Quote:
Originally Posted by MB140300SD
Doing a compression test to know if you should keep it or sell it is a total and complete waste of time. As I mentioned before, you are doing a compression test every time you start it cold. If it starts fine, you have good compression. If it is hard to start, then you need to make sure the valves are adjusted properly, there is not too much chain stretch, you have good filters with no air getting into the fuel system etc. After working on and owning hundreds of MB diesels over the course of 30 years, I have only checked compression on them 5-6 times if that. When I did it only confirmed what I already suspected after all of the above was done.
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Random opinions.
It depends on what you find during the compression test. In the past some found that they had cylinders lower compared to the rest and were beyond the balance spec between the cylinders.
Some that found that condition instilled something inside of the cylinders to free up any positions rings that might be stuck. That worked for some but not all. For a few the compression on all cylinders went up.
However, the cylinder soak thing takes down time of a week or more.
I did not take the compression because in the past I had rebuilt the sort block on my Volvo diesel, boring out the bores to oversized pistons so I my case I knew the compression was not an issue.
However, it sat unused over a year and when I got it started, I had hazy Gary smoke at all speeds and worse on acceleration. I took a chance that it was sticking piston rings and instilled Marvel Mystery Oil into to the cylinders for one week (threw the glow plug holes), I rotated the engine and did it again and was supposed to go for another week but had to get it running after 3 days.
I cranked the engine to blow out any excess MM oil and installed the glow plugs and drove from my back yard to the driveway and did an oil change. You don't want the marvel mystery oil diluting your oil. Remember the product has a history of use on gasoline engines but not on diesels.
Took the car out on the freeway and drove it hard with some decrease in the gray haze. It took 3 more days before the haze was gone.
I did the same on the Mercedes and it had no effect at all. Apparently, I had no sticking rings.
On the Mercedes because I did a compression check I know that #5 cylinder is below the balance between cylinder specs. That was many years back.