I have a Yanmar 3GM30 which has a rolling idle. It will start fine but when warmed up the idle speed will go up to about 750 or 800 and then it drops all the way to the point that the engine quits. I have eliminated fuel delivery as a problem. I was told that it may be a weak spring in the governor or possibly a fuel injection pump problem ( this from Yanmar ) Does anyone else have any other suggestions on what it may be ? The two possibilities that Yanmar suggested are both expensive fixes and I would like to eliminate any other possibility before laying out some bucks. Thanks in advance
The suggestions from the Yanmar mechanic are fair.
But only if all the other possibilities have been eliminated.
Does the engine operate perfectly at operating speed (ie around 2700 rpm - take Roger’s recent post about running a 3GM30 at engine speeds of 2600 - 2900 as golden, as is all of his advice)?
I was surprised when you said the ‘idle speed will go up to about 750 or 800 and then it drops’.
I would have preferred that you had said the ‘idle speed will go up to about 900’.
For any GM series Yanmar, idle speed (for a warm engine) should be between 825 and 875 rpm. The only way you get that level of accuracy of measuring engine speed is with a handheld tachometer. With the standard tacho, all you can aim for is somewhere below the 900 rpm mark.
Having an idle speed setting lower than 850 rpm runs the risk of damage due to excess engine vibration at idle, stalling when changing gears, and stress on the transmission.
So my #1 step would be to check that the idle speed is correctly set. And that there’s no excessive play in the engine speed control.
Some other questions before contemplating expensive action:
Is exhaust back pressure high due to coke deposits in the wet exhaust? How many engine hours on the engine and when was the last time the exhaust mixing elbow was checked?
When were the injectors last removed and tested? It’s pointless spending money on pulling the fuel injector pump if the injectors are worn.
Thanks for the feedback. I did not realize that the engine idle was in the 800 to 900 range. Maybe it’s just and idle adjustment that’s needed. The boat was repowered with this engine a few years back. It has 475 hours on the meter.
The engine seems to run fine when the engine rpms are over 1000 and very smooth at over 2400 rpm. Back pressure seems ok. Injectors look good as well.
Maybe it’s as simple as idle speed too low but would this cause the engine to have an uneven idle ( up and down ) ? Thanks again