tz7epuE.jpg
               
            • 67Dart273

              67Dart273 FABO Gold Member FABO Gold Member

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              That is surely a 1/8 pipe thread plug 1/8 NPT is roughly 3/8"OD
               
            • AJ/FormS

              AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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              The hose to the PCV operates at hi-vacuum, so it needs to be PCV specific thick-wall stuff, or else it will get sucked flat at hiway speed, and then quit working.

              The breather that goes to the air box must breath in both directions. If your engine is smoking, then the airbox should be full of oil, which is bad. If there is no oil in there, then the system is not working, and the pressure has to exit somewhere else. The dipstick is just the weakest link. and I have never seen an O-ring on it.
              Your carb should have a nipple on the FRONT of it, facing the rad. The CC gasses should enter the throttle bores in close proximity to the idle discharge ports and transfer slots, so the blow-by can properly diffuse into the air-fuel stream. If you plumb it anywhere else you will have driveability issues, or plug problems. I would replace that #6 plug right away..

              After you get it all plumbed correctly and working, don't be surprised if it still smokes for say 15/20 minutes, as the condensed oil inside the muffler, cooks outta there.
               
            • bob_o

              bob_o FABO Gold Member FABO Gold Member

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              I went and bought an 1/8th pipe thread nipple, installed it at the base of the carb and relocated the PCV there. While doing that I looked at the hose I was using and it was for trans cooler. I didn't have any PCV hose so I went and test drove it anyway, still spraying oil. I stopped at Napa on the way to work and picked up some PCV hose so I'll put that on when I get home. If that doesn't fix the problem, I'm not sure what else I should be looking at. Again, thanks for all the help!
               
            • 67Dart273

              67Dart273 FABO Gold Member FABO Gold Member

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              I've run lots of PCV "cheat" with fuel line. You can getr some idea of the PCV operation at idle, just feel the end with your finger. should feel strong vacuum, and covering it/ uncovering the port with your finger, the plunger in the pcv should suck in with a "pop" You may have some sort of engine problem, like a broken ring "or two"
               
            • RustyRatRod

              RustyRatRod Lemmie see your b00bs. FABO Gold Member

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              OK, that's a little better, but the rear needs to be a pcv valve, not breather.

              After reading more, I assume you have one of those combination breather/pcv valves. Only thing left is make sure it's operating correctly as mentioned. Moving it from the intake was a good start.
               
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              • 512Stroker

                512Stroker We are all here because we are not all there.

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                Have you run a compression test since this oil issue started?
                 
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                • MOPAROFFICIAL

                  MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                  No.
                  It's crankcase pressure removal, the rings will seal the same with or without it...your gaskets however, won't .
                  And if it helps anything aside from that ..it would be oiling.
                  Internal pressure fights oiling just like negative pressure helps it.

                  Thank you, come again.
                   
                  Last edited: Nov 24, 2020
                • RustyRatRod

                  RustyRatRod Lemmie see your b00bs. FABO Gold Member

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                  Alright sir.
                   
                • MOPAROFFICIAL

                  MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                  You schooled me on that strobe probe.
                  Haha. If we ain't learning whats the point.lol
                   
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                  • RustyRatRod

                    RustyRatRod Lemmie see your b00bs. FABO Gold Member

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                    I can point to tons if info that show you pcv does indeed aid in ring sealing. But what's the point? lol

                    Just remember, it all depends on WHAT you're talkin about.
                     
                  • MOPAROFFICIAL

                    MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                    Well the point would be you can't explain a PCV valve sealing the piston rings more.

                    More accurate statement would be evacuating the crankcase pressure could possibly relieve some of the blow-by or oil burning. It absolutely does not seal the piston rings more though.
                     
                  • RustyRatRod

                    RustyRatRod Lemmie see your b00bs. FABO Gold Member

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                    Look how I worded it. It aids in sealing. It does so because it gives the rings LESS blowby vapor to deal with. Right?
                     
                  • bob_o

                    bob_o FABO Gold Member FABO Gold Member

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                    Not yet, but a buddy just lent me his compression gauge and a Leakdown tester. So I'll be doing those tests tonight and/or tomorrow.
                     
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