onan 4000 dying

check the fault code. If code indicates starved for fuel most likely the fuel pump. They get too hot where they are mounted. Instead of moving them Onan replaces them. My Onan gas generator had fuel pump replaced on warranty. If mine goes out again I am going to move it over to the MH frame. The Onan repairman told me since those generators are not all installed on MHs they sell the unit as a one complete unit which includes the fuel pump. Looks like an easy enough mod. Some RVers are replacing fuel pump with a pump from Amazon. Much cheaper than Onan pump but slightly different in how it is mounted.

X2 on likely a bad fuel pump. Same thing happened to us. Our model 4KYFA26100K Onan has a "prime" function on the start switch at the generator which runs the pump without the noise of cranking the starter. After the generator shuts off by itself, try holding the prime and listen for the buzzing sound from the pump. No sound, bad pump.

Replacing the pump isn't a difficult DIY job, but the control unit had to be removed on our Onan to get access to the pump. Replace the filter while you are at it. Disconnect shore power and battery negative terminals before anything else.

Agree with "probably fuel pump" but ours was not. Prev owners had let it sit without use/exercise. It would start, pick up the rooftop A/C as a load, and run as long as the load didn't drop off. If I set the A/C to Beyond Cold, genset would run till shut off. If A/C cycled out, to Fan Only, genset would quit. Problem was carburetor. The "idle circuit" was plugged with gum. Remember, on these RV gensets, Idle RPM and Full Load RPM are the same. 3600 in this case, but the Throttle Settings are entirely different. At Load, throttle is open and gas pulled through the large Main Jet. At "idle" throttle is closed and a tiny amount of gas has to get through small Idle Passages.

But a Fuel Pump is subject to failure and an Amazon replacement at $50 is 1/4 to 1/3 the price of an ONAN carb. So if I was throwing parts at it, Pump would be my first toss.

You've heard from OP's who have had ONAN fuel problems that went the two common ways. Be reminded the Model and Spec Code are very important with an ONAN. Always post the full Model Number. The Spec is the letter at the end, "K" in CD's case above. Model/Spec will get you into the subtleties of a long run of these gensets. For example, people will shout "low oil switch" and duck behind a rock. Fact is, ONAN deleted that feature from 4KY series before production of the "K" spec units. That means that switch is a bunny trail in your case.

j-d wrote:
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Fact is, ONAN deleted that feature from 4KY series before production of the "K" spec units. That means that switch is a bunny trail in your case.

Great info, j-d! We've always referred to your "bunny trail" as the even more nebulous "red herrings".

Herring if it's a marine genset. Bunny if it's used in the woods.

First off, thanks for all the replies. So, I ran it yesterday until it died. Tried to prime and no fuel pump sound.

Model: 4KYFA26100K
S/N H050825005 SPEK 5

I have the same unit. Mine is a 99, but the carb had been replaced maybe 10 yrs ago, and yes I know about exercising it.
What year? I just removed & cleaned the carb, it wasn't too bad a job. Mine started sputtering & popping. After that I Ran it for hours with no issues.
search you tube for vids.

HBJBIRD wrote:
First off, thanks for all the replies. So, I ran it yesterday until it died. Tried to prime and no fuel pump sound.

Model: 4KYFA26100K
S/N H050825005 SPEK 5

If you want to replace the pump yourself, I can provide a description of how I did so. Just ask.

CloudDriver - Yes, please, show me the way.