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Leak Down Tester

I found the following article written by a Jag mechanic. He graciously permitted its reproduction here. Check down below for full particulars on the author if you would like more info on the really neat stuff that he does at work every day.

The cylinder "Leak down test" when performed in conjunction with a compression test (see last week's tip) is a conclusive analytical tool. You will need a cylinder leak detector such as Snap-On #MT 324. Or a shade tree apparatus can be assembled from a few common items as follows.

First, modify a good quality compression gauge by separating the gauge from its hose and installing a female air hose coupler so the gauge can be easily separated and reconnected. Secondly, use the following parts to finish the apparatus. A 1/4" NPT tee fitting, three male air hose fittings, a 1/4" stop valve, and another female air hose coupler. Assemble the parts so the gauge connects to one tee port, the stop valve and male air hose fitting connects to another tee port, (The stop valve controls air pressure to the apparatus) and the hose from the compression gauge connects to the third tee port


Third, remove the check valve from the fitting that screws into the spark plug hole. (Use the tire valve extractor from your car tool kit) If possible, run the engine to warm it up. Remove the spark plugs. Remove the oil filler cap and the dipstick. Remove the air cleaner or air flow meter, and open the throttle. Carefully remove coolant filler cap and top up with coolant to bottom of filler neck.

To apply air to the cylinder with the valves closed, crank the engine by hand in the direction of rotation until the piston is at top dead center with the distributor rotor pointing to the same cylinder plug wire position. Now screw the apparatus into the spark plug hole. With the gauge and air supply hooked up, start opening the stop valve. Note the maximum reading of the gauge.

Listen for leakage at the following places:

  • Adjacent cylinders sparkplug hole. Use a piece of small rubber vacuum hose, stick one end near the spark plug hole and the other end in your ear. Leakage, air hiss, heard here may be a blown head gasket between cylinders or it may be leakage heard through an open valve.
  • Exhaust pipe. May indicate a burnt or stuck exhaust valve.
  • Carburetor or throttle body. May indicate a bent or stuck intake valve.
  • Oil filler or dipstick hole. May indicate broken rings or a damaged piston.
  • Radiator filler cap. Bubbles here will indicate a leaking head gasket or cracked head.
Good luck, Walt Osborn
Walt Osborn's reproduction policy :

This is the Policy for reprinting of "Walt's Jaguar Tip of the Week"

Walt is sharing this information as a service to Jaguar enthusiasts and would ask that you follow the guidelines below when using these tips. You may reprint these tips with the following conditions and limitations:

1. Reprint for monetary gain is prohibited.

2. The tips must be properly accredited to Walt Osborn and his company Vintage Jag Works.  Walt would prefer the following format for accrediting the tips " This Weekly Tip Service is provided by Walt Osborn and his company, Vintage Jag Works. Internet: Email  to subscribe or unsubscribe."

3. Note that this is a free e-mail service and anyone can subscribe by request to

 Article By: David J. Morrow

Updated January 2005 

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