OM603 compression test result questions
 

Hi all,

In preparation for my cylinder head R&R, I thought I'd do a compression test before ripping it all apart and possibly spending $xxxx on a new head. I was not thrilled with the results (below). Engine was warmed (car driven) to 90C, shut down, injector lines removed. It was an hour or so before I actually began the compression testing, which then took another 30-45 minutes (for two runs.) I let each cylinder pump until the needle didn't rise any further. Here's the numbers:


Test run one:
====================
Cylinder #1 - 24 bar
Cylinder #2 - 28 bar
Cylinder #3 - 27.5 bar
Cylinder #4 - 25 bar
Cylinder #5 - 28 bar
Cylinder #6 - 27.5 bar


Test run two:
====================
Cylinder #1 - 25 bar
Cylinder #2 - 27 bar
Cylinder #3 - 27.5 bar
Cylinder #4 - 25 bar
Cylinder #5 - 30 bar (checked a third time and it was 28 again!)
Cylinder #6 - 28 bar


Service manual specs:
=====================
Normal pressure: 26-32 bar
Minimum pressure: approx. 18 bar
Permissible difference between cylinders: 3 bar


Now, I wasn't overly thrilled with the numbers, I was hoping for closer to 30 instead of 25-27. But what concerns me more is the variation between cylinders, which was 4-5 bar! Well, if you toss out #5 on the second run as an anomaly, it's just within spec. But on the first round it was 4 bar. What do y'all think? Would the engine being cooled down for an hour or more make that much of a difference? I can't see how it could be done much faster, maybe 30-45 minutes at warp speed...? The temp gauge was 50-60C during the test, I think (wasn't paying attention at the time.) I put on the battery charger a couple times while swapping to the next cylinder, but cranking speed never seemed to be an issue.


Side note: The stupid service manual neglected to mention that although holding the "Stop" lever down prevents fuel from being released through the 6 injector lines, it DOES allow fuel to be pumped out the return line! You know, the little one from injector #1 to the fuel filter? I didn't notice (or locate the source) until the end of the second round, after a 3-foot diameter diesel puddle appeared under the car. AAARRRRGH! Blocking the line should force it out the return to the tank. Details, details.... **sigh**

Anyway - should I be worried about the results? Particularly cyl. #1 and #4?


Thanks for "listening",

Dave M.
Sacramento, CA
1987 300D - 229kmi (just got compression tested)
1987 300D - 236kmi
1984 300D - 207kmi

although not the greatest results ive seen, by far not horrible. this engine definently has some time left on it. I wouldnt worry about the 4 bar differences.. or the numbers.. they are in the middle of the tolerance and your engine is in all likelyhood fine for another 200,000 miles.

If you suspect a bad head, these numbers aren't too spooky -- after all, if you do have a cracked head, compression will be a little low on that cylinder, with no leakdown indications or bad rings or bad valves. Ditto for a bad head gasket, not unheard of at your milage.

I'd expect some increase with a valve job anyway.

Peter

Hi Peter,

Yeah, that's kind of what I'm thinking too. After the valve job (or possibly new head + valves!) I'll have to re-test it. I just have a feeling the problem is the rings, though. Although I don't much burn oil (1 quart per 7,500 miles), there was a very noticeable amount of oil in the charge pipe when I removed it. My EGR is disconnected so the only source would be the crankcase vent into the intake elbow.

I'd love to do a leakdown but don't have the tools, time or $$$ to fiddle with that, plus the numbers aren't "bad enough" to warrant it IMO! :) If I ever get the head job done I'll post again with the "new" numbers! Sure would be nice if it is just the valves...


Regards,

Dave:

Valve guide seals going bad will also make oil appear in the turbo inlet -- the exhaust gas blows out past the guide and makes oil mist. Much less trouble on the 603 than on previous engines since the spring is covered by the tappet, but 1 qt in 7500 miles isn't much! Better than the 1 qt in 200 on the 220D!

Peter