Engine hunting at idle

[wpc691]

Started the engines the other day, and immediately noted the port engine hunting from 400 to 600 rpm’s with throttle at low idle. Advancing throttle reduced the hunting; it disappeared at about 900, but the engine was noisy. Reducing throttle brought it back. Checked boatdiesel.com, found several threads suggesting dirty fuel filter, air leak somewhere between tank and fuel pump, and broken governor spring. Interestingly, nobody ever reported what solved their problem. I won’t be that guy. Easiest thing first, I pulled the Racor...a little dirty, but not too bad. I changed it, then pulled the vacuum gauge to replace the little Buna-N seal on the stem. There was the culprit, a big divot in the seal. Apparently I nicked it at the last filter change, and cycling finally wore a leak path through. In any case, I replaced it, started the engine, problem solved.

[MVCaprice]

I would investigate the fuel system before anything else. Check your primary and secondary filters. Also look at your lines to see if you have any leaks. It's likely a fuel problem.

[oceanjake]

Good for you on starting simple before ripping into the engine.

[SEVEN]

You must have been sucking air right?

[MVCaprice]

I misread your post. I had similar symptoms and it was bad soft lines. I found that It’s easy to get a vacuum leak that will only show as a hard start or lumpy idle.

[wpc691]

You must have been sucking air right?

More than usual, yes :-)

[Briankinley2004]

I had similar problem after changing racors on port engine. Realized I forgot to change the positions of the priming pump valves back and was sucking through the pump

[Triskele]

Started the engines the other day, and immediately noted the port engine hunting from 400 to 600 rpm’s with throttle at low idle. Advancing throttle reduced the hunting; it disappeared at about 900, but the engine was noisy. Reducing throttle brought it back. Checked boatdiesel.com, found several threads suggesting dirty fuel filter, air leak somewhere between tank and fuel pump, and broken governor spring. Interestingly, nobody ever reported what solved their problem. I won’t be that guy. Easiest thing first, I pulled the Racor...a little dirty, but not too bad. I changed it, then pulled the vacuum gauge to replace the little Buna-N seal on the stem. There was the culprit, a big divot in the seal. Apparently I nicked it at the last filter change, and cycling finally wore a leak path through. In any case, I replaced it, started the engine, problem solved.

Was this engine a 671? If so, what is the vacuum gauge and where is it located?

[SKYCHENEY]

Was this engine a 671? If so, what is the vacuum gauge and where is it located?

It says Cat 3208s and vac gauge on the Racor.

[Avenger]

Lucky you. On mine it was a broken governor weight.

3208-09.jpg