If this is a stupid question, I apologize in advance, but I am not mechanically inclined. Can someone explain the difference leakdown versus compression testing? When ordering a PPI, I want to be certain I know what to ask for... Secondly, is there any testing that can ascertain the condition of the valves / valve guides as opposed to watching for blue smoke bellowing out of the exhaust?

How common is to get some smoke on start-up in the morning? Is it a "good sign" if later in the day, there is no more smoke on start-up? I am looking at a 95 '993 with 62M miles. I saw a heavy puff of blue in the A.M that did not appear later in the morning. Thanks.

 

generally the compression test gives you the condition of your rings, wheras the leakdown test also indicates leaks through the valve and valve guides as well as the rings. ideally you want to have both tests done.

most 993s do not smoke on AM start ups, and certainly not heavy smoke.

 

Smoke on first startup is normal on my car if it has been sitting for a while, it never smokes later in the day. Compression Testing analyzes many things, the bottom line is that you want the compression in the cylinders to be "about the same" as an engine should wear down similarly on each cylinder. Newer engines will produce higher compression, as parts wear, it will get slightly lower.
To do this test, you will want to remove all spark plugs and inspect them carefully for any poor signs of use. Signs of fouling may signify a bad valve seal. Signs of oil on the plug could indicate a ring problem. You will then put one spark plug back in each cylinder, (on a twin plug engine), and screw a compression gauge into the other plughole. By depressing the gas pedal to the floor, you will open the throttle plate and want to keep it fully opened for as long as you crank the motor. Cranking the motor for at least 4 full revolutions. You will see the needle jump slightly less on each rotation. It may take more than 4 revolutions to get a stable reading. The last number the gauge jumps to is the compression for that cylinder. Repeat this for each cylinder. If you are not satisfied with the results or find an issue with one cylinder, further investigation as to the cause might cause you to perform a leak down test.

You will need a constant supply of compressed air and a leak down gauge to perform this test yourself. One face of the gauge measures the pressure of the compressed air that is being forced into the cylinder, while the other measures the percentage of this pressure that is being lost. A **** allows you to control the pressure going in so you get even results. You begin by taking out one of the spark plugs and bringing the piston (each cylinder at a time) to TDC (top dead center) so that all valves are closed. You will then connect the gauge to the spark plug well and the to the air hose. Apply the compressed air and modulate the **** to get a steady reading from both gauge faces. Use the same pressure on all cylinders. You will want your compressor full and stopped to achieve consistent readings for each cylinder. You will then listen to where you hear the compressed air if it is escaping. If you hear whooshing/howling at the oil filler cap, you are losing compression through the rings. If you hear it or even feel puffs at the tail pipe, it is your exhaust valves. If you can hear near the throttle body/intake manifold, it is your intake valves or valve guides. If the results are inconclusive, it is probably your head gasket.

That's how you pinpoint where compression/oil burning issues are without tearing down the engine.