Onan Generator Doesn’t Start – Is The Fuel Pump Bad?

Why does the generator stop running after 20 minutes or so? If you wait a little while, it would start again without any trouble. So what’s the problem?

After doing a some research I was able to determine that the problem was a bad fuel filter. To be sure I tested the fuel pump using a simple test I read about online. The test really helped me diagnose and fix this problem. In this article I’ll explain how to perform this test on your Onan RV generator if you’re having a similar problem.

Why not a bad carburetor or fuel filter?

When cold, the generator would start up and run very smooth for around 20 minutes. If I let it sit for a while, the generator would start back up and run fine for another 20 minutes. This was pretty consistent behavior. This told me that the carburetor must be fine.

If the carburetor was bad, I suspect the generator wouldn’t start at all, or maybe start, run rough and stop.

If you’re saying to yourself “what about the fuel filter?” Well I figured a bad or clogged fuel filter would cause the generator to run bad all the time. So it was ruled out as well.

Once I suspected the culprit to be a faulty fuel pump, I wanted to test it before ordering any parts. Testing the fuel pump turned out to be pretty simple. The key was when to run the test?

When to test the fuel pump

I suspected that the fuel pump would function just fine if tested when the generator is cooled down. So I decided to do the fuel pump test just after the generator dies. That was my plan.

I started up the generator and waited for it to die. Sure enough, it died after about 20 minutes. That’s when I tested the fuel pump. Guess what? It failed.

How to test the fuel pump

Run this test immediately after the generator dies. The test will tell you whether or not fuel is being pumped to the carburetor at that time. All you’ll need is a bucket and screw driver to perform the test. Heres’ how:

1. Disconnect the fuel hose going into the carburetor (use the screw driver to loosen the hose clamp)
2. Point the hose into the bucket
3. Press the generator start or prime button (press left side of start button)
4. You should get a nice flow of fuel into the bucket if the fuel pump is working. If not, you’ll get a trickle or nothing
5. If no fuel comes out, order a new fuel pump since it’s likely bad (probably overheated). I had no fuel coming out when I ran this test on my generator
6. Attach the fuel hose back on the carburetor

Where to buy a new fuel pump

You could buy one from an Onan dealer I suppose. I bought my replacement fuel pump from Amazon. The parts you’ll need for this repair are in this Onan Generator repair parts list I created for you. It includes the fuel pump and filter as well as a few other common replacement parts.

Just click on each item to get more information.

Note: I suggest you order a new Onan fuel filter and replace it when you replace the fuel pump. It’s the only time you’ll be able to access it anyway.

Tools you’ll need

Before replacing the fuel pump, make sure you have these tools on hand:

1. T-30 TORX socket to remove the TORX screws holding the control board and other stuff (this generator has several T-30 TORX screws)
2. Vice Grips to pinch the incoming fuel line from the gas tank
3. A bucket or drip pan to place under the pinched fuel line in case it leaks
4. 1/2 inch socket or wrench to remove the solenoid or grounding bolt

How to Replace the Fuel Pump

Replacing your generator fuel pump and filter is a DIY project that you can perform. I’m not a mechanic and replaced the fuel pump and fuel filter on my generator without any trouble. The toughest part was removing everything just to get to the fuel pump.

The fuel pump is located behind and below a bunch of stuff on the left side of the generator compartment. To get to it, you’ll have to remove everything on top and in front of it. I suggest taking a picture of the area with your cell phone to help you when you put everything back.

Here’s a quick run down of how I replaced the fuel pump and filter:

1. Disconnect your RV from shore power
2. Disconnect the negative cable from your RV battery
3. Pinch the fuel line between the fuel tank and fuel filter (attached to the fuel pump) so that no fuel flows when you disconnect the hose
4. Place a drip pan below the left rear of the generator where the fuel pump is located to catch any fuel that runs out
5. Disconnect the fuel hose from both ends of the fuel pump and fuel filter (the drip pan should catch any gas that drips out)
6. Remove everything blocking access to the fuel pump (control board, solenoid, cables)
7. Remove the bolts holding the fuel pump and remove the fuel pump and filter
8. Screw the new fuel filter into the new fuel pump and screw it in place
9. Attach the fuel lines to the new fuel pump/filter
10. Put all of the other stuff you removed back in reverse order
11. Remove the vice grips from the fuel line and check for leaks
12. Do a quick inspection to see if everything is put back together and connected
13. Reconnect power to your battery and shore power (if necessary)

That’s it. You’re ready to test your fix.

Test The Generator

Left press the generator start switch and hold it down for a few seconds to prime the pump. Then press the right side of the start switch to fire up the generator. It should fire right up.

If you’ve done everything right, your generator will run and run and run. When it warms up, turn on a few things to put a load on it and let it run. Success!

Related Information

• How To Change the Oil In Your Onan 4000 Generator