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Onan generator fuel supply problem


a4epilot
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In recent years, the Onan is getting harder to keep running after starting when it hasn't run for a few days. It runs for only a few seconds and then stops. Apparently, there is enough fuel at the injector pump for it to start but that's all. Hard to start again and then quits. This cycle repeats about 3 times and then it stays running. To be clear, there is nothing wrong with the start sequence as it does start on the first attempt.

The problem is fuel supply. It appears that the fuel siphons slowly back to the tank therefore the generator fuel pump must operate for an extended period to get the fuel back to the motor.

A few years ago, I placed a check valve in the fuel supply line to the generator in an attempt to prevent this. It did seem to work for a while but then back to the same problem. Possibly, that check valve is leaking now.

The only way not to have this happen is to run the fuel pump at least 3 minutes before attempting the first start. It helps but still the gen will usually quit once and then run OK after the second start. All second starts are hard and take a long time cranking to get the fuel back.

I'm thinking there probably is a check valve in the fuel pump that is supposed to prevent this and has possibly failed and/or the fuel pump has become weak.

Anyone had the same problem and found a fix?

Thanks,

Gary K   06' Windsor with 8 KW Onan

 

 

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I am not sure that I would blame the pump itself because ours is just as low as the bottom of the tank, always below the workable fuel level and therefore should not cause reverse siphoning, I would be more inclined to an air leak somewhere in the fuel lines or at the tank pickup.

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This reminds me of our dearly departed Fred White ( former owner ) when he had a gen fuel issue .

IIRCC he found the fuel line was coiled on the tank to provide for the gen moving in and out had developed a pin hole allowing it to suck air   

A new piece of hose stuck in a can of diesel and if it runs properly that may be your problem too .             

I imagine it would be hard to see the hole so its probably better to just replace the entire hose .

BTW have you replaced the fuel filter ?

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31 minutes ago, Ivan K said:

I am not sure that I would blame the pump itself because ours is just as low as the bottom of the tank, always below the workable fuel level and therefore should not cause reverse siphoning, I would be more inclined to an air leak somewhere in the fuel lines or at the tank pickup.

Good point. Thanks. Gary K

1 minute ago, Ray Davis said:

This reminds me of our dearly departed Fred White ( former owner ) when he had a gen fuel issue .

IIRCC he found the fuel line was coiled on the tank to provide for the gen moving in and out had developed a pin hole allowing it to suck air   

A new piece of hose stuck in a can of diesel and if it runs properly that may be your problem too .             

I imagine it would be hard to see the hole so its probably better to just replace the entire hose .

BTW have you replaced the fuel filter ?

Yes. I now have an externally mounted fuel filter/water separator. Removed the factory unit.

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11 hours ago, a4epilot said:

Good point. Thanks. Gary K

Yes. I now have an externally mounted fuel filter/water separator. Removed the factory unit.

 You have probably already done this but if you haven't I would recheck all the hose connections and filter , any place it might be able to suck air .

  I like the external filter but the plumbing offers more places to suck air 

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Gary,  

I think both Ivan and Ray are on the right track. Based on the behavior you describe you have an air leak in the supply (not return) hose between the fuel tank and genny. Could be a pinhole in the line - on the supply side the hose is under negative pressure, so it draws in air and you wouldn’t necessarily find fuel leaking. (As opposed to the return, which is under pressure and would leak fuel). Could be the hose; could be a fitting.  
 

I agree with Ray...  replace the entire supply-side hose. Even if you found the leak, would you repair or replace?

Let us know how it turns out and what you found. 

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My generator is on a slide and had a similar problem.  After checking all the usual suspects, I noticed that the fuel supply hose had a wet spot on it. The flexing as the genset moved in and out had caused it to crack, causing it drain and leak air when I tried to start it.  Replacing that hose fixed the problem.

 

dick L. ‘04 HR IMPERIAL

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  • 4 weeks later...

I had a problem that was intermittent.  The 12 volt pick up pump was not getting power all the time.  I installed a switch to manually get 12 volts to the pump when ever the gen wouldn't run.  Turns out it was a loose pin in one of the connectors in that mess of wires and connectors on top, just under the cover in the front of the engine.  Maybe?

Woody Miller

09 Dynasty Regal IV

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