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Replacing Carburetor on Onan 5500 Generator 

 March 11, 2019

By  Derek

When we purchased our 2014 Keystone Raptor, the on board Onan 5500 generator only had approximately 50 hours on it. We think our rig may have been a repo unit and the previous owners rarely used the 5th wheel. 

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Onan 5500 Durability 

Onan 5500 generators are known for their durability and reliability.  Which is why most RV onboard generators are Onan 5500s. They usually do not require much maintenance and they really last.  

Two Major Issues 

Although they don’t require much maintenance, they do have two common issues that should be noted.

The first and most common issue with this generator is the fuel pump.  Although the fuel pumps on these generators frequently need to be replaced (compared to other generators), it’s not the fault of Onan and Cummings.  Onan actually makes great products and all of their generators are well engineered.


Generator Problem or RV Problem 

Ironically, the problem is with how the RV itself is engineered.

The fuel pump is designed to pump fuel (efficiently) at a distance of approximately six feet.  Most toyhauler rvs are over 38ft long and the fuel cell is usually located in the rear of the rig.  

That means that a well designed low pressure fuel pump is constantly straining to pump fuel from a tank set at a distance of at least 38ft. Our Raptor is 41.5 ft long so that little fuel pump works hard to pump that fuel.

Recommendation from Cummings 

I spoke with an engineer at Cummings and he recommended an easy fix for this issue.  He said that by installing an additional fuel pump halfway between generator and the fuel cell, the fuel pumps  would not work as hard. By adding an additional fuel pump, he said that it save you from replacing the Onan 5500 generator fuel pump.

The Second Common Issue 

The second problem is also a fuel issue.  But again this issue is not the fault of Onan.  The second most common problem with these generators are the carburetors.  Generators are meant to be ran under load. Unfortunately most RV owners only crank their generators when shore power goes out or shore power is not an option.

"Gunked Up" Is A Real Phrase 

By allowing the generator to sit for extended periods without running under load, the carburetor gets “gunked up.”  This means that when it’s time to actually NEED a generator, it either doesn’t run correctly or it doesn’t start at all.

Our generator had 50 hours on it when we purchased our rig and it was a 2014 model. That means that the generator was definitely not ran like it was designed to do.

Changing The Carb Is Easier Than You Think

The generator cranked when we purchased it but I did notice that it was running rough. I knew that the fuel pump was a common issue with these generators and I also knew that Onana designed these generators to be be easy maintained.  So I thought that it just needed a new fuel pump.

It turns out that it also need a carb.  I thought this was going to be a huge job but its was a lot easier that I expected.

I made a short video of the carb change that you can see below.  

VIDEO: Replacing the carburetor on an Onan 5500 Marquis 


Trouble Shoot These First 

If you have one of these Onan 5500 Generators and you are having fuel issues, Start with the fuel pump and then try changing the carb.  

If you need to see exactly how to change the carb, see the video and feel free to comment below with any questions.

Hopefully this post has helped.  Please like it and feel free to comment below.

About the author


Derek (Dad) is the tall, goofy guy that loves life and is a friend to everyone. He is also notorious for leaving his dirty underwear around the house, and then asks Steph why there are no clean ones.

  • Hi, I’m not mechanical at all. Trouble starting onan marquis 5500, I replaced carb yesterday, finally got it to run, yay! Today won’t start at all, crank but no start. Will run if I spray starting fluid for a moment. Any ideas? Thanks!

    • Gregg, Thanks for the comment. I had a similar issue with mine. Unfortunately, I troubleshot my generator completely backward. The 5500 gas gen sets are notorious for fuel problems. I’ll try to make this short because I know how frustrating it is to write back and forth with generator problems.

      Firstly: Is your Genset currently showing any fault codes when you crank it or when it turns off? If so please let me know what code it is showing…depending on the code, it might change everything that I’m about to write below…lol

      I would save yourself a ton of money and time and take the following steps. DISCLAIMER: This is what worked for me after I replaced two fuel pumps, two carburetors, countless fuel filters, added a fuel pump to the middle of the rig ( I added a fuel pump because an Onan Rep told me that the fuel pumps were designed to pump fuel at a distance of 6 feet. He said that the fuel pumps go bad so often because RV manufacturers install them to pump fuel at 30-40 foot distances).

      This is what I finally did to get my Onan 5500 running like a champ:

      1: Dropped the fuel tank and drained all the old fuel
      2: Purchased and installed all parts in an Onan 5500 Grand Mar. Tune-up kit (Purchased from Amazon)
      3: Changed the oil
      4: Replaced the Carburetor
      5: Replaced the Fuel Pump
      6: Added new fuel

      After these 6 steps, the generator starts right up and runs like a top every time.

      Keep in mind that these generators do not like ethanol gasoline. It will gunk up the filters and carb quick if the generator is left sitting for a short period of time (1-2 weeks). Try using ethanol-free gasoline whenever you can. I run mine under load about once every 10 days to prevent gunk buildup.

      Also, the oil dipstick is not accurate on these gen sets. If you put too much oil, the generator will sometimes show symptoms of a bad fuel pump when really it has too much oil. The problem is that the dipstick will show all good. If your dipstick shows that you are full, drain some oil until it shows slightly low. Sometimes, this will correct the issues if you have good gas, good plugs, a good carb, and a good fuel pump.

      Let me know if you have any questions about those 6 steps.

      -Derek

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