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How to Replace the Fuel Pump and Fuel Filter on an Onan Model 5500 RV Generator

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A standard Onan Model 5500 RV gas-powered generator.

A standard Onan Model 5500 RV gas-powered generator.

RV Generators Require Maintenance

Having a functioning power generator in your RV is so convenient when camping that most motorhome owners, cannot imagine not having one for their travels.

But, of course, a power generator, is a mechanical device and it will eventually require maintenance and care at some time.

It's a sad truth that all mechanical devices will eventually break down, even if they are maintained properly. So every RV owner who has an onboard generator should be prepared to have certain things go bad and need some level of maintenance.

What Happened When My RV Generator Died

Recently, my ONAN generator stopped running for me while I was camping. It had a little over 500 hours on the meter, and it had always been maintained properly.

It had been running fine until it just stopped running.

I could crank it, but it would not fire up at all. I had to fix this, of course, so after a little troubleshooting. I suspected it had a bad fuel pump or fuel filter.

Since this is a common problem with generators, I zeroed in and did a little more investigation to confirm my suspicion.

Troubleshooting My RV Generator

To confirm what I thought was wrong with my generator, I went through a thorough troubleshooting procedure and eventually confirmed that the fuel pump on my generator was bad.

During my diagnostics, I removed the cover of the generator and determined, among other things, that:

  1. There were no loose connections visible.
  2. There were no leaking fluids.
  3. The motor had oil in the reservoir.
  4. The DC power source was good.
  5. The fuel filter was clean.

The first step in troubleshooting just about anything is to remove the obvious things from the list of potential problems.

View of the fuel pump and fuel filter on an Onan 5500 RV generator.

View of the fuel pump and fuel filter on an Onan 5500 RV generator.

Finding the Fuel Pump

And I discovered one other thing; On my generator, an Onan model that is built especially for motorhomes, they didn't put the fuel pump inside the generator compartment.

Rather, for some unknown reason, they decided to mount the fuel pump and the fuel line filter onto the underside of the generator base plate.

I should note here that over the years, these generator manufacturers can change their designs numerous times so never assume the one in front of you is going to be just like the last one you worked on.

They might make a core generator, like my 5500-watt model, and then have a dozen or more configurations of the basic generator to fit numerous applications.

For instance, where mine is set up for my motorhome, the same core generator unit might also be configured as;

  • a portable model on wheels,
  • a home backup power source,
  • a remotely-controlled model for remote site backup power,
  • or any of dozens of other special applications.

In these specialty designs, peripheral parts like the fuel pump, fuel filter, carburetor, air filter, and oil drain and input, can take different forms, or, as is often the case, be mounted in different positions.

The position of the fuel pump and fuel filter on my RV though meant that to replace the fuel pump, or even just to replace the fuel filter, you had to get under the RV and work while lying on your back. I didn’t like this, but it was what had to be done.

Ordering the Fuel Pump and Fuel Filter

I decided to replace both the fuel pump and the fuel filter, as people often do because they are mounted close to one another and are cheap, and I wanted to make absolutely sure that I replaced whatever was causing the problem.

One of the great things about having a smartphone is that you always have a good camera on hand. So, even before I looked at the generator manual, I crawled under my RV and took several pictures of the fuel pump and the fuel filter that showed the labels as well as how they were mounted.

Back home, I sat down with my specific part numbers and pictures and shopped the web for the best deal on the parts I needed.

It turned out that Amazon gave me the best price and I was able to get free shipping within five days of placing the order. When I finally received my parts, it was time to install them and get my generator running again.

Installing the New Parts

Safety First!

Here is where I must mention that when you work on a machine like a gas-powered generator, there are certain safety procedures you should follow.

I disconnected the 12-V DC to the generator, shut off the fuel lines, and also followed the other manufacturer requirements listed in the owner's manual.

Once all of this was done, I was ready to perform the actual replacement.

View of the fuel cut-off valve on an Onan 5500 gas RV generator

View of the fuel cut-off valve on an Onan 5500 gas RV generator

Tools Required

By looking at the pictures I had taken, I was able to determine the tools I would need for this job:

  • Flat-blade screwdriver (for the hose clamps)
  • Crescent wrench (for disassembling the fuel pump and fuel filter from each other)
  • 5/16 socket and wrench set (for the two bolts that held the fuel pump to the base plate)
  • 1/4-inch diameter wood dowel (or another device, to plug the fuel line from the fuel tank)
  • A short piece of plumber’s tape (to assure a good seal between the fuel pump and the filter)

Knowing this ahead of time allowed me to take to my RV only the tools I would need, not a huge toolbox full of tools.

Fuel Pump and Fuel Filter Replacement Procedure

Since in my case the fuel pump and the fuel filter were on the underside of the generator, I checked and saw that there were only two things I had to disconnect on the upper side of the generator:

  • The two wires that provided power to the fuel pump,
  • The actual output fuel line from the fuel pump to the input of the fuel cut-off valve that is inline to the carburetor of the generator.

So, the very first thing I did was disconnect these two wires and this fuel line.

