How do you do a compression test? - PeachParts Mercedes-Benz Forum

JordaanDMC-12 · #1 09-25-2009, 03:35 PM

Hey guys, I did a search but didn't find a complete how to, to this subject....I've been wanting to do a compression test on my 220D for a long time but I have no idea how to do this and what tools are required or the numbers to compare with.

The car has 144,000 miles so it shouldn't be that tired, right? Well I know the only way to check for this is by doing a compression check, but I have no idea how to even approach this.. I don't think I even have the right tools. What are steps, tools required and numbers? This would help me greatly. Thanks!

Graplr · #2 09-25-2009, 03:43 PM

Here is a compression tester for $25-
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93644&xcamp=google&utm_source=googlebase&utm_medium=cpc&zmam=33951326&zmas=12&zmac=112&zmap=93644

Most people check it at the glow plugs vs. doing it at the injectors. I believe most do it with the engine warm.

You simply, one cylinder/glow plug at a time, replace the gp with the tool.

JordaanDMC-12 · #3 09-25-2009, 03:47 PM

Quote:

Originally Posted by Graplr

Here is a compression tester for $25-
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93644&xcamp=google&utm_source=googlebase&utm_medium=cpc&zmam=33951326&zmas=12&zmac=112&zmap=93644

Most people check it at the glow plugs vs. doing it at the injectors. I believe most do it with the engine warm.

You simply, one cylinder/glow plug at a time, replace the gp with the tool.

Awesome, thanks so much!

Sounds simple enough

bigblockchev · #4 09-25-2009, 04:48 PM

Be sure to wire down the stop lever while cranking or you will start it with the guage installed, very bad for the guage and you if you happen to be standing in the way. cheers

Junkman · #5 09-25-2009, 05:04 PM

Also look up leak down test before you do any tear down. Compression test lets you know that you have a leak. Leak down lets you know where you have the leak. It is very helpful for example to know that you have a bad valve or that the rings are bad before pulling the head or engine.

gatorblue92 · #6 09-26-2009, 06:36 AM

Quote:

Originally Posted by maxjlone

Buy a compression tester at your local auto parts store ,take out one spark plug, screw the thing in there, turn it over, and repeat for each cylinder. If it is a 4 stroke oil cant get everyere, and if it is a two stroke, and oil does get every were it is running two rich.

only problem is he doesnt have any spark plugs

toomany MBZ · #7 09-26-2009, 10:54 AM

^ Yeah and you want to remove ALL glow plugs or ALL fuel injectors.

junqueyardjim · #8 09-26-2009, 02:46 PM

Yaa, what is this blather about 2 stroke and four stroke. Man, I never heard anything so far out. Anyway, be sure to take out all the glow pugs before you start. Same with the injectors, though I think the glow plug way is easier.

kmaysob · #9 09-26-2009, 02:52 PM

Quote:

Originally Posted by maxjlone

Buy a compression tester at your local auto parts store ,take out one spark plug, screw the thing in there, turn it over, and repeat for each cylinder. If it is a 4 stroke oil cant get everyere, and if it is a two stroke, and oil does get every were it is running two rich.

? please stay away from anything i own

kmaysob · #10 09-26-2009, 02:54 PM

Quote:

Originally Posted by Graplr

Here is a compression tester for $25-
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93644&xcamp=google&utm_source=googlebase&utm_medium=cpc&zmam=33951326&zmas=12&zmac=112&zmap=93644

Most people check it at the glow plugs vs. doing it at the injectors. I believe most do it with the engine warm.

You simply, one cylinder/glow plug at a time, replace the gp with the tool.

i could be wrong, but shouldnt it be done on a cold engine? thats how ive always done gas engines. just like adjusting valves

junqueyardjim · #11 09-26-2009, 03:26 PM

Well I think warm means a recently run engine. Fresh oil on the walls, ect. By the time you get all the glo's out, it won't be hot, might be just a little warm. And that would be a better way to read the compression then on a dead, cold engine that hasn't been run for a while. But you could get a reading either way.

GWNorris3rd · #12 09-26-2009, 03:33 PM

To do the compression test according to the factory manual the engine needs to be at normal operating temperature. On my '83 300D I removed the hard fuel lines and removed all the injectors as the #5 cylinder glow plug is very close to the oil filter housing and very HOT to get the compression tester in and connected (even with the quick connector). I didn't wire the stop leveler so there was a little fuel spray when I cranked the car over (recommend 8 revolutions of the engine to get a good reading). Just remember to use new heat shields when replacing the injectors. You can also squirt 1oz. of oil or Marvel Mystery Oil in each cylinder before connecting the compression tester adapter and cranking the engine over. It's a very easy proceedure and helps you understand the internal condition of your engine.

Graplr · #13 09-28-2009, 10:21 AM

Quote:

Originally Posted by kmaysob

i could be wrong, but shouldnt it be done on a cold engine? thats how ive always done gas engines. just like adjusting valves

This gets debated frequently on here. I believe Mercedes says to do it with a warm engine. Many people on here insist it should be done cold. I don't think there has ever been a resolution but in my opinion the people that know these Mercedes the best say to do it warm.

vstech · #14 09-28-2009, 11:02 AM

there are several different ways to test compression.
as long as it's noted which way it was tested, the results are valid.
a cold engine will have lower compression numbers than a warm oil soaked engine will.
a quick way to see if the motor has excellent compression on a to buy trip is to insist the motor not be started prior to your arrival, and by hand VERIFY the motor is stone cold.
then, unplug the glow plug harness, and crank the motor. if it starts quickly, it's got good compression. if it will NOT start, (unless it's below 30°F outside) the motor has some compression issues!
a cold compression test (done with ALL injectors out, and the IP wired to shut off fuel) will reveal unequal piston compression.
not a valid test to get compression totals though. need warm engine for that.

Shawn T. W. · #15 09-28-2009, 12:45 PM

Quote:

Originally Posted by junqueyardjim

Yaa, what is this blather about 2 stroke and four stroke. Man, I never heard anything so far out. Anyway, be sure to take out all the glow pugs before you start. Same with the injectors, though I think the glow plug way is easier.

Well belive it or not there are BOTH two & four stroke diesels! Detroit made many two strokes, they are screaming machines!!

But your right about the having all the inj or gp's out at the same time when doing a test . . .