Maintenance of Your Onan Diesel RV Generator

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Published on Apr 25, 2012

How To Change The Oil In Your Onan RV Generator. We show the annual maintenance & service, including spark arrestor cleaning and air filter replacement, on our Onan 7500 Quiet Diesel.

One of the greatest freedoms we get from RVing is the ability go anywhere, any time, completely self-contained. Our Onan 7.5 kW QuietDiesel generator is a key part of that freedom. Proper care and maintenance of the “genny” will lead to a long life of trouble-free operation.

Each spring, we service our genset as part of routine spring cleaning. The oil and oil filter get changed every year, along with cleaning the spark arrestor. Some years also call for air & fuel filter replacement and coolant system service too. Generator service intervals are based on time or hours they’ve run, but we generally don’t use our generator enough to need service more than once a year.

We take care of all of these items ourselves, and in nine years of RVing, we’ve never had anyone else work on our Onan. We have never had a day of trouble with it, and the routine service is so easy that we’ve never needed any outside help. We’ll cover the three most basic service items here.

We just changed the fuel filter last year, so we won’t be demonstrating that today. And the coolant system service is involved enough to require its own video, so we’ll be covering that within the next few weeks. In this video, we’ll show how to change the oil & filter, replace the air filter and clean the spark arrestor.

From everything we’ve heard and read, the single biggest cause of generator trouble is lack of regular use. There are times when we’re hooked up for fairly extended periods and don’t need the genset, but we make sure to exercise it regularly anyway. The generator should be run at least once a month for about 2 hours under about half load. This means firing it up even if it’s not needed for power, and turning on both air conditioners (or heat pumps) and heating a tank of hot water, or plugging in our portable space heater.

Whenever staring the generator, be sure to let it idle for a short time to warm up before putting a load on it (turning anything on that draws power). Also, before shutting the genny down, be sure to turn off all loads first, then let it idle for about two minutes to cool down before shutting it off.

Our generator will likely see more use this coming year than ever before. After being hooked up all winter, we’re about to spend our first year out on the road with a new residential refrigerator that recently replaced our dead Norcold.

Our Onan (an extremely popular brand) is a diesel model, and runs off the same fuel tank as our engine. We have no experience with gas generators or other brands, so many things about them may be different than ours. Be sure to follow your manufacturer’s instructions for operating and maintaining your generator.

As a follow-up to our recent video on generator maintenance, when we cleaned the spark arrestor and changed the oil, oil filter & air filter, today we’re servicing the cooling system.

We flush the cooling system and replace the anti-freeze and radiator pressure cap on our Onan 7.5 kilowatt QuietDiesel generator every two years. Every other time (every four years) we also replace the thermostat. Unfortunately, the thermostat can only be replaced if you have access to the top of the generator’s engine, so the generator must be on a slide-out to do it yourself. The rest of the job can be done without a generator slide.

We use regular automotive radiator flush, anti-freeze and replacement pressure cap, all of which can be purchased at most auto parts stores. We purchased the new thermostat and gasket directly from Onan. We also chose to use pure antifreeze and mix it 50/50 with de-ionized or distilled water, as opposed to buying the 50/50 pre-mixed coolant. It doesn’t matter which you choose. Regular tap water can be used if it is low in minerals, but for such an important application, we don’t mind spending an extra dollar or two on bottled water (although we do the flushing and rinsing of the system with regular tap water).

The two most common challenges with this task are getting all of the coolant drained out of the system, and then getting all of the air back out of it. Onan published a service bulletin to address this, in which they recommend using care when filling the system, by avoiding pouring water, flush or coolant into the vent or overflow tubes. While the whole process is a bit time-consuming, the method we demonstrate has been very successful for us, and we’ve never had a problem getting our genset back up and running without incident.

Here’s how to replace the fuel filter on an Onan Quiet Diesel generator. Our previous Onan generator maintenance videos covered annual and bi-annual servicing.


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Full-Time RVers since April 11, 2003, we share DIY (do it yourself) RV maintenance, repair, travel, upgrade and operational tips & tricks.

While we’re not RV technicians, we’re very mechanically inclined and have learned a lot about RV systems over the years. We’ve handled most of our own minor service, maintenance and upgrade work on both of our RVs.

We meet lots of newer RVers who are eager to learn some basics about using, maintaining and caring for their rigs. After more than a decade on the road, we’re happy to share what we’ve learned (some of it the hard way). 😉 We hope our experience can help other RVers go DIY, saving time & money while experiencing the satisfaction of a job well done.

We are not professional RV technicians and do not pretend to be experts on any particular topic. We mostly know about maintaining our own motorhome, so be sure to confirm that all methods and materials used are compatible with your equipment. Every RV is different, so your systems may not be the same as ours. Regardless of what we recommend, consult a professional if you’re unsure about working on your RV. We encourage you to do your own research. Any task you perform or product you purchase based on any information we provide is strictly at your own risk.

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