This is known in the aviation world as a differential compression test.

You can purchase a differential compression testor for a little as $35
with a little shopping around. Makes doing the test as easy as pie.

One cautionary thing .... with the cylinder at top dead center - TDC -
on the compression stroke - when you hit the cylinder with a slug of
compressed air, the engine may turn, or it may just sit there for a few
moments, then suddenly turn over. Be sure all your hands fingers and
tools are not in over under or around any belts, etc, or you can get
hurt. A wrench on the alternator pully shaft - secured into position and
jambed against something- might prevent the engine from turning
inadvertantly.

John Rodgers]
88 GL Driver

Geza Polony wrote:
> I found this description of the leak down test procedure in the archives. Is
> it right? Anything to add? Any inaccuracies?
>
> QUOTE:
>
> The air test or leak down test is used to determine any wear problems within
> any given engine . I will explain the test very simpily to all . // You want
> to bring your piston to top dead center of each cylinder to be tested or
> actually the firing position NOT to the bottom center. // You will first
> remove all 4 sparkplugs from your engine,also your oilcap, and if
> watercooled your radiator fill cap // You will need a air compressor with a
> adjustable air regulator plus a air test hose that may be purchased at your
> local parts stores and it will have a end to thread into the sparkplug hole
> and a male quick release end to connect to your air hose. To run the test
> properly run and get the engine to operating temperature before removing
> plugs. // Your FIRING ORDER is 1-4-3-2 ~~ You have a V notch on your crank
> shaft pulley and that notch will show up on Number 1 cylinder and Number 3
> cylinder. // You will need to find number 1 plug wire on your dist.cap and
> that will lead to the drivers side towards the front of the van number 2 is
> on the drivers towards the back of the van Number 3 is on the pass.side
> towards the front of the van and Number 4 is towards the back of the van //
> if you have standard shift place van in neutral and ebrake IT // Remove your
> distributor cap and turn the V notch to the coresponding top dead center
> marks ~~refer to your Bently manual ~~ You can usually turn the engine on
> the alternator center nut ~ Turn engine Clockwise // Once you find Number 1
> TDC // Insert your test hose into number 1 cylinder then connect your air
> hose to the test hose with 0 air pressure at the air regulator,slowly turn
> air up to 75 to 100lbs. Now if you have a problem in the test cylinder the
> air will tell the tale // to go to next cylinder to be tested turn clock
> wise 90 degrees and make sure your rotor points to the correct wire on your
> dist cap // If a exhaust valve is leaking you will have air coming out the
> end of the muffler .// I f the intake valve is bad you will have air coming
> out the air intake or on carbed out the carb.// If you have piston ,ring or
> cylinder wear problems the air will blow past the rings and into the oil
> area and you will be able to hear it at the oil fill region // If your head
> gaskets are bad you will see air bubbles or antifreeze coming out the
> radiator fill cap .On horizontally opposed engines always replace the
> piston,cylinder and rings as a kit never replace just rings, you will be
> putting new round rings into 2 egg shaped objects = piston & cylinder NOW
> ??? The reason for having the piston at top dead center is to test the area
> at the top of the cylinder and the rings for proper combustion which is the
> area IN which your power and compression originates from == Combustion ) ALL
> these tests can be done on 4 / 6 / 8 cylinder engines.// You can have
> problems in all areas so listen in all area,s to be sure . You should also
> run a a compression test first to decipher any pre air test problems !!!
> Average good compression tis 150 to 170 lbs
>
>
>