ONAN runs, stops, starts again

I'm sure it's here somewhere...
http://www.twinslan.net/~n0nas/manuals/onan/

First, Congratulations and Thanks for posting Model, Serial, Spec information! We have 4KY Spec K also. If fuel pump had been replaced by previous owner, don't know. Carb HAD been and was already fouled with gum when we bought the coach. Luckily an OP here had a good used one he was willing to part with...
I met a fisherman/camper I'll call Old Joe. He was running his 4KY and it just hummed. Mine (before carb replacement) would start, and it would run as long as there was load on it. Once hot, and once load dropped off, it would stall and I could not re-start it. Old Joe told me "Every drop of gas that goes into my tank (which was the 55-gallon tank on a Ford E450 under a Class C) gets Stabilizer in it." I said "Even on long trips where you're only filling up for the road?" and Old Joe said "YES!" Since the new carb, I've added stabilizer every fill-up. I also try to exercise the 4KY regularly. But since I'm an irregular kinda guy, sometimes it's less often than monthly. Nonetheless, simply faultless genset operation.

Since you buy good fuel, add stabilizer and exercise the genset I'm going with the Fuel Pump crowd on this one. The ONAN Service Manual you'll probably find is 981-0530C. The 0530 without "C" is for older 4KY's. Others here can probably post a download link. I haven't had any luck finding one today, but if you PM me with your email ID, I can send you a copy as attachment.

0530C will give you Codes, Troubleshooting Chart, and Fuel Pump Pressure spec. Code'll probably be "36" which kinda means "Beats Me" and I'm concerned that by the time you connect a fuel pressure gauge, a bad fuel pump could have cooled enough to look good again.

Let me add this: Many Marinas pump ValvTect marine gasoline. It's stabilized, disperses moisure, cleans injectors, on and on. I am a Reformed Skeptic about ValvTect! Honda engine on my Pressure Washer needed carb cleaning every few months if I forgot to run it when I used automotive regular. Adding StaBil did a decent job but the fuel lines on the String Trimmer still kept failing since it was Ethanol gas. The ValvTect available in Florida is both Unleaded and ETHANOL FREE. I believe Octane is 89. I just bought my start-of-mowing-season supply. Paid $4 a gallon while plain old car gas is $2.50. Worth every dollar, every drop. If you can get to ValvTect, by all means DO!

Recently had the same problem...replaced fuel pump and works like a charm

All good advice and things to check. Vapor lock was one of the things I'd thought about but not some of these others. Thanks, I'll update soon.

TexasUltra wrote:

I use a fuel stabilizer and premium fuel for the genset at all times.

So called "premium" fuel is a waste of money in a generator......and anything else that doesn't specifically require it.

Your symptoms sound a LOT like mine when the fuel pump failed.
It would start and run fine......until it got hot, it seemed.

Have you had a tuneup done? There could be crud in the carburetor. Also, if you can get ethanol free gas it would be help. Small engines don't get along with either.

Check the air filter, fuel lines (as stated above).
Have you changed gas cap? Sound like you are sucking fuel from the tank, but the used gas is not being replaced by air (tank sealed to tight)Loosen the gas cap and see how long it runs.

Even though you have checked the oil level, if your Onan has an oil level switch it could be bad.
Can you tell if oil is foaming? This could also cause oil level or oil pressure switch to shut it down.
Along with "whemme" post the fuel pump could be sucking air through decaying rubber line.

My opinion. You may have a fuel pump getting weak.