I have posted a few time previously about how I am keeping a very old Volvo Penta engine in my yacht going but it has recently developed a curious issue that I would like your thoughts on.

The batteries have charge but when turning the engine on with the key it turns over but doesn't / won't fire. Here's the curious thing, if I start the engine manually by hand cranking it (told you it was old, don't think they make them this way any longer), it starts and runs normally.

On a recent outing we went through this process to get off the mooring, cut the engine and went sailing then when we returned to the mooring it started with the key as normal. The boat sits on the mooring for a few weeks, then next time it is started we are back to the hand cranking.

Thoughts?

Is it a fuel issue? Electrical? Something else?

Thanks for your input.

Gday Matt

Does it turn over when starting with the same speed when you come back to the mooring? Is it slower cranking? If it is slower, it could be that your alternator charges the batteries a bit after you hand crank it and then it can start fine with higher rotation. But after sitting for a week, the batteries get a little flatter and don't spin it as fast.

I know that some petrol motors used to have a thing where they would catch just as you turned the starter off. The idea was that the ignition coil had more juice to make a nice spark with the starter off. It seemed plausible. But this is a diesel with no spark plugs. Then again, does it have glow plugs? Maybe that is the difference.

A small solar panel (with regulator) of 5 to 10 Watts will keep the battery/ies fully charged.
Batteries will less readily deliver their power in the colder months, but a battery of sufficient capacity and a starting (not deep-cycle) will be able to do all that you ask.
How old is the battery? Do you have it hooked up to a solar panel? What is its capacity/is it about the proper size, etc for the diesel?
Does the diesel motor have a decompression lever?
I suspect an electrical issue. If it were fuel, it would be a bubble of air and you'd be bleeding the fuel lines regularly, which you are not doing.

Hi Matt,
Hmm..the only real difference between the two starting sequences is the decompression lever position.
If your rings/valve guides are toast, you might not be generating enough compression with the valves closed to fire the diesel, ie: its leaking too much pressure on the upstroke.. After it warms up a bit, the tolerances close and it has more compression.

Next time you go down, try starting it on the key but using the decompression lever like you would when hand cranking. If this works it points to the sudden jump in pressure giving you just enough compression to combust the diesel fuel.
Simple to try and might help you isolate the problem.

If it starts by hand cranking then the engine should be OK.
It sounds to me like the battery isn't turning the engine fast enough. But it starting on the crank handle you would think would negate this.
However, a new battery would be my first move.

Battery, cabling and starter motor in that order is the way I would approach it. As MB said you can hand crank it that's a real good sign that all is good with the engine.

If it's a mb7b or similar I would start by cleaning both surfaces of the earth terminal to the engine block.

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Ramona said..
If it's a mb7b or similar I would start by cleaning both surfaces of the earth terminal to the engine block.

Newb here - but is it worth re-terminating the battery terminals too? I had a similar problem with a car that was solved by re-terminating the battery connectors (after I'd replaced the alternator and the battery ) - there was a heap of tape etc covering the connector and when I ripped it all off it only had a small number of copper strands actually connecting.

Is it a cranking battery you are using to start it? They are designed to give high energy output for a short period. House batteries of course are designed to give out low amounts of energy over a long period. Just a thought.

Thanks for the suggestions I will give them a go and see how it goes.

I hadn't thought of checking the battery terminals as a source of depleted charge so I should be able to check them and give them a clean up, which will hopefully assist.

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JonE said..

Ramona said..
If it's a mb7b or similar I would start by cleaning both surfaces of the earth terminal to the engine block.

Newb here - but is it worth re-terminating the battery terminals too? I had a similar problem with a car that was solved by re-terminating the battery connectors (after I'd replaced the alternator and the battery ) - there was a heap of tape etc covering the connector and when I ripped it all off it only had a small number of copper strands actually connecting.

On a boat your looking for lost amps. In a car, especially a modern one with computers the earth strap is critical and sometimes needs replacing every 5 years or so. Lots of sensors operate between 0 and 5 volts and any error caused by a crappy earth strap or dirty terminals could have serious outcomes.
Don't use electrical tape!