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I have a M320B three cyclinder diesel in my Catalina 30. I just recently bought the boat. When engine is cold it fires up first try. If I turn it off and restart it again it starts first try again. If I let it run 20 minutes and engine gets hot, turn it off and then try to restart it...it cranks really slowly and wont start. I have to let the engine sit for half hour or longer before it will start again. I believe the problem is electrical because it cranks but just cranks too slowly to start. I had starter rebuilt, I replaced solenoid thats on top the engine, I cleaned/sanded all the wire and connections, batteries are new and fully charged. My next step is I'm gonna change the solenoid on top of starter. If that doesnt fix it I dont know what else to try. Any suggestions would be great! Thanks
 

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My guess is its the wiring harness connector in the engine bay - if you follow the wires from the engine to the panel and find a big wrap of tape around a lump unwrap it and check the connectors. Might be corroded, possibly melted. If the engine cranks okay when first starting the solenoid and starter are working. When cold possibly the harness has a usable connection but once hot it expands enough to create resistance in the circuit.
It might be the battery connection but you mentioned you cleaned all the wire connections - maybe you missed the harness connector hidden under the ball of black tape.

Search "mainesail M3-20B universal wire harness" (Mainesail has a ton of information online at marinehowto.com). Also lots of information available online from the Catalina 34 owners group.
 

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Could also be that the starter had been abused. They get incredibly hot and the stator and coils expand during normal starting. If they overheat, they can also bind up from the expansion. (Had that on a car.)

If the starter is marginal, the additional heat from the running engine could keep it swollen up and that would make enough friction so it might turn slowly--but not fast enough to start the engine.

The simplest way to diagnose that, if you can access the starter without too much trouble, is to pull it, open it up, examine the wiring for grinding and other physical damage.
 

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Sounds to me like you are getting a vapor lock somewhere in the fuel system. When the engine cools it dissipates and returns when the hot engine has been sitting awhile.
Also, a warm engine has expanded and will have a bit more compression than a cold one, making the engine turn over slower, but to get started now there's no fuel.
It can often be a combination of several little things and solving one might not end the problem.
 

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Measuring voltage directly at the starter whilst cranking and then at the battery for comparison during the issue will help narrow the potential causes of the problem. If it's electrical, I'd be real sus on the ground connection to the engine block.
 

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One bit of preventative maintenance I did on my old Universal diesel was to have the starter motor rebuilt at about 1000 hours.
It was starting OK, but a mechanic friend advised me to fix it before it broke. (!)
Interesting thing was that when reinstalled, it turned Noticeably Faster ! Parts like that can have their performance degrade so gradually that you do not realize how bad they are because you have no recent frame of reference.
The shop that did the work replaced the solenoid too, since on my model they are kind of a one-shot device.

Not expensive by "boating standards"... at around $135.
Worst part of the job was the R&R. That upper bolt took an extension and a wobbler. And some mild cursing.

OTOH, if yacht maintenance was easy they'd just let anyone own one!
:)

Also, as noted by other posters here with band aids on their knuckles, DO remove all of the ground lugs and sand paper every one and reinstall tightly. 12 volts is not all that eager to push its way thru a tiny bit of corrosion or dirt..........
 
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