Onan Marquis 5500 surging

Question

The Onan in my 2000 Holiday Rambler has about 400 hours on it. When I first start it, it will run smoothly for about 10 seconds, then start surging then eventually quit. It may do this for about 10 starts, then will smooth out and run fine. If I give it a very short shot of Carb cleaner just before the surging would start, it will continue to run smoothly as long as I do this. At some point, it will run fine on it's own. I have the breakers off while doing this. The problem is definitely fuel related, but what is it? Is it running out of fuel but the carb cleaner keeps it going? If so, why does it eventually start running fine on it's own? Is it because it has warmed up a bit? any ideas?

ranger_smith · Answer

Fuel pump is a common problem with them. Try putting the fuel line in a can of gas with some seafoam in it to clean the carb.

drewe · Answer

Do you prime it thoroughly before starting it? Possibly it's just some air bubbles in the long gas line back to the gas tank, either from a minute air leak somewhere or (more likely, in my opinion) from gas draining back to the tank eventually when the generator is turned off. On some generators, you prime by holding the switch in the off position for a spell (say 30 seconds, or by ear—you should hear the fuel pump operating).

It could be a fuel pump that's weak, too, though I think the more common symptom is that the fuel pump stops working properly when it gets hot rather than when it's cold.

jluke · Answer

I haven't tried priming, will get the manual next time there to see if that works. The gas definitely drains back to the the tank. Takes about 20 seconds of cranking in 10 second intervals before it will start. I will try the seafoam in a gas can method, thanks.

darsben · Answer

And get an inline fuel filter with check valve

christopherglen · Answer

Do the priming thing first.  I hold the switch in the OFF position for 5-10 seconds, then start.  Fires in 3-5 seconds usually.  If I haven't run the genset in a while, or we moved the trailer then I prime it closer to 10 seconds.

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Onan Marquis 5500 surging

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