There is:
http://www.isuzumarine.co.jp/english/index.html

And:
http://boatdiesel.com/Engines/Isuzu/Isuzu-Manuals.cfm?PDFID=3968811400575
but it looks like you might have to subscribe!

It's a strange one , the engine in my pal's narrow boat  , a four cylinder, two litre isuzu diesel was missing on one pot , I should have said "sorry, I know nothing about them" but instead I suggested cracking the injector pipe.
Nothing there !
Try each in turn , yes all working OK , except for No.3.
We decided to bleed the injector  and there is no fuel at the faulty cylinder.
By coincidence he had changed the fuel filters the day before so we decided some debris might have got through and be blocking something.
I am used to auto diesels, having owned several commercial vehicles and driving diesel Peugeots for the last four cars , but this is completely different from what I know.
It is a common rail system with a separate pump for each cylinder , so we took the pump for No.3 cylinder apart , only a spring , a loose jet and a collet , no obvious fault , put it back together , engine won't even fire let alone run.
Over the next two days took every component off the fuel supply , got soaked in diesel , flattened the batteries and had to get a petrol generator to re-charge them but not a fart.
I rang every boat hire firm , Isuzu dealer and even JCB agents as the engine was fitted to some diggers , but no-one had any information or a manual.
The engine has an electric lift pump and that was working and a stop solenoid which also works so we were totally stumped.
We decided to remove the stop solenoid and see what it did , my old Bedford van had a stop solenoid which was a needle valve , the tip  broke off and blocked the jet.
The solenoid pushes against a bar , but the bar didn't move.
Removing the side cover from the block we could see that the bar was really a lever attached to each pump along the engine , and it was jammed in the closed position.
After pulling the lever back we gingerly tried the starter and she fired up instantly and runs smooth as silk , well as a diesel.
responds slow to  the throttle now though and sometimes the lever closes on it's own , guess we will have to look at the other end now.

Hay Abb what you need is a marine engineer!

Abby

You have lots of room. I wish I had a picture of me changing the starter moter on the port engine on a Coast Guard 44' Motor Life Boat. You have to stand on your head wedged bewteen the engine and the side of the hull. Was not a fun job but I was 23 at the time so I could do it, today no way.

Just a point. I think you have a unit pump injection system where all the injection timing is done at the pumps. As I recall common rail systems had one high pressure pump and the timing and speed was controled at the injectors. GM on the 71 series engine used a unit injector that had the injectors feed with a low pressure pump and the unit injectors supplied the high pressure and the cam shaft provided the timing. It has been a long time sence I worked on a marine diesel engine.

Dave

Abby

I reread your posts. My guess is that the engine has lost one or both of the springs on the govenor fly weights. At least it is not a great big v8 engine so lifting it high enough to work on is less of a problem. Of course you will have to do a new shaft alignment when you set it back in place. Good luck hope the weather stay good or you can rig tarps to keep you dry while working on it.

Dave