  1. Then I crawled under the RV, removed the fuel line from the fuel filter, and stuck a piece of the wood dowel into the hose to prevent it from dripping onto the ground and me.
  2. I then removed the two screws that held the fuel pump in place with the socket wrench.
  3. Carefully, I pulled down the two wires and the fuel line with the fuel pump-fuel filter combination attached, and with this assembly in hand, crawled out from under the RV.
  4. I assembled the new fuel filter onto the new fuel pump making sure that the connection was sealed well with a piece of plumber’s tape.
  5. I pulled the short piece of fuel line off of the output of the old fuel pump and attached it to the output of the new fuel pump, using the same hose clamp.
  6. I took the time and compared the two assemblies to make sure they were essentially the same dimensions and that the new one should fit properly.
  7. I crawled back under the RV and fed the two wires and the fuel hose up and through the gap through which I had removed them.
  8. I then mounted the new fuel pump where the other one had been, with the same bolts.
  9. I connected the fuel line from the fuel tank to the input of the new fuel filter and reused the same hose clamp on this.
  10. Then, checking that everything fit properly and was firmly mounted, I crawled back out from under the RV.
  11. All that remained now was for me to reconnect the two wires, and then slip the end of the hose from the fuel pump onto the input connection of the cut-off valve. I reused the same hose clamp on the hose connection.
  12. Having had finished the replacement of the fuel pump and fuel filter without incident, I reconnected the 12-V DC and got ready to test how the generator worked with the replacement parts.
After installation, the new fuel pump and fuel filter looked like this.

After installation, the new fuel pump and fuel filter looked like this.

Testing the Generator

I was now ready to run a quick test on the generator to assure that my new parts had solved the problem and the generator was now operating properly.

Using the two-position switch that is installed with my generator (with a parallel one on the RV dash), I pressed the lower section and primed the generator. Do see the operator's manual if you are not familiar with this commonly-used switch. I noted that the pump was making a sound and vibrating.

Then I pressed the other side of the switch to start the generator, After a couple of seconds of cranking, the generator fired up and started idling.

I then let the generator run for almost ten minutes.

Because it was running evenly after that period of time, I went inside and checked that my RV's power control panel indicated that I had 50-amp service and that power was available for ALL of my AC appliances.

I then turned on one of my air conditioners, to check the generator under a load—the air conditioner uses a lot of power—and after the AC had cycled and was running properly, I went outside and watched my generator run for a couple of more minutes as the overall load shifted on and off with the AC compressor.

Since the generator seemed to be operating properly after several minutes under a load, I pronounced my task done.

I went inside, shut down the AC, and turned off the generator. I was now ready for my next trip confident that I had a functional AC generator in my RV ready to provide me with the comforts I am used to.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

Question: I have a 2008 Fleetwood terra with an onan marquis 5500 generator. The generator will start and come to full power for about 3 minutes then it starves the engine and stops. I was told the control box on these units causes this problem however I don't want to spend 500 bucks on a new one if I don't have to. Any thoughts?

Answer: First of all, remember that motorhome generators have fuel lines into the main fuel tank, but it only goes down to the 1/4-tank level. This is because so many motorhome owners would out out and "rough camp" and use up all of their gas running the generator and then not be able to get out of the wilderness and refuel.

Generator rule #1 - Make sure you always have over 1/4-tank of gas.

Next, the fuel filter and fuel pump, which are mounted below these generators are notoriously cheap, and the filters gum up and the fuel pumps go bad. You can pick up replacements almost anywhere even on Amazon.

See my Hub on this problem for directions on how to easily replace these parts.

Question: The fuel pump makes the sound that it is pumping. That said, how do you know it is pumping at the proper pressure for the generator? Is there any other test to confirm the pump is, in fact, working to specification?

Answer: The fuel pump and the fuel filter attached to it are both pretty cheap, and the filter often gets "gummed up." I would not bother with the pump until you were sure you had a good clean fuel filter on it.

In fact, I hate crawling under the generator area of my motorhome so much that I usually purchase both parts at the same time and replace them at the same time.

As to the actual speed of the fuel pump, it is designed to provide adequate fuel pressure for the generator to run well, as long as it has fuel. That said, I again recommend replacing this cheap part if you have doubts about it operating properly.

Question: After installation, my fuel pump will not work. I tested the pump with power and ground on auto battery. The pump is good. I suspect a grounding issue. Do the mounting bolts need to be a specific material and size? Mine mounts to a welded plate on the side of the generator support cage. Should it mount to the bottom of the generator?

Answer: The mounting bolts for the generator would typically be steel.

When in an RV, the fuel pump gets its ground from the chassis ground of the RV via the black wire bolted to the generator chassis.

If you place your hand on the pump when it is operating, you can feel the vibration.

If you suspect your ground then clip wire to the ground wire of the fuel pump and the other end to the chassis of the generator and then try to start the generator. While it is cranking the pump should operate.

If it doesn't pump at all then get a multimeter and see if you can measure the voltage from the hot wire of the pump to its ground. You should read the same as you do across your COACH battery terminals, around 13.5-VDC, while the Gen is cranking.

Question: I have a 5500 Marquis in my 5th wheel trailer. Someone stole my fuel hose that goes to the stand-alone fuel tank. I hooked a hose to the generator and into a tank of fuel, it will start and run for about 15 seconds. Does the fuel line have to have the special quick-connect hooking to the tank to run properly?

Answer: From your symptoms,I assume you have an external fuel tank and an external generator that you use for your FIVER?

With this being the case, I can say that your generator's fuel pump pulls the fuel into the generator. So, if the Fuel Pump is operating and the fuel filter is clear, the generator should operate OK.