Yanmar 3GM30 Hot Starting

[cdrmike43]

Occasionally I have had difficulty starting the engine when hot. The starter tries to spin the engine halfheartedly but not enough to get it going. I have then had to lift one of the decompression levers and the engine then spins over freely and bursts into life and I can then close the lever. This has only occurred when the engine is hot. When cold the engine starts readily. The start battery is new and fully charged so this phenomenon is somewhat of a mystery and the local Yanmar Agent is also puzzled. Is there someone out there that has experienced this problem? I previously had a common Yanmar start problem with intermittent starter button operation but since running a new thicker wire from the start button to the solenoid, the original problem has gone away.

 

[Baddox]

My guess is that you have a problem with the earth cable or possible its connection to the engine. As the engine heats up, the resistance in the cable or connection will increase resulting in less current to turn the starter.

Some things to try, assuming the system is earth negative:

Connect a heavy duty jump lead from your battery’s negative post to the engine earth post. Make sure the leads are attached to clean metal, you’re looking to see if bye-passing the boat’s negative cable fixes the problem.
If it does fix the problem, it indicates that the cable, switches or connections at fault. If you have a volt meter, do a drop test across parts of the system when starting. Very simple to do, ask google if unsure, and it will help pin-point to the cause.

 

[lw395]

measure the volts and current of the starter motor.
If it's getting plenty of volts but not pulling enough current it's the motor.
If it's not getting volts it's probably wiring or earth. Measure the volts at the battery while cranking
If it's pulling lots of amps, with goodish volts and not turning the engine well, the starter motor is duff, unless the engine is partly seized.
But if it's partly seized, decompressing won't help much, so I'd discount that.
Measure what's happening and go from there.

 

[cdrmike43]

Thanks Baddox and Iw395. What you have suggested opens up new possibilities for rectification and will follow up when next down on the boat. Will let you know what I find.