Hi, I have a 1985 3GM30F installed in a Hunter 336. It's been perfect for 21 years. This year, the RPMs have started to fluctuate. When the engine's cold and for the first 20 minutes of operation, it behaves normally. After about 20 minutes at cruise speed around 2,500 RPMs, RPMs fluctuate down and up intermittently several times a minute. It gets progressively worse, and the top of the fluctuation gets progressively lower. There's a little more white vapor than usual in the exhaust when symptoms occur.
I thought maybe the head gasket was the problem, but fresh water coolant isn't being consumed and I assume the power loss would be more constant if it were that.
I checked the fuel filters and they seemed normal. I treated the fuel with biocide. Other hypotheses I've read about are fuel pump, injectors and the governor.
Has anybody experienced this? Any suggestions on pinpointing and fixing the cause? Thanks!
I thought maybe the head gasket was the problem, but fresh water coolant isn't being consumed and I assume the power loss would be more constant if it were that.
I checked the fuel filters and they seemed normal. I treated the fuel with biocide. Other hypotheses I've read about are fuel pump, injectors and the governor.
Has anybody experienced this? Any suggestions on pinpointing and fixing the cause? Thanks!
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