Ask Sail: A Faltering Start

My Yanmar 3GM30F diesel engine has about 650 hours on it, and this season, it’s been a little hard to start it. Are injectors or compression the problem?
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James Macallister of Toronto, Ontario, asks:

My Yanmar 3GM30F diesel engine has about 650 hours on it, and this season, it’s been a little hard to start it. I have to put the throttle at one quarter open before cranking, and once started, the engine surges between 1,000 and 2,000 rpm for about 30 seconds. I can then drop the throttle down to idle speed. Are injectors or compression the problem? There is no smoke at all, and the engine runs fine once it settles down. Restarting after a couple of hours of downtime requires no special procedure.

Nigel Calder replies:

NigelCalder

This might be a compression problem. Cranking at quarter throttle will increase the initial fuel-injection rate, with the fuel in those cylinders that aren’t firing dribbling down onto the piston rings, raisingw the compression to the ignition point. However, once a cylinder fires, it typically kicks right in, and you would not see the erratic operation you describe, so I doubt this is the true cause.

It also doesn’t sound like an injector problem in that things run fine once you’re over the initial starting hump. Most likely air is finding its way into the injection system when the engine is idle, which is causing problems until it is driven out. Has the secondary filter recently been changed or the system bled? Check the filter seal, the bleed screw and the injection line connections. Finally, I’ve also seen this happen when the injector leak-off pipes somehow manage to siphon fuel out of the injection system, so check for that.

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