View Full Version : Yanmar 2qm20 stalls at idle, slow to respond throttle

04-19-2011, 06:57 PM
I have a 33' sailboat with a 2qm20 Yanmar diesel. l had the engine completely rebuilt sixteen months ago in Ft. Lauderdale by a great diesel mechanic, and I connected her back up in the boat myself. While she runs well, she stalls when I idle her down, which is a pain for docking. She runs okay down to about 1000 rpms, but stalls after a few minutes with the throttle in that position. She is very slow to respond to increasing the throttle, and I have to go to full throttle to save her when she wants to stall. Then she gets fuel, runs at full rpm, and I can bring her back down again to about 1000 rpms (estimate, no tachometer), before she again starves for fuel. To check if it is in the throttle linkage, I made all these changes by hand at the engine lever, and the engine acts the same without the throttle being involved.
I took her out once, shut her off for two hours, and she absolutely refused to start. She cranked over, but would not even begin to fire. Had to get towed in. I bled my engine carefully at the dock and she started right up again. Thus, it s e e m s to be an air leak problem but I can find no evidence of air leakage (though I cannot get at a small part of the copper fuel line leading from my fuel tank to the manual fuel valve). All of my "rubber" fuel lines are sixteen months new.
Relevantly, I cleaned my tank out three years ago, have regularly put bio stuff in my fuel, inspected my fuel and see-through bowl for any particles, changed my primary and secondary fuel filters repeatedly, and carefully bled my fuel lines, in sequence, repeatedly, and put in a new electric fuel pump.
The boat has only gone out only three times since the overhaul sixteen months ago because of this problem, but I run it frequently at the dock. She starts without hesitation and sounds smooth. Some white smoke at the starting for a few seconds only. I opened the 2 engine inspection covers to check the linkage between the throttle and the governor. All parts are solidly connected, and there is no binding or friction in the sliding mechanism when I move it manually (though in the cramped quarters of a sailboat, I can't see very well even with added light and a mirror). I have not messed with the governor nor the settings on the governor housing, as these are beyond my understanding.
I have tried repeatedly since last December to get a diesel mechanic (my engine rebuilder or another mechanic), but they are very tied up and and don't like cramped sailboat engine spaces much, I suspect. So, it is up to me, I think. I have a decade experience as owner of my Yanmar, love it, have fixed a lot on it, but this baffles me. I have time and the will to persevere, but I need guidance. I have the added incentive of planning a trip at the end of May.
I can't figure it out and would appreciate very much hearing the procedure I should follow to isolate where my problem lies.
I have tried to get my Ft. Lauderdale diesel mechanic who rebuilt my engine, but he is booked till June!
Hope to hear from someone who might be able to move me through this problem to a solution! Thanks very much!

Walter Renn
Lighthouse Point, FL

Walter Renn

04-19-2011, 07:27 PM
I would install ( for diagnostic purpose only) some clear plastic line in the fuel line before the engine and look for air bubbles. You can also put a tee in the line and install a vacuum gauge to see if you have a blockage.

If you have a mechanical fuel pump, I would test the fuel pressure coming out of the pump. May not be enough at idle.