Leakdown Test - Catastrophic Failure?!?

Discussion in 'Fox 5.0 Mustang Tech' started by JasinC19, Jun 25, 2013.

  1. I have an old Matco leak down tester.

    I tested cylinders 5-8 at TDC compression stroke.

    Unhooked from cylinder:
    30psi into tool (left gauge)
    ~29psi right gauge (hey it's old)

    Hooked up to cylinder:
    right gauge was reading between 20-22psi, which if I understand correctly is 33% loss.

    IMPORTANT: The car was coldish... I did all this with the upper intake and valve covers off so by the time I got to everything, it wasn't anywhere near operating temp. Could this cause this much blowby?
    Because I second guess everything, I went out and bought a harbor freight leakdown tester.

    Same psi, the leakage was right around 30-35%.

    No air escaping through radiator, no air from intake manifold, I hear rushing past rings into crankcase.

    This engine was just built with all forged internals by a local machine shop that I've heard from several people is good.

    I'm going to talk to him tomorrow, but I wanted to share what I've found first and hopefully someone unbiased here can clue me in to what I might be seeing.

    I know that he asked me when I had the bottom end built if I was going to boost it. I said maybe in the future. He said he would install the piston rings a certain way so that it would be boost ready but guaranteed me it would not affect NA performance and functionality.

    What he hell could be going on here?
  2. Your leakdown test sounds very wacky. The pressure you put in the cylinder seems to be very low. With that pressure, even a small amount of air lost will make a very large percentage difference.
    See http://bellsouthpwp.net/l/r/lrichke...ession_tester/Blow_down_comprssion_tester.htm for a description of an aviation type leakdown tester and the proper procedure to use it. The test is most accurate when done on an engine that has come up to operating temperature.
  3. Thanks for the reply J. Tomorrow I'll crank the psi up to 80 and make sure I don't go past TDC and try again.

    I know my engine has been running rough, but i think if there was 33% blowby it would be a little worse than it is.
  4. Update: I tried 80 and 100 psi and the engine turned (not in gear).

    I did the test at 50 psi and got the following results on my Matco tester (the harbor freight one seemed pretty flakey so I stopped using it)

    1. 42/50
    2. 44/50
    3. 44/50
    4. 44/50
    5. 42/50
    6. 44/50
    7. 44/50
    8. 42/50
    The only thing worth mentioning is that the right side gauge measure around 48 psi without the tool plugged into the piston chamber. So I'm guessing the readings were more like 44/46.
    This is a pressure loss of between 12 and 16% (8 and 12% if you add the missing 2 psi at the default value)
    This is again with the engine cold (didn't have time tonight to put it back together.
    I'm assuming these numbers are not terrible and not bad enough to cause a rapid fluctuation of a vacuum gauge needle?
  5. The engine turned because the pistons were not at TDC (Top Dead Center). That means the rings were not necessarily positioned in the ring lands for best sealing.

    If you are concerned about the accuracy of the results, where's a tip.

    On aircraft engines, you use the propeller to help position the cylinder under test to TDC. With a car, use a square to mark of the balancer into 90 degree sections using the timing marks as a starting point. Put the transmission in 5th gear and rock the car back and forth to help line us the marks. Use the emergency brake or some wheel chocks to keep it from moving if necessary.