Diagnostic question

Small Block Mopar Engine

  1. jos51700

    jos51700 Green Bearing thread connoisseur

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    I have a stumper that someone here (RRR) will know the answer to.

    I have a 5.9 Magnum in my Dart, dead stock, hooked to a 4-speed.

    I missed a shift. The next day, when cranking, it sounded like a cylinder or two were out on compression when cranking. wha-wha-WHA-wha-wha kinda thing. Seemed down on power but had no funny noises.

    I pulled the valve covers and rocker arms expecting to find bent valves. Pushrods were all straight, and there was some variance in the heights of the valves but nothing major. With the rockers off, I pressurized the cylinders with air via leakdown tester. Six cylinders were at 10%, 2 were at 15%. This blows up my theory about valves and/or pushrods.

    Any ideas? Anyone?
     
  2. CFD244

    CFD244 FABO Gold Member FABO Gold Member

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    Timing chain?
     
  3. jos51700

    jos51700 Green Bearing thread connoisseur

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    Timing chain is a new double-roller, with maybe 150 miles on it. I used the Magnum tensioner as well. I've been wondering if there's an easy way to check time while I've got the intake off.
     
  4. furrystump

    furrystump Well-Known Member

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    Be careful with the tensioner. I had one come apart and try to eat its way through the cover. Won’t use one again.
     
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    • yellow rose

      yellow rose Overnight Sensation FABO Gold Member

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      Do a compression check. You may have bent a valve. Or a leak down test.
       
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      • RammerJammer75

        RammerJammer75 Well-Known Member

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        He did do a leak down.
         
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        • jos51700

          jos51700 Green Bearing thread connoisseur

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          I did. The numbers were acceptable. That's where my confusion comes in. Cranks funny, compression seems fine...?
           
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          • PRH

            PRH Well-Known Member

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            A compression test and a leak down test aren’t the same thing.

            For example, you could have a malfunctioning lifter........ wouldn’t show up on a leak down test.
             
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            • TrailBeast

              TrailBeast AKA Mopars4us on Youtube

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              Or snapped in half cam.
               
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              • yellow rose

                yellow rose Overnight Sensation FABO Gold Member

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                Dang I missed that. Thanks.
                 
              • 408 swinger

                408 swinger FABO Gold Member FABO Gold Member

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                You say there was "some" variance in the valve tip heights. Is that to say there wasn't any before? With or without the rockers torqued down?
                 
              • 67Dart273

                67Dart273 FABO Gold Member FABO Gold Member

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                Sometimes broken valve springs are a little "non obvious" to spot
                 
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                • jos51700

                  jos51700 Green Bearing thread connoisseur

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                  There was maybe up to .010" in variance. That's with no rockers. I don't know about before.

                  Imma check that. Thank you
                   
                • famous bob

                  famous bob mopar misfit

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                  10--15% AINT ACCEPTABLE TO ME !
                   
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                  • AJ/FormS

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

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                    After the missed shift, did it idle normally?
                    or run rough/ stall?
                    If the latter;
                    My guess is bent valves, lotsa bent valves.
                    and the 10 to 15% is the proof.
                    Normal is closer to 4%.
                     
                  • jos51700

                    jos51700 Green Bearing thread connoisseur

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                    That's helpful. I'll tell my car, and see if it straightens up a little

                    I've yet to see a reference that says 4% leakdown is normal and 10% being bad. I can see all the valves being bent resulting in fairly even numbers, but most of the reference I'm seeing is like this:
                    ?u=https%3A%2F%2Fst.hotrod.com%2Fuploads%2Fsites%2F21%2F2005%2F10%2F116_0406_leak09_z.jpg

                    Personally, I'd figure something was up if I was at 20%. When I did the leakdown test, I listened to the intakes (Intake manifold was off) and never heard any air there. All the air I heard was in the crankcase.

                    I guess I need to go back and do a compression test.
                     
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                    • 408 swinger

                      408 swinger FABO Gold Member FABO Gold Member

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                      Possible that some lifters are bleeding down upon cranking?
                       
                    • AJ/FormS

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

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                      jos, don't bother; repeat the LD with a different gauge. If you get the same results, the engine has been hurt.


