Compression Test on Cummins?

[Marshall Mahoney]

I think I have too much blow by. My engines do smoke a bit and I have lost fuel economy. I can see a diesel sheen at the dock at idle (injectors tested fine). I can't do a blow by test or performance checks because my boat is out of the water. So -- I think I could get some good information from a compression test -- at lease a good comparison between cylinders. 3/4 of the info on line says that a compression test on a diesel is a waste of time. I tend to disagree. I searched the topics and is seems that some of you have done compression checks. Your thoughts?

[Ironworker]

How many hours are on your engines and how often do you run them? Do they ever get up to operating temperature for extended periods of time?

[Marshall Mahoney]

Unknown hours (running take outs). 1990"s vintage. I start them at least one every 2 weeks, run them to the fishing grounds (hours) 4-8 times a year and short cruises a few times a year. I run them at 2200 rpm. Noticing the issues in the past 1-1/2 years.

[Bruce]

Without a total time, diagnosing from normal wear and tear is a little tough.
Over the years I've pulled very few diesel compression tests, because when rebuild time came, it was pretty clear. All the engine surveys I did, I never needed to do one.
That being said injection pump wear and timing, boost psi, ive seen many a diesel sheen the water a bit at idle but clear up up throttle increase, air flow, valve adjustment(nobody checks this) are all things that can be checked to.

Excessive blow by is usually apparent out the breather tube and I would do a crankcase pressure test.

[scenarioL113]

If you have excessive blowby then I would absolutely do a compression test. It is not that difficult on the B-series. It will take some time to pull injectors and loosen the fuel lines but it is not difficult to do.

Check you valve lash as well, if you have never checked it then that could be out of whack. Inspect the rockers to make sure they are all traveling properly, basically just compare to make sure nothing stands out.

Some blowby can be OK to deal with for some time. Blowby "can" be tricky to evaluate on the Cummins B if your not familiar with them. I think they all have some to some extent...

The crankcase breather is also in a bad spot and they can be messy and leaky from it. B's are old school engines compared to the new stuff
Are the engines pitched a lot? The "normal oil level" on the stick has been suggested to be kept a bit lower. Go to sbmar forum and Tony Athens will fill you in on why and what to do if that is the case.

[Marshall Mahoney]

That settles it -- going to check compression (right after I get back from my wife's week long family reunion -- god help me). I checked my valve lash. Some were way out but made no noticeable difference. Lot of pitch and roll tied up to oil rigs offshore -high probability of getting water backing into the engines. Anyway, will see what the compression test reveals.

[Marshall Mahoney]

Finished the compression test (cold engine). I could not find official compression numbers, but Googling showed new engines in the 400 psig range and older engines in the 300 psig range. Test gauge is accurate. So stbd is on the low end. Port #6 cylinder way low. Since it is closest to the exhaust outlet, it would be the first cylinder to be exposed to sea water backing into it. I will try a blowby test when I get her launched. Also, looks like I can relieve some blowby by installing a crankcase vent contraption tied to my air inlet like on Seaboard.
Port Stbd
1=320 1=300
2=300 2=285
3=315 3=300
4=315 4=295
5=310 5=295
6=270 6=285

[scenarioL113]

Your numbers are all pretty close across the board. Since you do have blow-by then suspecting tired rings is probable. Excessive carbon build up on valves and tops of pistons could also span equally across the jugs. These engines need to be run wide open for a bit also every trip out in my opinion.
If you are propped correctly then you will not hurt anything running them hard for a few minutes here and there...

see what others have to say...