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How to build your own Leakdown Tester

Discussion in 'Mechanic and Repair' started by Dirty Water, Nov 9, 2006.

  1. Dirty Water

    Dirty Water LawnSite Fanatic
    Messages: 6,794

    Since a lot of you guys work on your equipment here, and since the mower engines are so simple, I figure most you would appreciate this tool.

    A store bought leak down tester will run $100+, but you can build your own for much cheaper.

    First off, you need to get a pressure regulator. This one from Sears (part number 282-16025) is the same one that Snap-On uses on their leakdown tester. It even comes with the gauge on it already.


    Attach a male Quick Connect fitting to the inlet side of the regulator and a female quick connect fitting on the outlet side of the regulator.

    Now all you need to do is make a small hose that adapts from a male quick connect to spark plug threads. A easy way to do this is to butcher up a old (or cheap) compression tester to use its line. Or you could have any store that makes hydraulic lines make you one.

    If you do use a compression tester hose, you need to remove the check valve out of the fitting.


    To use it, attach your air compressor to the inlet of the regulator and adjust the regulator until the gauge reads 100 PSI, this is your 0% leakage indicator.

    Make sure the cylinder is at TDC of the compression stroke so all the valves are closed and attach the leakdown tester to the cylinder head.

    All engines leak somewhat, so even on a healthy engine, the gauge will drop down to 90 PSI (10% leakage) or so. If it drops a large amount, then you have a leak (from chipped valves or a bad headgasket etc).

    If it leaks significantly, you can feel the air coming out of the intake or exhaust manifold, indicating which set of valves is damaged. If the headgasket is leaking, you can sometimes feel it between the head and the block.

    Anyways, have fun building a tool for $20 that is pretty much the same as the several hundred dollar Snap-On equivalent.
  2. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    Just to add to Jon's instructions, If no air is heard or felt out of the intake or exhaust remove the oil dipstick checking for air escaping there. That would be a sign of worn rings and or cylinder.

    One other thing, Before the air is applied to the cylinder the flywheel MUST be pinned, If not the pressure will push the piston down the cylinder opening the valves. Do NOT put a large screw driver or pry bar into the flywheel blower fins expecting to hold it, The pressure is so great it will either snap it out of your hand or break the fins on the flywheel.
  3. Dirty Water

    Dirty Water LawnSite Fanatic
    Messages: 6,794

    I only work on car engines and larger, so I forgot to tell you to pin the flywheel :) Good call.

    I'm just used to putting it in gear :laugh:
  4. tomo

    tomo LawnSite Senior Member
    Messages: 660

    hello, use a large guage 2.5 - 4.0 inches easy to read and more accurate.
    A guage graduated in 2 psi increments .
    I have personally found 3-5 psi drop after setting guage to 100psi u could also take this as 3--5 %
    Interestingly i used a over the counter leak down dual guage and also set at 100psi, the leak % guage indicated 7-10 % weird ????
    When only using the pressure guage 3-5 as i stated . After reading some race books there expectations r upto 5% ok over this eng requires work.

  5. Jim@MilkyWay

    Jim@MilkyWay LawnSite Senior Member
    Messages: 472

    I am interested in this procedure as I have never learned how it is done. What is the time frame for reading the gage after pressurizing cylinder. I get the impression you are "listening" for air bypass from cylinder, indicating immediate, steady drop in pressure. But what if you can not hear it "hiss"; then how much time do you give it to leak down?
  6. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    You need to go down and get a hearing aid. ;)

    However long it takes you to find the source of the leak. There is really no given time frame, If there is a problem is will show up quickly.
  7. scholzee

    scholzee LawnSite Member
    from NY
    Messages: 130

    A single gauge is not very acurate and a poor way to do a leakdown test. Any quality leakdown tester uses 2 gauges with an orfice between them and reads the differential pressure of the flow through the orfice. If you have a good compressor it will be able to maintain the 100 psi even with a leaky cylinder in the engine. Your dual gauge should 7-10% because it is acurate. If needed I can post the way to make a dual guage and still save some money over a bought one.
  8. pugs

    pugs LawnSite Gold Member
    Messages: 3,024

    You should also watch out for breather hoses that attach to the air intake. You may hear air coming out and think its a leaking intake valve when its really the breather hose you hear :)

    Also if you are really good you can get the piston at TDC perfect so that you dont need to pin anything.

    For these small engines the accuracy is not that important. Its either good or its leaking pretty bad. And if you are to the point of needing to do a leakdown test you probably have something leaking pretty bad.
  9. Jim@MilkyWay

    Jim@MilkyWay LawnSite Senior Member
    Messages: 472

    Huuuh, Say what,, speak up there, sonny.
  10. tomo

    tomo LawnSite Senior Member
    Messages: 660

    Hello, i agree with your thoughts . In my experience on one of my engines i use the dual guage leakdown test in conjunction with other tests .
    example..... ch25s kohler in reasonable condition
    1/ i conduct tests every 6-9 mths
    2/ compression wet and dry 10psi lower on 1 cylinder
    3/ slight oil consumption present
    4/ leak down 2% diff on dual guage set up
    5/suspect cylinder has oil build up on plug over 6-9 mth period
    6/ bypassing air "hissing" is slightly louder from suspect cylinder
    7/ put all these together and my assumption is i have a sus cylinder,but not of a major concern at this stage .


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