View Full Version : Yanmar 2GM20F Compression Check

10-21-2007, 02:18 PM
How do I check compression for the yanmar engines? What size fitting do I need?

10-21-2007, 08:35 PM
"since there are no glow plugs in the yanmar, you must use a "dummy" injector in place of the regular one. make sure the stop cable is pulled to avoid spraying fuel all over your engine room while cranking."

10-21-2007, 09:03 PM
"Thanks for the info, but where can I get the dummy injector? I'm pretty certain I've got a compression problem. I picked up a diesel compression tester from NAPA, but it does not have the right fittings(size) Does anyone know of somewhere online or locally where I can order a compression tester for this engine? I would ilke to keep it on the boat at all times as part of my spare parts kit.

My next step is to test compression and determine of the loss is a result of rings, or valves. I do have fuel flow, and both injectors check out.

Thanks in advance."

10-21-2007, 09:41 PM
"check with your local dealer, or distributor. I made my own out of an injector that was no good. snap-on might have, but probably pricey. might be able to steer you to something if I knew where you were. wish I could be more help. S."

10-21-2007, 10:36 PM
"I'm in Clearwater area, I keep the boat in Fort Pierce, FL accross the state.

I think I understand the idea, not sure where I can find a spare injector, but I would need to remove the injector from the houseing, plug the fuel line, then I would be able to utilize the threads on top of the injector houseing where the bleeder line connects. Only concern I have is that I would not have a one way valve, and as such, would have to watch the guage as the engine is turning over.

Thanks for all the help..."

10-23-2007, 01:16 PM
"Let us know how you eventually accomplish this.

FWIW, Peter Compton warns (in "Troubleshooting Marine Diesels") that the presence of the dummy injector, the length of connecting hose, and the gauge itself all affect the compression test outcome -- i.e., it's hard to arrive at "absolute" values for diesel compression.

He suggests taking an average, and that the values derived are comparative, rather than absolute. He recommends the test be performed by an experienced technician who has a "feel" for how diesels behave with his/her equipment.

That all seems to make sense. I'm all advice and no experience, but I was thinking I'd set myself up to do my own compression tests, and first time out I'll compare my values with those of an experienced guy, to establish my own personal baseline."

10-23-2007, 01:54 PM
"Yes Robert, Paul has good points here. have it done, along with a "tune up" Change filters and adjust valves to .oo8. Yanmar say the compression is about 460. We have never seen it that high.We have found if all are around 390 to 410 ie all the same within 10 psi thenn you are off to the races. Good luck and get your self a Yanmar service book, well laid out and well worth the bucks and a great read."

10-23-2007, 09:33 PM

Many Many thanks for the advice. I always love advice and other peoples opinions. It can only help to establish a greater knowledge base.

I am of the mindset, that I would like to understand my diesels inside and out. While I'm at the dock, if anything happens, it is easy to make a call to the mechanic, however, I'm usually off sailing for months at a time, and things always seem to go wrong at the worst time. It's pretty darn expensive to get a mechanic in a foriegn port or 100 miles offshore.

Thus far, I have found and ordered the yanmar adapter($120) for doing compression checks. I've also ordered the service book($76), and both are expected to arrive tomorrow.

Thanks again"

10-23-2007, 09:48 PM

Just read through the post, and realized that I didn't ever really explain what the reason for the compression check was.

So if anyone is curious, this is what is happening.

The forward cylinder (furthest from flywheel) is losing compression. The engine starts in about 10 or even 5 seconds(fires right up), but will only do so if I have the compression lever off on the first cylinder. It will start with the compression lever down on first cylinder, but requires about 1 minute of turning the engine over. Above 1000 RPM's the first cylinder will begin to ignite, at 1200 rpms, it runs super smooth, and both cylinders are firing consistently. Both injectors appear to have a nice spray pattern, misty circle, and I have rotated the injectors so that #1 cylinder is using #2 injector, and vice versa, so I'm pretty certian that fuel is not my problem. I have plenty of fuel hitting both cylinders.

The engine has been sitting for approx 1 to 2 years since it has last run. Currently, the total hours on the engine are 1700, so it's seen some use, but nothing excessive.

In my opinion, there are two likely problems, possibly others,

#1 would be loss of compression due to pitting on the valve seats, or valves, probably on the exhaust for cylinder 1.

#2 would be a stuck, or frozen ring.

Compression test would be crucial in determining my next step, do I remove the head for rebuild, or do I look at cylinders, and install new rings."

10-24-2007, 08:00 AM
"I would adjust valves at .008 because that will effect timing. Get your unit started and then place in gear at about 1500 rpm "in gear". Better yet is take it for a run. This engine is designed to run at 3000 rpm max 3600 rpm, see if it will do this. Remember all "in gear" forward. Sitting is not good and using that engine to shage batteries is a no no. It must have a load.
Check the exhaust elbow and make sure they are clear of carbon. The goodneck elbow love to get blocked up especially if you run a light loads. Light loads cause glazed cylinders and you will need a re-ring and hone. This may clean it up"

10-24-2007, 09:55 PM
"OK, everyone, again thanks,

I've finally checked compression, as well as recieved the service manual. Time to do some reading.

At present, the #2 cylinder (the one I believe to be losing compression) reads at 410psi, while the #2 cylinder reads at 400-405psi.

Guess it's time to go look at fuel again. As this seems to be the only thing that would make sense. I suppose timing could have something to do with it as well, but i'm not sure yet."

11-01-2007, 06:05 PM
"Make sure your stop cable is not on partial. Also check that elbow. You can remove the injectors and have them tested, and the pressure adjusted to specs. Get new injector insulators and packings. You will have to dig the old ones out."

11-06-2007, 04:55 AM
"Just to give everyone a heads up, I had to do the following, and the results were as follows.

Contacted mechanic from yanmar, and he suggested a different compression tester. I purchased compression tester suggested, and found that compression on both cylinders was really around 470 on both cylinders.

With this infomration, looked at fuel system. and sure enough, my problem cylinder was not getting enough fuel. It was getting some, but the pressure coming out of the plunger for injector #2 was very low, and the result was misfires on that cylinder at lower rpms.

I removed injectors, and injection pump, brought them in and had all rebuilt. Recieved them back, and engine started right up, and purrs like a kitten now. Idle is set at about 750 rpm.

So now I'm just doing the little things. New fuel lines (due to minor drip), new vale cover gaskets, and new oil pan gasket(removed to flush oil system and make sure there was no sedemint in the oil pan, all was very clean.

Thanks for all the information everyone has provided, it was very helpful."

11-06-2007, 07:47 AM
"Thanks Robert for the info, change both fuel filters every year, and make sure the secondary is a good fuel water filter seperator. Racor make excellent units."

08-14-2010, 09:14 PM
Please post what compression tester and adapter the Yanmar mechanic asked you to purchase.