Yanmar 3YM30 mystery: I can turn the engine over by hand

[danielhwolf]

Hi, everyone, longtime lurker here.

I came into possession (cheap!) of a Yanmar 3YM30x1 engine in externally nearly perfect condition. The first mystery I faced was why I was able to get it so cheap (from a 3rd party so no story came with it). I think I solved that mystery when I found that I could turn the engine over by spinning the crankshaft pulley -- by hand. Hmmm, not good, it's supposed to have compression! And, mysteriously, all the cylinders (apparently) lack compression.

I know I'm going to have to start taking things apart to diagnose the problem, but before I start going down low-probability channels I thought I'd ask for ideas about what might be the problem, and whether I'm on the right track.

Here are my ideas about the lack of compression, more or less in order of simple>>complicated:

1. The compression relief switch is stuck open (but I don't see such a switch on it and the downloadable manual on this site doesn't mention one)
2. The key locking the crankshaft to the pulley is sheared and the two are turning freely and independently
3. At least one valve per cylinder stuck in an open position
4. The crankshaft is cleanly and smoothly (unlikely!) broken between the pulley and the first con rod

That's all I can think of. I can't imagine all three pistons having holes in them. What else could it be?

Thanks!

Dan

 

[kloudie1]

There is no compression release on that engine. First check the valve clearance. It would be strange to have 3 that are holding the valves open, decompressing the engine. With the valve cover off, you’ll be able to verify that all valves are operating at the correct time (flywheel markings) and are not stuck. From that no cost look-see, things start getting expensive. Good luck!

 

[jssailem]

On my Perkins, with the valve cover off, the crankshaft can be turned with a large wrench and big bar using the nut on the end of the shaft. As it turns the valves open and close.

If yours do not open and close then something is causing the valve rods not to do their thing. As Claude suggests you may be looking at a rebuild if the motor is important to you.

 

[danielhwolf]

Thanks very much, guys. This gives me some confidence about the place to start. If there's no valve problem then my next step will be to see if the pulley is slipping on the crankshaft. If not, well, now we're talking work and money.

 

[justsomeguy]

we're talking work and money.

Hopefully it'll be a quick fix on the engine, but I hate to break this to you...
Edit- do you have a boat to put this engine into?

 

[danielhwolf]

Okay, edit: lots of money.
I have a boat but it has an engine. I will either fix and sell it or part it out.

 

[Hollyberry]

Is there a gear box attached? does the output shaft turn along with the engine? Does transmission position (F/N/R) change anything?

 

[kloudie1]

If someone has been messing with the engine, you may have loose fuel injectors and/or glow plugs..

 

[Alan Gomes]

If someone has been messing with the engine, you may have loose fuel injectors and/or glow plugs..

Wouldn't that be great if that's all it turns out to be!

 

[Ken13559]

When will we know your findings? We await with bated breath