Compression Tester setup

Bill Roosa · Mar 3, 2014

I'd go with the auto parts store compression tester. It has a hard rubber boot that you hold over the injector hole while somebody cranks the motor.
Much easier to use and you can do all the cylinders in short order.
FWIW they are not that expensive.

srhummer · Mar 3, 2014

Not sure what tester Bill is using but do watch out for the max PSI that the tester will do. Some of the cheaper auto testers only do 300 PSI since they are made for gas engines. Since your looking at a diesel and diesels do between 16:1 and 22:1 compression - not sure of 2gm cylinder pressure - which may result in pressures of around 500 PSI. Which can be above the ability of the cheap $20 auto test kit. You can pick up a diesel test kit but they do cost more - under $200.

Ron20324 · Mar 3, 2014

IIRC from back in my car-tinkering days of car/gasoline engines, the compression ratio, times 15, gives you the PSI.

Bill Roosa · Mar 3, 2014

My experience is if you can hold the rubber fitting on with +300 psi then the rings are OK (and your nick name should be Conan) and you need to look else where for the problem. Rings do not typically go "half bad". they are either shot and will not hold even 100 psi or the are good.
so start with only about 50 pound of pressure on the tester and the excess pressure will just pop off the rubber and not damage the gauge. if the indicated pressure is below the max just press harder (remember to eat your Wheaties before testing). When the tester indicates 300 psi you can stop increasing pressure as the rings are OK.
Now if you are trying to document the pressure for future use in indicating a change the you certainly need a higher pressure tester.

203 · Mar 3, 2014

Ken13559 said:
I've the US General Compression Tester for Diesel. I think it goes up to 1000 psig. I expect my diesel to be in arrange of 350 - 450 psig. The key thing I believe is to remove all entrials of the spare Injector so that nothing blocks the pressure from reaching the gauge. Right?

This is correct, you must get to the point that nothing blocks the pressure from reaching the gauge. I'm under the assumption that the US General device has a threaded connector that will tightly seal to the spare injector ( gutted ). Taking your injector apart should give you a good idea of what path the pressure must take to get to the gauge.

My experience has been with Mercedes diesels, not marine motors, but all are about the same. If there is concern about messing up your spare injector, you can get a bad one from the diesel shop close by and not be concerned about ruining it. The Bosch injectors I messed with required some modification to fit the hose with my tester. I ended up drilling one out and building an interface for the fuel line to my tester.. I don't remember the exact reason I had to drill it out, but it was not just a situation of taking out insides. Yours may be different.

I finally ended up with a 'leak down' tester. Much cleaner and easier on the starter. Oh, and don't remove all the injectors in an effort to lower the load on the starter when you use your compression gauge. It's a bad idea. Please don't ask how I know... bwahahaha

A good dissertation on a leak down tester is at

http://en.wikipedia.org/wiki/Leak-down_tester

You still have to have an injector ( or glow plug body if your motor uses them ) that will pass the pressures to the cylinder.

Don't be too surprised if your numbers don't add up. All cylinders should be roughly equal.. like within 10% or so,but you may be way low of the 300+ PSI you expect. If there is a huge difference between cylinders then there may be an issue. Diesels are notorious for giving bad readings on leakdown and compression tests thanks to the high pressures normally encountered in operation. If you are concerned about the reading, squirt some light machine oil in the cylinder and rotate the engine then retest. If the numbers come back better, then consider doing the test at or close to operating temperature and see how that affects your outcome.

Richard

203 · Mar 3, 2014

So I went and looked at the injector for your motor.. Figure 34 on

http://j30.us/files/Yanmar-2GM20-Parts-List.pdf

This brought back part of my memory.. what are you going to do with the return line?? I had to block the return lines on my 'scrap' injector. This was one of the modifications that made the injector useless to regular service after modification. I plugged the return ports with an oxyacetylene torch and a bit of brass, as I remember.

203 · Mar 4, 2014

It should be fine. Yes I removed all the internal pieces for a clear shot thru the injector. In my case that was springs, shims, a needle valve looking assembly, etc. It all came apart by unscrewing the injector with the Bosch units.

203 · Mar 5, 2014

That looks accurate. That injector is a lot easier to 'modify' than the Bosch unit that's in Mercedes engines. Mercedes uses a press on return line, no clamp or threads. It's a small hose that is constantly getting baked by the heat and always leaking (c: . On the Bosch injectors, the return fitting must be sealed off, and since there's no threads etc, it has to be done in a non-reversible manner.

woodster · Mar 5, 2014

i have a question about this testing......are you going to check the compression on each cylinder with the compression release on the open position or the closed position on the other cylinders....

regards

woody

MSter · Mar 7, 2014

I think each cylinder/piston chamber should be a sealed unit if rings etc in good condition. The other cylinders should not impact on the test results. Therefore it would be OK to use the decompression lever to help the engine spin better.
Mster

203 · Mar 12, 2014

I referred to the owners manual

http://www.yanmarmarine.co.uk/pdfs/owners_manual/128270-4E.pdf

( which has a very useful troubleshooting section in the back )

and the engine service manual

http://www.yanmarmarine.co.uk/pdfs/service_manual/EPB5494.pdf

and on page 2-26 of the latter book there is a diagram of the decompression system. It appears that you might be able to remove the link rod between the decompression levers and thus have decompression on the cylinders not being tested, while having full compression on the unit under test. I guess with your motor ( 2 cylinders ) that's not so difficult to do.

Since you are looking at about 4 or so compression revolutions of the motor to get a useable reading with a compression gauge, I'm not sure it's worth the effort to disconnect the linkage. But I'm a lazy sort, as well (c: ..

Note that a compression test is not 100% effective on a diesel ( I've mentioned this before ) and will only give you some relative numbers. The leakdown test mentioned earlier will give you information that is at least as useful as compression testing, and you wont be spinning the motor for the test, and the decompression levers would remain in the 'run' mode all the time.

One advantage of leakdown tests is that you have at least a bit of diagnostic information provided as a function of the test. If you have burnt/bent exhaust valves ( heaven forbid ) then you'll hear the hissing in the exhaust of the engine. If an issue exists with intake valves, you'll hear something going on in the vicinity of the air intake. If the issues exist with rings, the hissing will come from the oil fill cap.

And of course, if you just like playing with diesels ( this is my situation ) you'll end up with a baseline of info about your motor for future troubleshooting.

Compression Tester setup

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