Leak-down tester (Formerly "Is the compression tester over the hill?")


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:p Every decent maintenance schedule will have you doing a compression check every 15,000 or 30,000 miles. One way to tell engine wear & health. It should also be in every potential buyer's check-out of a used Westy. But the old compression tester may be on the way out.

Most better mechanics now have a "leakdown" tester. I once worked with a factory race team whose engine man didn't even own a compression tester. With 4 straight 24-hours of Daytona class wins, and a number of national and world championships or records with his engines, I tend to listen. He uses a leakdown tester exclusively.

The principle is the same. A compression tester relies on the pumping action of the engine to build up pressure in the cylinder. A gauge with a check valve measures the cumulative pressure, giving an indication of how good the engine's internal seal is. A low cylinder may have a burned valve, blown gasket or scored cylinder. All low compression probably means worn out rings.

The problem is that it's difficult to get accurate and consistent readings. Engine & oil temperature affect the seal, as does whether the throttle is wide open (preferred method) or not. Cranking speed, how many cycles cranked, and even atmospheric pressure & shop temperature affect the readings. Getting the same readings a few days apart or with a second gauge is impossible. Consider it a one-time snap-shot.

A leakdown tester basically does the same, except it relies on a continuous supply of compressed air. The air is regulated, typically 50-100 pounds depending on instrument manufacturer, and then goes through a calibrated orifice in the tester, past a gauge, and into the spark plug hole. The higher the compressed air loss, the lower the leakage gauge will read. Better gauge sets will have two gauges -- inlet and outlet; other's (economy models) will rely on one.

With a two-gauge set, 5% leakage of the psi applied is excellent -- there has to be some leakage around the ring gaps. 10% is still pretty good. At about 20% you should start thinking why, where & are repairs due. One-gauge sets, though they may have percentage numbers on them, are more of a Good-Warning-Bad measure (Milton Industries is even colored green-yellow-red) as they can't measure the actually air drop in PSI. The gauge manual will give you reading interpretation.

Unlike a compression gauge, the leakdown test helps diagnose the problem. Using a mechanic's stethoscope (without probe) a leak at the tail pipe indicates exhaust valve leakage. At the FI or carburetor air supply, an intake valve leak. At an adjacent spark-plug hole or to the edge of the head, a blown head gasket. Gurgling back through the radiator can indicate blown head gasket or cylinder wall crack. There will always be some through the oil dip-stick (ring leakage) but excessive hissing there indicates worn rings or cylinder wall leakage.

Performing a leakdown test differs from a compression test in that you test each cylinder at top dead center (TDC), much in the way you adjust valves. Remove all plugs; turn the engine by hand to #1 TDC; insert plug adapter and apply shop air per gauge manufacturer's instruction. Record reading. Rotate engine to TDC on the next cylinder in the firing order and repeat until all are done. A nicety of the leakdown test is it can be done with the engine out of the vehicle.

My maintenance log has a place for both compression & leakdown readings. [I'm not quite ready to chuck the compression gauge yet!] A running record of both can tell you a lot about where you're at in the engine's life cycle, and warn of impending problems. Like a compression check, there should be only minimal difference between cylinders.

Capt. Mike


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Capt. Mike

My cousin-in-law is a factory Peterbuilt technician. Last time we visited them, he & I discussed my Cummins diesel and that it appeared to have lost a little oomph & mpg (156k). I asked about a compression test and he said they no longer use them, relying solely on the leak-down test. They do have a special guage that taps the crankcase breather that is more accurate than the instrument gauges but the same principle. So I guess when Snap-On made it part of the big B-series Cummins tool kit, they got me for a $200 gauge I probably will never use . . .:eek:!

BTW: That set is now up to $1,012 in case you want to fuss about the cost of the tools necessary to do the VW diesel.


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