Onan Generator Troubleshooting and Maintenance

By Kevin Quiambao

When we first looked at Wanda, we identified a few problems and knew she would need some work before she was ready for the road.  However, since we are both new to RV life and are unfamiliar with how a lot of the systems work, we decided it would be smart to get a pre-purchase inspection to ensure there were no surprises.  Based on that inspection, we purchased Wanda.  The day we picked her up, we were super excited to explore and test our new house on wheels.  Unfortunately, we quickly discovered that the company who performed the pre-purchase inspection failed to identify several major issues, including a problem with the on-board generator.  
 
Wanda's generator is an Onan 4K MicroQuiet Generator.  It has relatively low hours with 168 hours.  When we started the generator, we noticed that it sounded funny and after a couple of minutes sputtered and died.  We could not get it to turn back at all for for several hours and once again it immediately shut off. 
 
After many hours of doing research, I found several possibilities that could be causing the problem.  It is important for generators to be consistently exercised and in our case, Wanda sat unused in the prior owner’s driveway for over a year.  This sitting caused a lot of damage to the generator (and depleted the batteries).  Since I couldn’t find a comprehensive list for troubleshooting generator issues, I decided on a step by step process to diagnose our problem, fix the issue and perform some necessary maintenance.  To start, I purchased the following: an air filter, a spark plug, fuel filter, fuel pump and two quarts of 15W40 Oil:
 
Airtex E11007 Electric Fuel Pump for Onan Generator Set
Cummins 1670275 Onan Spark Plug
Cummins Onan 149-2457 Fuel Filter
Cummins Onan (140-3280 Spec B Air Filter)
(2) Quarts 15W40 OnaMax Oil
 
To dissemble the generator and replace the above items, you will need the following tools: 
T-30 Torx Bit
3/8" ratchet with an extension
9/16" Spark Plug Socket
Spark Plug Gapping Tool
Oil Drain Pan
Funnel
Paper Towels
Rubber Gloves
Permatex High Temperature Thread Sealant
Wire nut or something similar to plug a gas line

First things first.  Remove the negative side of the all the batteries for your safety.  Then, remove the generator door by opening both levers.  I highly recommend taking a picture with your cell phone as a reference point.  You can look back at the picture to see where wires and parts are orientated.  I forgot to do this and had some trouble remembering where some of the wires went.


Let’s start with the easy stuff.

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Clean the Spark Arrestor: 
There are two plugs on the bottom of the muffler.  Using an adjustable wrench remove the two plugs and turn on the generator.  It will be louder than usual.  Let it run for a minute or two under load, turn it off and let cool.  After the mufflers are cool reinstall the two plugs. 
 
To replace the Air Filter: 
Unscrew the wing nut and remove the air filter cover, which will reveal another wing nut. Unscrew the 2nd wing nut and pull out the air filter.  It could be dusty inside, so do a quick clean and replace with the new air filter.  Return the wing nut, put the cover back on and replace the last wing nut.  Do not over tighten. 

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To replace the spark plug: 
Identify where the spark plug is located (for our model, it is located to the right of the oil cap).  Grab the end of the spark plug wire and pull the boot straight out.  Using a spark plug socket and a ratchet remove the spark plug.  Take note of the condition of the spark plug and compare the old one to the new one.  Make sure the plugs are identical by checking the gap on both plugs with a spark plug gap tool (ours is .025, but check your owners manual).  Once you identify they are the same, hand screw the new plug and use the ratchet to tighten.  Do not over tighten.  Replace the spark plug wire by pushing the boot straight in.  You should feel it snap in. 

To Replace the oil: 
The Onan 4k generator does not have an oil filter so oil changes are super easy.  Under the generator is a small yellowish cover.  Using a T-30 Torx bit, unscrew the bolt from one side of the cover and loosen the other side but do not remove bolt.  Rotate the cover away exposing a 9/16 drain plug.  Now prep your work area.  You need to be able to catch the oil coming from the generator.  I used an upside down bucket and placed my oil drain on the bucket, so it didn’t splatter.  Using a ratchet, a 3-inch extension, and 9/16 socket loosen the drain plug.  Once the drain plug is loose enough to hand turn be ready to catch the oil by moving the oil drain under the hole.  Let the generator drain completely.  Once every drop is out, grab a paper towel and clean off the drain plug and the hole making sure there's no debris or sand left on the mating surfaces.  Next, screw the drain plug back into hole and tighten with your ratchet.  Again, do not over tighten.  Rotate the cover back, align the bolts and re-tighten using the T-30 Torx bit.

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Inside the generator door will be a yellow oil cover, unscrew it and place your funnel down the hole.  Pour in 1.6 of 15w40 OnaMax Oil.  Remove the funnel and clean the cap dip stick with a paper towel. Screw the cap in all the way and then remove to check the oil level. I've read a couple of articles that say these generators perform best when the oil level is just above the fill side.  Overfilling the generator can cause your generator to shut off intermittently.  Check your owner’s manual for your specific model. 

To Replace the Fuel Filter and Fuel Pump:
Under the generator on the left side there will be a fuel line that runs into the fuel filter.  Using a flat head screw driver loosen the hose clamp and pull the line off the fuel filter. After the hose line is off the fuel filter, use something to plug the hole to prevent fuel from leaking out. I used a wire nut which fit perfectly. Then prop the hose line out of the way facing up so it doesn't leak. 

Next, move to the front of the generator and remove the front two screws on the bottom left using a T-30 Torx Bit.  Take a picture of the solenoid.  Note which wires go where.  Remove all the wires and unscrew the solenoid.  Next, remove the circuit board. This may be tricky to remove, but keep working it and it will eventually come out.  Unplug the cables and put the circuit board aside.  You can now see and access the fuel pump and fuel filter.  Remove the screws holding the fuel pump in and unplug the fuel line.  There are two wires coming from the fuel pump.  The black wire is the ground.  Follow that line to the ground and unscrew the bolt and release the wire.  The next is a red wire that you can pull apart.  The fuel pump and fuel filter will come out as one unit.  Next, take your new fuel filter and apply Permatex High Temperature Thread Sealant on the threads.  You don't need to apply a lot, just enough to cover all the threads.  Screw the new fuel filter into the new fuel pump.  Use a ratchet to tighten.  Plug the fuel line coming from the generator into the fuel pump.  Run the new ground to the existing ground and attach the red wires.  Reinstall the fuel pump by screwing the fuel pump back down.  Retrace your steps and put everything back.  Plug in the circuit board and maneuver it back into place.  Next, attach the wires back into the solenoid (use the picture on your phone as a wiring diagram) and then screw the solenoid back into place.  Then replace the two screws in front of the generator.  Remove whatever you put in the hole to plug it and shimmy the fuel line back into place.  Tighten the hose clamp over the fuel line and the fuel filter and secure tightly.  Make sure to put the cover back on, clean up and dispose of the oil properly. 

Plug the negative side of the battery back in and you should be ready to test.  Turn on the generator and cross your fingers.  For us, the problem was the fuel pump and fuel filter.  We tested the generator after each of the above steps, but the problem still existed until we replaced the fuel pump and filter.  When I started the generator that time, it started right up but nothing inside had power.  I had to restart the line circuit breaker on the generator and then everything worked perfectly. 

I then took a picture of the hours of the generator as a reference for the future so we know when the generator was last serviced.
 
Nice job you deserve a beer and hopefully saved yourself hundreds of dollars. 
 
And remember, exercise your generator with a load once a month to preserve the life of your generator.