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Diesel This is a forum dedicated to diesel engines and their applicable accessories.

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Old 07-06-2010
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Another Exhaust Smoke Diagnosis

I have a Yanmar 3GM30F motor vintage 1992, with 613 h on it. I have owned the boat for 4 years now and it had about 480 h when I bought it. This year we have not used our motor that much, electing to sail off the mooring 90% of the times, and sailing almost to the mooring and then firing up the motor for the last 5 minutes while we take down the sail. I have probably put only 7 h total on the motor for 20 or so sails.

Yesterday I wanted to get back to the hook and decided to motor iinstead of sailing in very light winds. The goal also was to work the motor a little and burn out any build up. Fired up the motor, allowed it to warm-up for a few minutes and then cruised in for 20 minutes at 2800 RPM, occasionally getting the rpms up to 3200 and 3600. Landed back on the mooring, allowed the motor to recharge the battery a few minutes. Before I turned it off, while in neutral, I reved the engine and notice brown/chocolate/rusty exhaust with slight oil sheen to it. Once the initial pulse was over, the water was clear. Did it again and same result, and again same result. After 4 or five times, it looked like it may have been reducing in intensity, but I didn't want to keep doing it. Under normal idle flow, no brown exhaust. Shut the motor off and allowed it to cool.

Went below and checked the oil dipstick and nothing unsual. Black and no froth. Level was about 1/2 way between the upper and lower level marks, which is where it has been since the last month. Checked strainer and water was normal with a few pieces of grass in the strainer. Water flow was not a problem. I changed the air intake sponge/filter last year, but did not look at it this time. Motor starts in less than two seconds and sounds normal.

I had a air/fuel leak for a few weeks this year that I fixed. And a few days a ago, I saw a whole bunch of white smoke the at last time we reved the motor, but that cleared up and I thought it was just from underusing the motor. Fuel looks clean too, no evidence of water or contamination in it.

I haven't had any major engine work down on the motor. Change the oil every 50 h and this year, changed the coolant. I was planning to have the injectors cleaned over the winter.

Ideas before I fire off a email to Yanmar?


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Old 07-06-2010
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In our experience 'brown' smoke is a dirty injector... a really bad one will produce a loud "tick/knock" as well as the engine tries to compress liquid fuel that hasn't been properly atomized.

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