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Monday, November 9, 2009

Fuel for Thought

I have an Onan generator in my Class C motorhome. It has been running fine until a few weeks ago. Now it will not start. The fuel tank is full.
After several tests I have done to troubleshoot the problem, I have determined that it is not getting fuel. When I use starting fluid it will fire, but will not continue to run. The fuel line from the main tank appears okay. No fuel comes out of the hose that goes into the carb when cranking.
I have narrowed it down to a clogged filter, or a bad pump. Is there a possibility of another issue like a safety fuel shutoff? I want to fix this myself. I think I can get at the fuel pump and filter without pulling the generator out of the coach. Is there a test I can do to verify that the pump is good or bad? Thanks for your help!
Randy, (Ossian, IN)

Randy, try this; run a new rubber fuel line from a portable gas can directly to the fuel inlet on the generator. If the unit starts and runs fine, you’ll know the problem lies within the fuel delivery system. If it still will not start, then you can focus your attention on the genset itself. If it starts and runs as it should using the gas can, replace the entire length of fuel line from the fuel tank to the inlet on the generator. An aged hose is likely to have cracks resulting in the generator fuel pump sucking air even though no apparent leak exists. The fuel line may look good, yet still allow air to enter while cranking the generator. You may have to drop the fuel tank slightly to reach the take-off tube on top in order to replace the whole length. 

Look also for hidden in-line filters somewhere between the tank and the generator. To be safe, replace all existing fuel filters. If the unit does not start using the portable fuel supply, remove the fuel solenoid temporarily and connect the fuel line directly to the carburetor. It is possible for the solenoid to become faulty, fouled or plugged. If the generator starts, replace that solenoid. In some extreme conditions, it may be necessary to measure the actual fuel pressure at the carburetor. A weak fuel pump may be at fault. I’d recommend a professional RV shop if it comes to this. If you have access to another fuel pump, you can swap it out also to be sure. But I’m betting on cracked a cracked fuel hose between the fuel tank and the generator or a plugged filter.

Follow-up from Randy Fields:
Gary, I did as you suggested and no fuel is being supplied to the carburetor, even when I try to draw fuel from a portable container. I believe it is the fuel pump or as you call it, the fuel solenoid. Is there a way to test them before I pull the generator?

Randy, to test the fuel pump, you’ll need to attach a pressure gauge to the fuel pump outlet hose. While cranking the generator the pressure should read about 3.5 PSI on the gauge. Anything less, replace the fuel pump. You can bench test the fuel solenoid by applying positive 12-volts DC from a battery source to the non-grounded contact on the solenoid and negative 12-volts DC to the grounded terminal. When connected, you should hear an audible “click” as the internal plunger is activated and the solenoid valve is opened. While activated and open, you should be able to blow through the solenoid. If the plunger does not move or you cannot blow through the solenoid, replace the solenoid. This is, of course, assuming the pump and solenoid are both receiving the proper voltage during cranking when installed on the generator.


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