No Compression after Spring Replacement

Discussion in '2005+ Mustang GT 4.6L Tech' started by Shua, Dec 29, 2018.

  1. Shua

    Shua Junior Member

    5
    2
    2005 GT 4.6L 3V
    Supercharged
    FRPP Hot Rod Cams (purely for the lope)
    Stock Heads otherwise

    I recently broke a valve spring on the #7 forward intake valve. Thankfully damage was limited to just the spring.

    After replacement of all three #7 springs and without the roller cam followers installed, I performed a satisfactory compression leak down check of 77/80.

    Installed roller cam followers, and now I’m getting 0 compression and I can hear blowby in the intake and exhaust.

    Is this caused by the lash adjusters being pump up a bit? Will they automatically adjust back down once I fire up the engine? Or should I attempt to pull them out a bleed them down? Thank you!

    (Also, I apologize if this has been answered a million times, I get an error every time I “search”)

    -Shua
     
  2. Steve Waters

    Steve Waters Member

    57
    12
    How long ago did you do the compression test on it? Have you tried running a second test on it. Seems if they were pumped up they wood bleed down after sitting under some load for a while. Could have a bad hydraulic lash adjuster too.
     
  3. Dino Dino Bambino

    Dino Dino Bambino I have a red car

    Age:
    56
    513
    52
    Yeah, it sounds like the lash adjusters are pumped up and causing the valves to hang open. If they don't bleed down after a few hours under the tension of the valve springs, replace them. DO NOT even crank the engine, let alone fire it, if they haven't bled down and normal compression has been restored 'cause you'd risk piston to valve contact.
     
  4. Shua

    Shua Junior Member

    5
    2
    Update: I let the car sit overnight with the followers installed. The lash adjusters must have bled down as I’m now getting excellent compression.

    Steve,
    I did my compression test immediately after installing the springs. Should have waited longer for the lash adjusters to bleed down to position.

    Dino,
    Thanks for the warning. I’m going to slow down a bit and be sure all my compressions and lash adjusters are good before I install valve covers and crank it.
    I have a red car too, small world!

    -Shua
     
    hammeron likes this.
  5. Dino Dino Bambino

    Dino Dino Bambino I have a red car

    Age:
    56
    513
    52
    That's good news. After you've confirmed there's normal compression in all eight cylinders you should be able to fire up the old girl.
    My only remaining concern would be what caused a valve spring to break in the first place.
     
  6. Shua

    Shua Junior Member

    5
    2
    I suspected the following combination of stuff on those stock springs:
    10 psi boost.
    Higher lift/ramp rate on the Hot Rod Cam.
    May have oversped it.

    I thought I asked Lito to set the rev limiter at the 6k, but a couple days before it broke I saw a lot of red to the left of the needle. I may have been turning 6300 or 6400. Think a combination of all that may have killed it?

    Not Lito's fault, I reviewed my communication with him and I never asked. I will now though!

    -Shua
     
  7. Dino Dino Bambino

    Dino Dino Bambino I have a red car

    Age:
    56
    513
    52
    You could upgrade to beehive springs and not have any of those worries. The only limiting factor would then be the maximum supercharger rpm, and that's specific for each supercharger head unit.
     
  8. 46addict

    46addict 13726548

    1,820
    54
    The directions on compression tests say to perform the test on a warm engine. If running the engine pumps up the lash adjusters, does that mean we have to run the tests on a cold engine?
    Also what does 77/80 mean on the leakdown test?
     
  9. Shua

    Shua Junior Member

    5
    2
    M122. I’m spinning it slower than a stock GT500 does at the same engine RPM.
    Car is running great now!
     
  10. Shua

    Shua Junior Member

    5
    2
    Warm wasn’t an option at the time. After letting it sit overnight I “spun” the engine by hand (ratchet) through a dozen rotations. Then performed a leak down check. All was good.

    77/80 on a leak down or differential check, not a “compression check”. With an 80psi supply it held 77psi. It’s an aviation thing, and I’m an A&P.

    I don’t understand the automotive compression test. I did perform it later, got 155-160 psi on all 8 at 4500ft elevation. The problem with this test is determining where the leak is. You can’t hear the air in the exhaust, intake, oil filler or coolant filler. Leak down/differential is superior for diagnosis.
     
    46addict likes this.
  11. Juice

    Juice forum member

    495
    52
    An "automotive" compression test is a little easier to do than a leakdown test. Its not so much about how much psi on the gauge, but that all cylinders are close to equal. (10% or more difference is 'bad') So if you are close to spec AND equal, no further testing needed.
     
    Shua likes this.