What is a Leak Down Test? - PeachParts Mercedes-Benz Forum

Mr Goodfahrt · #1 04-27-2004, 11:24 AM

Newbie question.... someone asked me this and I didn't know the answer.
What is a Leak Down Test?... I know there is one for the cooling system and that vacuum sytems can leak, but what is a leak down test for the engine and how do I do it? I am about to do my 1st valve adjustment and compression check this week and would like to know if it is something I can do at the same time.

My main symptom in my 82 300D is large blow by and oil consumption... 500 miles - 1qt. This has gone down since I replaced small seal under air filter housing, but still is high.

Starts well when cold, 28mpg, 200K+ miles. No idea as to injector condition, timing chain condition, etc.

#2 04-27-2004, 11:41 AM

The leak down test involves applying pressure to each cylinder while valve is closed. The percentage of pressure lost is monitored.

The most valuable part of a leak down test is in the case of a low compression cylinder. You can apply the pressure to that cylinder and listen for where the pressure is leaking. If it is heard through the oil filler cap the leak is past the rings. If heard at the intake manifold, the leak is an intake valve. If heard out the exhaust, the leak is an exhaust valve.

Hope this helps,

leathermang · #3 04-27-2004, 12:40 PM

Have you considered replacing your valve stem seals ? Not TOO hard... the seals are chaep..... but might make a lot of difference....

TonyFromWestOz · #4 04-27-2004, 12:40 PM

Quote:

Originally posted by dieseldiehard
I thought if you hear the leak through the valve cover it could be a loose or worn valve guide (that is the problem I had in a previous engine)
here is a link to a good leakdown tester at a good price:
http://www.thetoolwarehouse.net/shop/STR-CLT2PB.html
DDH

As the test is done with the valves closed, there is no way the compressed air should be able to get to the valve guides - unless there was a valve sealing problem as well.

JT8Mech · #5 04-27-2004, 01:52 PM

This is also used on aircraft piston engines to identify air leaks/compression loss. Brown Tools

#6 09-21-2004, 03:01 PM

WIth an auto trans, how do you prevent the engine from rotating due to the pressure on the piston? With a manual, you could stick it in gear.
Steve

JT8Mech · #7 09-21-2004, 03:20 PM

considering MOST leak down testers only run about 100psi and a typical diesel will run upwards of ... what 300PSI? Besides, I don't know many people that have an air compressor that will put out that much.

Brian Carlton · #8 09-21-2004, 03:40 PM

100 psi would exert 829 pounds of force on a 3.25 inch piston. This would be more than sufficient to drive the piston to the bottom of its stroke. However, at the bottom of its intake stroke, the intake valve would still be open a bit and, at the bottom of the power stroke, the exhaust valve would be starting to open a bit.

It does appear that you would need to prevent the engine from rotating in some fashion.

Tirebiter · #9 10-01-2004, 08:07 PM

If you set the piston at TDC the air pressure injected will not turn the engine. A degree or two off and it will move.

#10 03-16-2006, 04:30 PM

So is there anyone who has actually done a leakdown test and can comment on whether it is necessary to lock the engine somehow? I can't see how you can get each cylinder at exactly tdc and then apply 100+psi - the engine position will be out by a little & the engine will rotate - that's what I think - but i've never tried it.

Has anyone tried it ? Do you have to fiddle for hours getting 'exact' tdc - or am I making a lot of fuss about nothing!

Steve

krs · #11 03-16-2006, 04:55 PM

Quote:

Originally Posted by sdelasal

So is there anyone who has actually done a leakdown test and can comment on whether it is necessary to lock the engine somehow? I can't see how you can get each cylinder at exactly tdc and then apply 100+psi - the engine position will be out by a little & the engine will rotate - that's what I think - but i've never tried it.

Has anyone tried it ? Do you have to fiddle for hours getting 'exact' tdc - or am I making a lot of fuss about nothing!

Steve

No, it's not fuss about nothing, Steve. No question is. TDC can be found by observing the valves (with the cover off), and by looking at the piston if the injector hole allows. To do it with any precision means a dial indicator is mounted to measure the precise moment of movent for the valves or the indicator is used with usually special tools that allow a setup to measure the exact position of a piston. Which method works best depends on the particular engine.

Or an engine can be immobilized too by a variety of methods. One uses a specially designed tool that will lock against the flywheel or torque convertor ring gear and bolt (usually) to the bellhousing.

For diesels particularly the method is often a factory procedure using factory designed tools that only have that purpose.

i'm answering with gneralities because each engine probably requires it's own instructions. But yes, it can be done without what you said.

#12 03-16-2006, 05:04 PM

Okay - anyone actualy done it on a 300d 617 engine with auto trans?

I have not even managed to get a socket on the crank end - I use the PAS pully to rotate the engine!

Steve

dieseldiehard · #13 03-16-2006, 05:13 PM

I've only done a leakdown on a gasser. Its a lot more difficult than taking a compression test but worth the effort
Here is a link that describes the process pretty well. and yes you have to be right at TDC or the piston will move and your back to looking for TDC!
I would not use the PS belt to turn the engine its not that precise, a 27mm on the pulley is much more "in control" and I've turned them backwards a few degrees with no ill effects. YMMV. (Please don't try that yourself and say I said it was OK!)

http://vmaxoutlaw.com/tech/leakdown_tester.htm

dieseldiehard · #14 03-16-2006, 05:16 PM

Quote:

Originally Posted by krs

No, it's not fuss about nothing, Steve. No question is. TDC can be found by observing the valves (with the cover off), and by looking at the piston if the injector hole allows. To do it with any precision means a dial indicator is mounted to measure the precise moment of movent for the valves or the indicator is used with usually special tools that allow a setup to measure the exact position of a piston. Which method works best depends on the particular engine.

Or an engine can be immobilized too by a variety of methods. One uses a specially designed tool that will lock against the flywheel or torque convertor ring gear and bolt (usually) to the bellhousing.

For diesels particularly the method is often a factory procedure using factory designed tools that only have that purpose.

i'm answering with gneralities because each engine probably requires it's own instructions. But yes, it can be done without what you said.

The OM603 uses a flywheel locking device to remove the crank dampener. I don't know if its all that necessary for a leakdown test but its possibly a timesaver because if you lose the TDC (were off a few degrees) and air pressure moves the piston down you have to crank it again and shoot for another TDC (PITA)