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Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 14906
Experience:  Certified on Onan and Generac generators
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I have a Cummins Onan generator set in my race car trailer.

Customer Question

I have a Cummins Onan generator set in my race car trailer. I purchased it new with the trailer on 09/02/10. It is a 5.5HGJAB-1038D, s/n I100152202, and presently has about 1,100 hrs. on it. I've changed the oil and filter every 200 hrs. and new plugs once so far.
Now the problem: It has always had a "surging-pulsing" when running. You can see the engines governor linkage moving up and down. Until now, it has not been unlivable although very annoying. Now, it's pulsing so bad that the lights and air conditioner are also pulsing. The surging then shuts the unit off and codes (sorry I didn't record the code). I've tried adjusting the governor and spray cleaner in the carb, but no help..........
Submitted: 1 year ago.
Category: Small Engine
Expert:  Dan replied 1 year ago.
Hi my name is***** will be glad to help. please grab the codes the generator is throwing so we can help narrow things down in troubleshooting. Click on this link for the manual and fault codes list and let me know what you have for codes. There is some troubleshooting information covered in this manual as well. Let me knowManual
Expert:  Hank F. replied 1 year ago.
Hello, my name is ***** ***** I am going to assist you with this. I am a certified Onan technician. The manual in the link above is not the correct manual for your unit. Most of the information is the same, but there are a few differences. Surging is caused by a lean running condition. Either too little fuel, or too much air.Unless you have taken parts of the engine apart, the chances of an air leak are very slim.Most of the time this is caused by a bad carburetor, but can also be a weak fuel pump, or a clogged fuel filter (extremely rare). The first thing to do is to check the fuel flow from the pump.To do the test, remove the fuel line from the carburetor. It will be necessary to cut the crimp-type hose clamp.Place the fuel line in and empty container.Press and hold the 'STOP/PRIME' button for about 10 seconds.When you first press the button, there will be a 3 second delay before the fuel pump activates.When the fuel pump does activate, it should make a faint 'humming' sound. It should not make a loud 'clacking' sound. A loud 'clacking' sound indicates that there is air in the pump, and can indicate either a faulty pump,or no/not enough fuel getting to the pump.There should be an immediate strong, steady flow of fuel in a pulsing flow. If there is no fuel flow, or if the flow seems very weak and thready, locate the fuel supply line.The fuel line attaches to the fuel filter, and the fuel filter is screwed directly into the pump, which is located underneath the front left corner of the genset.Remove the supply line from the filter, and attach a length of fuel hose to the filter.Place the end of the hose into a gas can.Now press and hold the 'STOP/PRIME' button again, for about 10seconds.If you still do not get sufficient fuel flow, the pump is bad and needs to be replaced.If you now get good fuel flow, but did not on the first test,either your coach is low on gas (there must be at least 1/4 tank for the set to run), or the fuel lines between the tank and genset are bad..You might also try replacing the fuel filter, although with today's manufacturing processes, it is rare to see a dirty filter. If none of this takes care of it, or finds the problem, then you are looking at a bad carburetor that must be replaced.