Can someone describe what exactly a compression test and leak down test entail?

In general does a seller have anything to be concerned about if I ask them to get this test done? I'm guessing they have to really trust the mechanic?

How long does this process take usually?

 

In a compression test a gauge is used to record the pressure of each cylinder as the engine turns over. The spark plugs are removed and a hose with threaded end goes into the spark plug hole. The compression is measured for each cylinder. In a leakdown test the same thing but the engine is turned over with the piston stopped at TDC to pressurize the cylinder. It's left in that position and the gauge is used to see how the pressure reduces over time. It's not hard but does take time. You need special tools to do it. I have a really nice one I bought long ago. A kit will include the pressure gauge and various fittings to connect into the spark plug hole.

 

It's also around $400.

 

Related to this, I saw the following compression and leak down results on a PPI report on a 2010 987.2 (2.9L base) with >40,000km:

"Compression test results all found to be normal, an average of 190 psi.
Cylinder leakage test results all found to be normal, an average of 6% leakage.
All specs OK".

I did a web search which implied anything less than 5% leak down is very good, but I imagine this depends on engine design - piston ring pressure against the cylinder walls?

 

The important thing in a compression test is that all cylinders are very close to one another in the pressure reading. If one cylinder is quite a bit lower than the others, that's bad - worn rings, stuck valves, scored bore, etc. Looking at an average isn't very useful as one could still be really low, but when you average all 6, the number isn't bad.

With the 997 a compression test isn't completely necessary as any low readings you'd get would also be manifested by misfires in that cylinder. You can have one done for extra peace of mind, but you can also run the car with a Durametric attached and check for misfires. If you do go ahead with a compression test, might as well buy a set of new plugs and replace those at the same time - also inspect the coil packs if they haven't been done recently.

You might actually talk this over with the owner - maybe offer to split the cost of the parts with him if you don't buy the car. That will give him an advantage when selling it to the next potential buyer since it will have just had new plugs and coil packs. If you buy the car, you'll have done most of a tuneup and will likely have years of trouble-free driving before you have to do them again.

 

The seller might be concerned if he has something to hide. I am used to the principle that the potential buyer pay for the test.

 

Generally a compression test/leak down test is what is done when one suspects a problem -- due to other symptoms -- and is attempting to narrow down the problem.

For a used car purchase both tests are rather unnecessary.

As an aside I can also tell you were I selling one of my cars I'd be very leery of letting just anyone at the engine to do this. One is I don't like to have old plugs reinstalled so there's the cost of new plugs. There is also the risk of a plug being incorrectly gapped or installed incorrectly -- cross threaded or over or under torqued -- or a coil not being connected properly.

In the case of my Turbo bumper cover has to come off and part of the exhaust system which means the turbo metal o-rings have to be replaced or the turbos can leak where they bolt to the exhaust. The intercoolers are in the way and and the some intake disassembly is necessary.

While this gets done when I have the plugs replaced it is done at Porsche dealer service department where the tech knows the car very well and does the job right.

It is not something that I would entrust to a shop just because it had a compression or leak down test gage.

As the engine runs the DME constantly checks the output of each cylinder and if it finds one under or over performing will turn on the CEL and log one or more error codes.

A compression/leak down test can't really tell you the engine runs nor how well it runs and sounds.

So really instead of a compression/leak down test you are better off doing a thorough used car check out and this involves a real road test over a route that is long enough -- 15 miles or so -- and gives the seller/driver a chance to demo the car as you intend to use it followed by you then driving the car over the same route the same way.

You must experience the car on the road. You ensure the CEL comes on when you turn the key to start the engine but it goes off when the engine starts and then remains off throughout the road test.

This is a good indication not only is the compression/leak down acceptable but teh engine is in good health. Additionally you get to listen to the engine at cold start and after as it idles warms up and you want to be sure the engine makes no abnormal sounds. A compression/leak down test can't do that for you.

 

A compression test is SOP in aircraft engines. A standard 80 lbs of air pressure from a compressor, is introduced into
the cylinder, on the compression stroke, with the crankshaft held so it does not move. In a perfect world, the results would be 80/80...in an excellent engine, the result would be 70 to 78/80. Cylinders in the 60s and 50s are losing compression. Any irregularity is to be investigated further. Good results would look like: 77/80 79/80 77/80 78/80 76/80 78/80. By listening to either the intake or exhaust manifold with a stethoscope, the mechanic can hear the loss by a leaking valve. Listen in the oil fill opening and hear the hissing of air bypassing the piston rings. A combination of compression leak down, then camera inspection through the spark plug hole, assess the condition of each cylinder, and diagnose problems prior to a tear down. Gauges to do this are reasonably prices from aircraft tool suppliers.
The adapter to fit Porsche spark plug threads might have to be fabricated.