Differential pressure compression check Demystified

[James Brown]

So many people have asked what's involved with the differential compression check and why is it so expensive? As I was going to do it on a aircraft, I might as well explain the steps. I will be doing it on a aircraft engine but the procedure is the same.

This is the tool you need. The end that goes in the spark plug hole for a Porsche is connected to a hose with a O ring on the ent to seal it. The rest is the same.

The adapter is screwed in the spark plug hole (lower plug in place)

the gage is attached to the adapter and the other end connected to a air source.

With the cylinder at TDC (holding the prop but putting a Porsche in gear with the parking brake on) The air is turned on to 80 psi. there is a small orifice thats lets a metered amount of air to enter the cylinder. If any leaks, the cylinder gage will read low and you can hear air coming from the intake, exhaust, or crank case. The amount of air loss is the differential expressed in % (like 80/72=8 psi=10%). If the air was coming from the exhaust, it would be likely be an exhaust valve that was leaking or burned. The rest of the cylinders are checked in the same way. This check gives you a good indication on the health of the engine. Hope that helps. BTW, it takes about a hour to perform this check.

[KCinBR]

Just to be clear, this is the test I would call "leak down test"?

Thanks for the pics and explanation, I had just purchased a leak down tester but have not used it yet.

[James Brown]

Yes, same thing, auto vs. aircraft.

[RoninLB]

i read the the prop may be turned a few degrees from TDC to get best results

[gshase]

Is that an o235 on a PA16 @108 hp

[JFairman]

I use 120psi from my compressor and use the attached regulator to adjust the leakdown tester to 100psi on the left side gauge with the piston at TDC.

If the piston is really at TDC no amount of air pressure will push it down and turn the crank.

That way is much easier and more accurate with no math because every single psi that leaks down from the cylinder is displayed as 1 pound or 1% on the right side 100psi gauge.

[James Brown]

Gshase, thats a O320 E2D on a Cessna 172 @150 hp.

[TRE Cup]

Since there tends to be carbon build up on the plugs, a tip before doing the test is to crack the plugs loose, reconnect the cables, start the car up. Rev it a few times to blow off the carbon. then do the test.
We have found that if you don't blow out the carbon, it can get caught between the exhaust valve and seat. Obviously this gives you erroneous readings

Not sure where the leakage is coming from ? Wet your finger, place it in the exhaust tip, oil tank opening, or the opened intake ( push the gas pedal down) . Whichever area (could be more than one ) gets the finger cooler is the problem area

[JFairman]

Originally Posted by TRE Cup

Since there tends to be carbon build up on the plugs, a tip before doing the test is to crack the plugs loose, reconnect the cables, start the car up. Rev it a few times to blow off the carbon. then do the test.
We have found that if you don't blow out the carbon, it can get caught between the exhaust valve and seat. Obviously this gives you erroneous readings

Not sure where the leakage is coming from ? Wet your finger, place it in the exhaust tip, oil tank opening, or the opened intake ( push the gas pedal down) . Whichever area (could be more than one ) gets the finger cooler is the problem area

Thats correct, and it is why you shouldn't touch the sparkplugs when doing a compression or leakdown test. Turning the sparkplug can knock a little chunk of carbon off and the exhaust valve and seat is right where it will fall... at the bottom edge of the combustion chamber.

Also it's alot easier and more accurate to hold the end of a 3 foot long piece of around 1/2" ID hose in your ear and hold the other end in the wide open throttle body to listen for the hiss of intake valve leakage, the oil tank or crankcase oil drain plug if the oil is drained out for piston ring leakage, and in the tailpipe or oxygen sensor bung for exhaust valve leakage.

[James Brown]

Funny you should say that TRE, One of the cylinders I was checking has a bad leak from the exhaust valve. On a aircraft engine, you can access the rockers and tap them (called striking) with pressure on them to knock off the carbon. This cured the leaking valve. You can do this on your Porsche but the valve covers must come off and it's ard to get in there to strike the rocker.