Onan 4000 pulsating

[Chucklesg]

The generator in my '16 Aspect pulsates when run, and when under a heavy load (AC), sputters and dies. Under no load or light load, runs but pulses. What might be the solution? Only 39 hours on it, third owner for only 2 months. In AZ at 4400'.

[Reniram]

In 6 years and only 39 hours you need to run the heck out of it under load for at least an hour or so to get fresh fuel into the system.It should get better the longer you run it. A new fuel filter and new spark plug and oil change is about the only maintenance you should be aware of.Sometimes if the carburetter is really gummed up you may have to take it off and give it the cleaning solvent treatment...Run it Run it...

[DavidM]

Originally Posted by Reniram

In 6 years and only 39 hours you need to run the heck out of it under load for at least an hour or so to get fresh fuel into the system.It should get better the longer you run it. A new fuel filter and new spark plug and oil change is about the only maintenance you should be aware of.Sometimes if the carburetter is really gummed up you may have to take it off and give it the cleaning solvent treatment...Run it Run it...

David

[AZAspect]

When I took delivery of my 2017 Aspect 30J, it had only about 80 hours on the generator and it was only 18 months old (second owner). I had a similar issue and I ran the generator for about an hour under full load (A/C on high) and the problem went away. I was expecting to have to pull the carb and clean it out but got lucky. Haven't had an issue since. Good ol' La Mesa RV assured me they ran it under load and it was all good. Right....my bad that I didn't check in before taking delivery. Good luck!

Al

[pastordon]

ON our onan generator, there is an altitude adjustment near the top of the carburetor that sometimes needs to be adjusted for altitude difference. While the generator is running and surging, adjust the altitude and see if it takes care of your problem.

[Wyatt]

Originally Posted by Chucklesg

The generator in my '16 Aspect pulsates when run, and when under a heavy load (AC), sputters and dies. Under no load or light load, runs but pulses. What might be the solution? Only 39 hours on it, third owner for only 2 months. In AZ at 4400'.

It should have closer to a MINIMUM of 140 hours on it and that’s assuming it was only exercised and never actually used.

Gas can go bad fast and varnish in the small carburetor bowl, if yours is a gasoline model.

Take the advice of the previous posts and see if running it for a few hours works. If not, next you may have to clean out the carburetor.

[moabdds]

I had a similar problem with my Onan 4.0 after sitting a few months without stabilizer in the gas. I called the closest Winnebago dealer (100 miles away) and they told me they don't work on them and referred me to the Onan dealer there. I called them and they said the EPA does not allow them to clean a carburetor, so they could only replace it. Interesting. Kind of like the ban by the EPA on recharging your sealed rooftop air con. If it needs recharging, you have to replace the whole unit! How is that environmentally sensible?

Anyway, I found a local RV repair guy who pulled the carburetor and cleaned it for me and it now works properly. Since my generator runs off the chassis fuel tank, he suggested installing an inline shut-off valve to be able to run the generator dry when I put it in storage. Haven't done that yet, but do run the generator once a month for about 1/2 hour or so to try to prevent gumming up.

[Wyatt]

Moabdds I agree with your sentiments regarding recycling ac units.
I looked into the install of a shut off for the carburetor bowl, but it won’t work. There will still be residual fuel in the bowl.
It’s either run it as Oman recommends monthly, or risk it getting gummed up.

[Geosman]

My old class C had the 4.0 Onan gas generator. 50 gallon gas tank etc. over its 15 years I replaced two carburetors due to clogged passages and the pulsing problem was the first indication that there was a problem. I tried carb cleaner even removed a carburetor and used a syringe to try forcing carb cleaner through the passages and soaked it overnight to no avail. A total carb replacement was the only solution with that design. Using stabile in a 50 gallon tank has its issues also as it must be mixed with the gas and then yet feed to the generator. I finally gave up with the process and spliced a gas shutoff valve into the gas line just before the carburetor so I could run it dry when not in use and never had another problem.

[DavidM]

My 2021 Onan QG 4000, spec R generator has a float bowl drain valve on the bottom of the carb. That will let you get all of the gas out of the carb and will avoid ethanol/water gunking it up. Much better than using a fuel shut off valve and running the engine until it stops. That will probably leave some fuel in the float bowl. But lots better than doing nothing.

David

[Rick 99037]

I too have a 4000 on my unit. I got all the paperwork from the previous owner had replaced the carb only a few run hours before. They charged him over $400 for the little carb alone! But a couple years had passed, and it came to me surging.
I haven't addressed my problem yet, and after I do, I don't think I can bring myself to do the required hours of running it under load, to keep it running smoothly.
I understand the problem is unleaded gas. There's a YouTube with a guy running double strength Stabilized gas, through a temporary bypass, that worked for him to clean the carb; And I understand a fuel cut off doesn't completely empty the carb, but it seems to have worked anyway for many anyway.

[dickb46]

I had the same problem on my old gas generator. I made a short fuel line for it and stuck in it a gallon of fresh gas with lots of Seafoam mixed in. It almost instantly gave up surging but I let it drink the mixture for an hour and never had a problem again.

[Swampy]

Same as dickb46 I made the short fuel line into the heavy sea foam/gas mixture and let burn 3/4 Gal of gas. Surge gone and haven’t had a problem since. La Mesa (used rig) promised the carb had been rebuilt. My bad for not running up it under load during walk through. You Tube for how it is done.

[Gentracker]

You can drain carb, at least on my 5500 I can, and eliminate need to cycle monthly. But I prefer cycling it monthly.
There is a screw at bottom, front, of carb that drains on mine.
Hope this helps

[Okimar]

Something else that can cause it is using a fuel pump, which I believe is probably an electric one, like mine.

What I learned fron Cummins is there are specific part numbers based on both the model and the VERSION.

The version, which is identified by a single letter, must be taken into account when ordering a repacement fuel pump.

(This problem left me in the Mojave Desert without AC for 3 days earlier this month...)

And, while you're at it, yours most likely has a fuel filter that screws into the fuel pump too.

You should absolutely go ahead and change that while you're at it.

Because of what happened to me, I'm now carrying a spare fuel pump and filter for mine in my kit from now on.

[4x4van]

Onan's fuel pump for the 4000, in addition to being ridiculously expensive, is also a royal pain to get access to. After having 2 go bad, I replaced mine the second time around with a generic (under $20) low pressure fuel pump, mounted on the RV chassis just outside the generator compartment. (I used a cheap generic fuel pump on my previous RV's Onan (BGE) as well, and never had another problem).

I also added a "Y" fitting with 2 shut-off valves in my fuel line: https://smile.amazon.com/dp/B09TD4J8...roduct_details
This allows me to run the generator from the onboard fuel tank, OR drop a short line into a gas can and run with a strong fuel/Seafoam mixture to help clean the carb. I suppose I could also shut both off and run the unit dry, although I generally don't do that since I try to exercise mine every month.