View Full Version : Air getting in fuel system

11-27-2002, 02:05 AM
" Hi there,
I have added a new fuel tank to my boat, by placing it in the bilge, in my steel yacht. The motor sits pretty well directly over the new tank. However, I cannot stop the engine sucking air occaisionly. The motor will run no problem at all, but every now and again, will just cut out a few minutes after starting. I have become an expert in bleeding it in only seconds because it always happens at the worst possible time!!
I have changed and checked everything (Believe me I have!) including all the connections,copper washers in filter, fuel pump you name it...
All I can think of is that because the tank is well below the engine(from 1 foot full to 3 footat empty)there is a tiny air leak somewhere jus allowing enough air to build up under vacuam to strike just when I'm beginning to think I've got the problem solved.
Any one got any suggestions in finding an air leak,short of fitting an electric pump, or using only the top tank? "

12-01-2002, 08:14 PM
" Try pressurizing the tanks/fuel lines with the line plugged at the engine. Do not put more than 1 pound or so in the tanks. Check for leaks with a soapy water solution in a spray bottle. Also double check fuel pump pressure, may be weak enuf to cause air bubbles but not fail completely. Also check depth of pick up tube in new tank. It should be approx 1/2" of bottom of tank and cut at an angle. One other thing, do you have a return line to the tank. If so this needs to be located at least 12 inches away from pickup tube so as to avoid possible bubbles. Good luck and post your results so we all can learn. Kim "

12-04-2002, 01:54 PM
The engine return line my need to be set up above the clyinder head so the fuel will not run back to the tank when the engine is off.also my have to install antisphone value in the sucksion line best would be at the tank.The reason for this is the tank is bellow the engine.

12-09-2002, 01:53 AM
" Kim,
Thanks for your reply.
I will try the pressure idea, and see what that comes up with.The pickup has a brass one way valve on the bottom of it,(With no valve inside it now!) so it cannot cause a vacuam on the bottom of the tank.
The return is about 6 inches from the pickup, but as the tank is so deep,(about 3 ft) I woudn't think it coud cause any problems? There is never much fuel coming out the return, so I think it would be difficult for it to cause air bubbles also.
The fuel pump is brand new, as I wondered if this may have been a problem, but no change.
Whats your thought on Charlie's ideas?
Thanks very much, I appreciate your input!
Richard. "

12-15-2002, 06:12 PM
" I think his ideas have merit but I am not sure they address the real problem. If your fuel line and return line are tight, antisiphon valves are not used. As far as the return line being aboe the head, I agree. This can be done with a simple anti siphon loop that at any point is above the head to prevent draining back to the tank. If you replaced the fuel pump, double check all connections on the pump, and yes it is possible that a new (or rebuilt or remanafactured) pump can be defective. Good luck and keep us updated. Kim "

Ron B
12-28-2002, 08:57 AM
" I had a problem with stalling with my new 20 hp kubota diesel in a small sailboat especially when I got into rough water. An air leak was suspected and the entire fuel line system was replaced.No help. The tank was also below the engine. After all the experts failed to correct the problem, I was experimenting with the new raycor filter and found I could stall the engine by unbolting it, and tipping it duplicating what happened when in rough water. I called Raycor who were great. They suggested the problem was a bubble of air in the dome of the filter caused by minute particles of air caused by sloshing in the tank. They volunteered to send free a replacement filter that has an outlet port in the top of the filter rather than on the side, which prevented the buidup of a bubble. I've had no trouble since. The problem was so simple, no one saw it. The small diesel just needed a small gulp of air to quit, and would run OK for awhile until the bubble built up again.
Hope this helps Ron Brassord Lighthouse Point Fl "

K.E. McDaniel
09-06-2003, 08:39 PM
" I had a similar problem with a Yanmar 3QM30-F. After 3 sailing seasons of playing the game, I finally discovered a weak lift pump diaphragm. Replaced pump, no more problems. "

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