motor dieseling (sp)

[sal]

Mariner 75 hp ... runs great but when I turn the key off... it keeps running for a bit.... any suggestions before it goes to the shop?? Thanks

[Valv]

Does it cool properly ? Sounds like it gets hot.

[Surface Tension]

In order for it to diesel like that it needs to be getting ignition from something other then spark. The first thing I'd look into is like Val said, heat. Being water cooled the outboard shouldn't be getting that hot. In fact I've never heard of an outboard dieseling before. That high temp would normally cook the motor.

Here are some things I'd check right away that'll cause heat.

Is it pumping water.

Bad Impeller

Bad thermostat(normally they stick open though not shut)

Obstruction in the cooling.

Correct spark plugs.(incorrect heat range)

Anyone of those will make the outboard overheat.

Next would be a dirty carb. Running lean will make the outboard run hot. Thats probably the most common cause for Dieseling.

A very dirty spark plug can make an engine diesel.

Those deposits on the plugs can glow and give ignition.

Like I said I've never heard of an outboard dieseling before.

This could also be caused from an ignition switch thats not cutting off the spark completely. For that matter it could be a corroded or loose connection anywhere between the switch and ground.

[sal]

Thanks.... pump is pumping... will change plugs... etc etc.

Voltage regulator was replaced last year.... would that have any bearing ???

[Macgyver55]

Have you checked the idle speed adjustment? Quite often run-on (dieseling) is caused simply by the idle speed RPM being set too high.

Another thought would possibly be carbon build up in the cylinder head. Most common in motors that troll a lot or use low quality oil. It can hold enough heat to ignite unburned fuel in the combustion chamber.

[eurolarva]

Have you tried to do a decarb on the motor? Dieseling can be caused by carbon build up in the cylinders and they get so hot that they actually glow. This creates a burn with no spark and the motor could run indefinetly. Seafoam makes a product called deep creep that will decarb the motor. After decarb a maintenance dose can be used regularly with regular seafoam to prevent further carbon problems.

Sorry should have read Mcgyvers thread. He pretty much said what I did.

[shamrock7]

All these things could do it but there is also the possiblity of the sync-n-link is out of wack. There is nothing that you can do for that. Its a dealer thing. Basically, the carb and the ignition has to be correctly syncronized for the engine to run correctly and they probably aren't.