                      One second at 6000rpm is 100 revolutions. If the springs couldn't take it, the lifters pumped up and the cam drove the valves into the pistons, the usual result is all bent intakes, and some bent pushrods; because the valves are opening as the piston is coming up to TDC, at the top of the exhaust stroke; BAM!
                      If you exceed the springs ability to keep it together, under load and the valves float, the engine usually stops revving. It just goes flat.
                      But if you overspeed it in neutral, like in a missed shift, flywheel inertia will continue the revs way past the onset of float. It will take a while for the rpms to come back down below valve float, and even longer for the lifters to bleed down. Sometimes taking several to many minutes with the engine at high idle, to keep it running.
                      If it stalls, you might have to leave it overnight, and at least two cylinders will likely still be dead in the morning. If the valves survived, you will be able to get it running on the six, but just wait; keep it running at say 1500 and soon the other two will come on line.
                      But if the engine stalled, and won't restart, you can bet the valves are bent. It stalled because of the high rate of leakdown/ lack of compression.

                      An engine will run thousands of miles at 8 to 10% LD, but it sure won't make power. It will be hard to start, may suffer from detonation, and is guaranteed to make lousy fuel mileage; and if it's the ringseal gone bad,will dirty up the oil pretty quick.

                      If I ever noticed my engine trending towards 8%LD, I would already be taking it apart. Last time I checked, it was at ~4%, with ~80,000 miles on it.
                       
                      Last edited: Apr 14, 2021
                    • yellow rose

                      yellow rose Overnight Sensation FABO Gold Member

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                      I agree with AJ. It’s hurt. Those leak down numbers are not good. 3-4% for a good street engine with gapped rings is good. Every point after that just says how bad it is.
                       
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                      • jay27

                        jay27 Well-Known Member

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                        I would think the leak down would be worse with bent valves would it not?
                        Are you running stock push rods? If so you said they are straight and not bent?
                        Cant remember ever bending a valve and not bending a stock tube style push rod
                        If your going to do a compression test pay attention to the first bump 75 , 85 , 100 or whatever the number is and the following 3 or 4 afterwards , record those #s as it will tell you something.
                         
                      • yellow rose

                        yellow rose Overnight Sensation FABO Gold Member

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                        Depends on how bent they are. Here’s a good one that some guys don’t think of.

                        I’ve seen valves so bent you’d think they wouldn’t seal, but if you put enough spring load on them, they will seal up.

                        So it’s possible your valves are bent a bit, but even 120 pounds of seat old can partially seal up a bent valve.

                        That’s just an FYI because I’ve seen it many times.
                         
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                        • dirty white boy

                          dirty white boy 50 yr old Juvenal delinquent

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                          whats your cam lobes look like???? less a couple lobes went flat, with no bent push rods or broke springs or collapsed lifters, good rockers,..sure ignition systems all good? then havta agree with rest of them, bent valves!
                           
                        • jos51700

                          jos51700 Green Bearing thread connoisseur

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                          Thanks all for the replies. I'd always considered 10% acceptable, and the only concession I'll make is that this a 100% stock engine with stock rings. It's mileage is unknown.

                          I have extra heads and some hot rod valve springs that I got from toolman. I guess it might be worth pulling heads.
                           
                        • RustyRatRod

                          RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                          20% on a leak down is unacceptable for a "healthy" engine. It does not necessarily however indicate engine damage. What I would do and have advised before is, remove the rocker shafts. Remove the plugs. Pressurize each individual cylinder one at a time and listen to where the air escapes. Through the exhaust, an exhaust valve is not seating. Through the carburetor, an intake valve is not seating. Through the valve cover area, hole or crack in a piston or bad or broken rings. Through the open radiator cap, head gasket, cracked head or block. Leak down testers are a good tool, but often times they are "too specific" and cannot diagnose the "where" of the problem. Been there too many times with it. I don't even own a leak down tester. I personally think they SUCK for a diagnostic tool. They are more of a performance indicator. If I had a full time race car, I'd own one.

                          Having said all that, If I had to "guess" I would guess that your valve springs already sucked and you over taxed a few and now they are weak.....that is, since you didn't find any bent pushrods. I would also guess that with no bent pushrods, you'll likely have no bent valves. Pushrods normally bend first or right by themselves. I cannot remember any engine I ever saw where the valves were bent and the pushrods not. However, I have seen several with bent pushrods and no bent valves.
                           
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                          • jos51700

                            jos51700 Green Bearing thread connoisseur

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                            With each cylinder, I did exactly what you describe, I just passed the air through my leakdown
                            Tester. I only heard air from crankcase.
                             
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