Thread: Correct procedure for doing a compression test on a 300Tdi

disable the FIP by disconnecting the solenoide
crack all 4 injector unions
remove all 4 glow plugs
install pressure tester

crank engine, note peak reading after X cranks
take reading on all 4 pots at X cranks.

make sure you have a good battery.

Originally Posted by uninformed

Hey all,

I have a compression tester, I have an adaptor made out of an old gutted injector.

What is the correct procedure to do a compression test?

From your post, it appears that you have the gear to test using injector holes instead of via glow plugs.
So do as Dave advised above but remove the four injectors instead of the glow plugs.
Also take two readings for each cylinder and crank for same number of revs each time.

While you're at it you should also do a 'wet test' by squirting a small amount of oil into each cylinder before repeating test readings. The oil will 'seal the rings and if worn or damaged will result in a significantly higher reading than obtained in the original (dry) tests. A wet reading similar to the dry reading should result if rings are OK.

Originally Posted by Xtreme

From your post, it appears that you have the gear to test using injector holes instead of via glow plugs.
So do as Dave advised above but remove the four injectors instead of the glow plugs.
Also take two readings for each cylinder and crank for same number of revs each time.

While you're at it you should also do a 'wet test' by squirting a small amount of oil into each cylinder before repeating test readings. The oil will 'seal the rings and if worn or damaged will result in a significantly higher reading than obtained in the original (dry) tests. A wet reading similar to the dry reading should result if rings are OK.

Is there a particular reason for removing the injectors instead of the glow plugs (as Dave said), I have only ever removed the glow plugs to save the seals on the injectors, in case everything is OK with the compression, Regards Frank.

Originally Posted by Tank

Is there a particular reason for removing the injectors instead of the glow plugs (as Dave said), I have only ever removed the glow plugs to save the seals on the injectors, in case everything is OK with the compression, Regards Frank.

Just that 'uninformed' stated in his first post that he had an old injector adapted to do the compression test. I can't see that it matters whether you connect your gauge via injector hole or glow plug hole - dependant on access and what you have to connect the test gauge.
More common to do it via the glow plug hole due to more univesal adaptors that are available for there and avoiding the need to replace the injector seals. However, if compression testing a Puma, access to the injectors is a lot easier than access to the glow plugs.

Originally Posted by Xtreme

While you're at it you should also do a 'wet test' by squirting a small amount of oil into each cylinder before repeating test readings. The oil will 'seal the rings and if worn or damaged will result in a significantly higher reading than obtained in the original (dry) tests. A wet reading similar to the dry reading should result if rings are OK.

You should most definately NOT do this on a diesel....

especially not a direct injected diesel with an in piston combustion chamber that will fill first before any oil you can drop in down the glow plug or injector holes gets to the rings and that has a piston protrusion above the deck of the block.


you wouldnt use this method of compression testing on the Puma, and you DEFINATELY wouldnt test the puma by removing the injectors because that would require you to break the HP side of the common rail injection system which IF you are lucky would only require you to replace all the injector lines and sealing unions and if you are unlucky would give you an injection of diesel at about 10,000PSI....

Originally Posted by Blknight.aus

You should most definately NOT do this on a diesel....

especially not a direct injected diesel with an in piston combustion chamber that will fill first before any oil you can drop in down the glow plug or injector holes gets to the rings and that has a piston protrusion above the deck of the block.


you wouldnt use this method of compression testing on the Puma, and you DEFINATELY wouldnt test the puma by removing the injectors because that would require you to break the HP side of the common rail injection system which IF you are lucky would only require you to replace all the injector lines and sealing unions and if you are unlucky would give you an injection of diesel at about 10,000PSI....

Interesting comments - thanks Dave.
Especially having witnessed compression test (both dry and wet) being done on Puma yesterday.

Originally Posted by Xtreme

Interesting comments - thanks Dave.
Especially having witnessed compression test (both dry and wet) being done on Puma yesterday.

probably by some stripey apron wearer who was either

A, using someones tools that werent his
b, didnt care about his tools
and
c, shouldnt be allowed to play with common rail stuff if he didnt have all the right gear to do it in the first place.

Most commonrails (in fact most modern electronically managed engines) can give you a "good enough" compresion test if you use the proper diagnostics gear. essentially it cranks the engine over on the starter and reads the changes in crank velocity as each pot comes up on compression does some maths and gives you a reading.

If youve got a fast enough cro and you only want to make sure all the compressions are even you can do the same thing by reading a winding off of the alternator and watching the spacings on the spikes.

if you want to do the "wet" test on a diesel you start it up, run it till its warm and then go again.