I know that alot of gearheads perform compression test to get a profile of the cylinder leakage and condition. I had asked about doing a leakdown test like they perform on small aircraft engines. This is supposedly the most accurate because it is not subjective to starter speed and oil splash in the lower cylinder bore. I had mentioned this in a later post.
Leathermang cautioned about having it at Bottom dead center, because it could turn over, but to perform a leakdown test, both valves have to be closed, and the piston at dead top dead center. This way the engine can't roll over. You can also check for exhaust and intake valve leakage by listening,by the exhaust or intake and you can check excessive ring blowby by checking the oil filler cap. With leakdown test it is not subjective as in a compression test, because of how fast the engine is turning over with the starter as you perform each cylinder test. Leakdown is a method of putting a fixed amount of air to the cylinder at T.D.C. and recording the difference of leakage from cylinder to cylinder. You can also look for the above. For instance, you set a regulator to regulate at 100 PSI with the end blocked off. You hook it up to the cylinder and you record how much the gauge is when hooked up to the cylinder. It should not drop more than 20% for gas engines and should not have a large deviation from Cyl to Cyl.
I am just asking. Maybe some experienced M.B mechanics know a better method and that may be a compression test, and know why.


PeterG



79 300 SD

I also stressed that it should be MECHANICALLY LOCKED (bolted solid) .... if you did it in the Top position...for YOUR safety....

On normal (read pushrod) engines it is easy to loosen the pushrods tension (thus closing all valves) and do it at whatever piston position you want to ....

I assume this can be done on our engines.. but the amount of work compared to any advantage a leakdown test has over a normal (properly done) compression test might negate the effort....

Where did you get the idea that having the valves closed would prevent the engine from rolling over ? If the crank is not perfectly centered (and unlocked) with respect to the force you are putting on the top of the pistons with the compressed air... it WILL TURN.... and whichever direction you are off from exactly TDC... and our engines are NOT supposed to be rotated backwards.

The amount of force is easy to calculate... in general terms... the diameter of the piston divided by two multiplied by itself times 3.14 times (let say ) 100 psi....

So a four inch piston with 100 psi on top of it would be about 1200 lbs of force !!! That is why it is so important to lock the crank if you do it an anything other than BDC...

A leakdown test is a great test...because it gives you more specific information as to problems..... I remember Smokey Yunick suggesting it in Hot Rod magazine as far back as the 1960's or 70's.... It is just much more work than the pretty darn useful compression test results ..... but which only gives you an overall condition indication.

OK, LOL , This is my last edit of this post.... it seems to me that the logical thing to do would be a compression test first... if no problems are indicated go on about your business... if anything is askew then do the leakdown to pinpoint your problems...

QUOTE]Maybe some experienced M.B mechanics know a better method and that may be a compression test, and know why[/QUOTE]


Thanks leathermang you answered my question. I forgot about the engine rolling backwards. I will do a compression test first.



PeterG


79 300SD

Peter, My pleasure. What we need is a section ( a few inches long) of ring gear bent backwards.. just a section long enough to fit into the torque converter access panel.. which would mesh with our starter ring gears and bolt to the car...like the VW tool....