Here's the deal, I was in the middle of tearing down a 400 when I realized I forgot to perform a compression test. I'm pretty sure there is a cam/lifter issue, but would still like to perform a compression test. The issue is, I have the oil pan off as well as valve covers and the timing cover is off and I'm down to the timing chain and gear. So my question is, I read somewhere that if I squirt some oil down in to each cylinder, that would suffice and I could do the compression test. Is this true or not? Will I be able to squirt some oil down the cylinders and still perform the compression test? T.H.I.A.

 

A compression test is a dynamic test.What you could get away with is a leakdown test,provided the heads are still in place.Pressurize each cylinder and see what the leakdown rate is.

 

So that's a no then. I can't oil up the cylinders, bump the ignition and get a reading?

The heads are in fact still on, never used a leak down tester. Where does that hook to? And to pressurize the cylinder just turn the crank with a breaker bar and a 15/16 socket.

 

It is done with a leakdown tester which is an inline pressure regulator and supply shut off or one way valve before the regulator.Do this one cylinder at a time,and leave the other 7 spark plugs in.You fill the cylinder with 90-120 psi.,shut off the air supply with a shut off valve so no air can escape back thru the line or regulator.I usually do this with the cylinder being tested at BDC.If you don't,the air pressure in the cylinder will force the piston down in the cylinder and the engine will start to spin.Not good with the lube supply gone!Also,be sure to have the rockers off or very loose so the valves are closed.With the supply in the cylinder,note the pressure loss.An engine in top shape will lose approx. 5% or less over 2-3 minutes.An engine OK shape will lose 6-14%.15-22% is significant.The beauty of this test is you can usually hear a leak in the intake or exhaust ports of the heads.If the pressure is escaping past the rings,you will hear it in the crankcase.Hope this helps.Also meant to tell you they are inexpensive at Harbor Freight.They cost about $40.00 for the kit.hereis the part#-Item # 94190.

 

IIRC you want a continuous 100 PSI regulated air supply during the test. if the supply air pressure drops you won't get an accurate leakdown reading

also, if you take out the other 7 plugs you'll be able to ID a blown head gasket by the air coming from another plug hole

 

If you are using an inline check or shut off,why the need for the constant pressure?You are looking for a leak.I guess you keep the engine from rotating by holding it with a socket and bar at the dampener fastener.He stated that he has removed the pan,I don't think he would want that engine spinning with no lube suppy.And they will spin with the spark plugs removed and a cylinder under pressure.

 

a leakdown test is not a 'timed event', you'll have your results in a matter of seconds. the reading will be the same in 5 minutes or 5 hours give or take for engine temp, that's why the inlet pressure is maintained stable

with a two gauge tester the first gauge reads the inlet pressure, 100psi makes it easy to calculate the percentage of loss. if the 1st gauge reads 100psi and the second reads 95psi, you have a 5% loss. if you use a 50psi inlet pressure and the second gauge reads 47.5psi you have the same 5% loss... you'd need to double the reading on the second gauge to compensate for the 50% lower inlet pressure. if you're a math genious you could use any inlet pressure you'd like and do the math, 100psi is just too easy as percentages are based on 100

with a single gauge tester the only gauge will read the loss. if the inlet pressure varies during the test the results are innacurate

if you read at BDC you may be missing some cylinder damage at the top, or getting a reading where the cylinder wear is minimal. i'd expect better readings at BDC on a worn or damaged engine

i wouldn't worry about 1/2 a revolution with no load on an engine that's been run, there's gonna be oil on the crank

bottom line, at this point of being partially torn down a 'poorman's leakdown' would suffice... just use an air line into the plug holes and listen for intake, exhaust or crankcase sounds, take notes and tear it down

 

I believe that in a lot of cases it is not a timed event.A number of failures become pretty apparent quickly.Other tests are timed.Some tests are made to a known performing engine and the test is made as an evaluation of the condition of the cylinder simply by the leakage rate over a given time frame.I won't argue with you over the procedures,as we each have our own.

 

Sorry for taking so long to get back to guys, I had family come in to town unexpectedly, so I'm still at the same point. Thanks for all the help.


But I do I have a question about usage with the leak down. Is this something I just hook up to my air compressor, shoot 100psi in to the cylinder and then figure out what the gauges are telling me?


As for the ppor man's leak down tester...what's the set up look like at the end of the air hose?

 

Building and Using a Cylinder Leakdown Tester

the diagram doesn't show the gauge for the input pressure either on the regulator or between the regulator and the damper orifice