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Diesel This is a new forum dedicated to diesel engines and their applicable accessories.


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Old 05-11-2014
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Compression test 2GM20F

I tested the compression of each cylinders of my 2GM20F. Cylinder #1 270psig and Cylinder #2 250psig. These numbers seems way too low as expected is 370-450psig. I should have squit some oil into each cylinders before trying again to see if the pressure will come up. If it does I suppose it means my piston rings are wore. But I forgotten about this step. I'll schedule anither day for this test again.
Comments anyone?
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Re: Compression test 2GM20F

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Originally Posted by trantor12020 View Post
I tested the compression of each cylinders of my 2GM20F. Cylinder #1 270psig and Cylinder #2 250psig. These numbers seems way too low as expected is 370-450psig. I should have squit some oil into each cylinders before trying again to see if the pressure will come up. If it does I suppose it means my piston rings are wore. But I forgotten about this step. I'll schedule anither day for this test again.
Comments anyone?

Have not had this experience, but have read that it might fire if you squirt extra oil in the cylinder. Maybe warm the engine up a bit before taking the readings?

If it starts OK when cold, the compression may be OK? But if it is below spec it may be an indication that the rings are worn or the valves are leaking.

I have read that an oil analysis may show where a problem may be? It would be interesting to know what you find.

Paul T
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Old 05-11-2014
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Re: Compression test 2GM20F

That the two cylinders have roughly the same compression is more important than the compression ratio per se (if you find more than a 10-15% difference, then I would worry). Getting 270 psi means that you still have about 18:1; not bad for an old engine. Try the "wet" test and see if they are still pretty close to each other. If so, you're probably OK.
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Re: Compression test 2GM20F

Puddin', "wet test" by that I suppose you meant squirting abit of engine oil into the chamber. I'll do that.

Paul, I've no problem cold starting. Cold here in SG is ...like 25degC. I guess that's pretty warm for some sailors. Day temperature is 30 degC in shades and 35degC under sun. Anyway next time will 1)warm up engine before test 2)Squirt some oil then test again.

Appreciated your comment/reply.

Lesson learnt:
Remove only the Injector of cylinder to be tested. Rest of Injector/s to be left tighten inplace. I made a mistake of remove both Injectors first then run test on one. In 2-3 cycle of engine chamber gaskets, insulators and all blew right out of the other cylinder and include a fair amount of carbon dust
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Re: Compression test 2GM20F

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Originally Posted by trantor12020 View Post
Puddin', "wet test" by that I suppose you meant squirting abit of engine oil into the chamber. I'll do that.
...
That is exactly what a "wet" compression test is. If there the engine has really low compression, and there is a huge difference between the "wet" and "dry" compression, then the rings are suspect. But given the relatively high "dry" compression that you're getting, I wouldn't worry too much about it.

Why did you do a compression test in the first place? Is the engine running badly? If it seems to be running OK, I would just put everything back together and leave it alone. If it ain't broke, don't fix it.
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Re: Compression test 2GM20F

I'm not getting the power I expect of the engine. Most of the time, I've to run at 3000rpm to make 5kts in slack tide. I've tried changing props several time (pitch & diameter) and in the end summise that I probably have some wore rings. I do return couple of years back, I made a 500nm passage (engine most of the way) and at the end of trip found the oil level to be alomost zero. I could only drain out 0.5-1 litre. I could have abused the engine on that trip. That's my suspect.
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Re: Compression test 2GM20F

bingo thats a lot of oil

suspect rings...
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Re: Compression test 2GM20F

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Originally Posted by trantor12020 View Post
I'm not getting the power I expect of the engine. Most of the time, I've to run at 3000rpm to make 5kts in slack tide. I've tried changing props several time (pitch & diameter) and in the end summise that I probably have some wore rings. I do return couple of years back, I made a 500nm passage (engine most of the way) and at the end of trip found the oil level to be alomost zero. I could only drain out 0.5-1 litre. I could have abused the engine on that trip. That's my suspect.
I think you're mistaken by blaming the engine.

All the engine does is spin the prop. If it's doing that but you're not getting good performance, it's not the engine's fault. Full throttle is usually 3400-3600. If you can get there, then you don't have power problems with the engine. Period.

Clean your prop and hull and cut down weight.
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Old 05-12-2014
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Re: Compression test 2GM20F

500 naut.miles would be 100 hours at 5 kts. A 2GM20 only holds 2 liters of oil in the crankcase. So, you burned about a liter, maybe a bit more, in 100 hours. That's probably more than a new engine will burn, but not a huge amount. I would keep a closer eye on the oil, check it every 24 hours or so of run time. And try running as heavy an oil as the manual recommends. In Singapore I would think you could run at least 40-weight, maybe 50-weight (but check the manual). In any case, avoid multi-grade oil, as it tends to "thin" more easily over time. And make sure you are using diesel engine rated motor oil, NOT oil for gas engines.

Also, I agree with asdf38. If you can get the revs up, but the boat isn't getting close to hull speed, it's not the engine that's the problem. A 32' Hunter shouldn't displace enough to over-burden a 2GM20, IF everything else is OK (if she's seriously over her designed displacement, that's another story). What size/pitch prop do you have? Are the bottom and prop clean? Is there something else on the bottom that is making it "un-fair"? Are the clearances between the prop and the hull, and the prop and the rudder adequate? Were you towing a dink? Was there a serious headwind?
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Re: Compression test 2GM20F

Prop I used are: stock 2RH 15"Dx10"P, CSP 3RH 14"Dx9"P, & 7"P, & 6"P, KiwiP 3RH 16"Dx20deg, & 18deg.
I've tried SAE40 and SAE15W40. I don't find any difference. I've tried adding STP Oil Treatment (Silicone base). It helps raise max rpm by 100-200rpm.
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