What is a Leak Down Test?

[Mr Goodfahrt]

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.

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]

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

[TonyFromWestOz]

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]

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

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]

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]

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]

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.

[hey_allen]

With a good light, you might be able to see the piston through the bottom port on the pre-chamber, but I couldn't swear to it.

[barry123400]

Degree wheel installed on the damper. Calculate the other cylinders degree positions is one set up. Top dead centre for # 1 is marked on the damper already as a fixed refference for your calculations. So you can be deadly accurate this way. Fair to say it's a pain in the posterior though.
The only other way I can think on this engine is to set each cylinder to it's start of injector point for each cylinder in turn. I would dry out and use the point at which fuel starts to creep into the injection pumps exit port for that cylinder. By experimentation you then have to figure how many more degrees to turn the engine until top dead centre is found or close enough. I imagine about 14 degrees.
If you use # 1 cylinder you will get the physical distance measurement right plus a usable refference for the other cylinders. Just measure how much further the damper had to be moved to get to top dead centre past the injector pump fuel appearing on number one as it does indicate true top dead centre with a mark.
Mark the damper at the point of initial filling for the other cylinders in turn. Mark the distance to go on the damper that you aquired from your # 1 cylinder effort. Move the engine forward to your new mark will put you close enough to top dead centre for that cylinder.
You have a small degree of latitude here as you can be off a few degrees and the piston will not decend with air pressure. This way you do not need any special equipment.
I am not saying it is impossible to do it totally manually but insanity might develop over it.
Fortunatly I am concerned most the time with only one cylinders deficiency. For that injecting compressed air after fiddling around for awhile to put that piston at the top dead centre position so it does not move only takes a bit of effort.
On these indirect diesel engines I am probably still going to use the well up method on the pump to find it quickly though. I am only going to increase the air pressure until I hear where the air is going as well. Others will have better approaches perhaps.
For random home use I would use the initial fuel entering each exit port on the injection pump for simplicity and usable accuracy. Do not use kleenex to wick out existing fuel in the exit port on the pump. You do not want to add any contaminates at that point. A tiny blast of low pressure air might blow the fuel out instead for example.
It has to be dry to find when fuel first enters remember. Plus since there is no injector pop pressure to overcome even a worn pump is going to be fairly accurate at showing you when flow initiates into that cylinders element. Hope this is of some help as it's about the best I can do. It will work.