Change the Coolant in Your Onan Diesel RV Generator

TheRVgeeks Annual Maintenance, Generator, Maintenance 11 Comments

As a follow-up to our recent video on generator maintenance (Servicing an Onan RV Generator), 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 ( Onan Coolant Service Bulletin 981-1049 ) 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.


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Comments 11

  1. Just completed the service. Thanks for the video, sure made it easy. One question, how do you keep your genset SOOOO clean? Seriously, it looks like it just came out of the delivery crate. :)

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      Author

      That’s great Mark! It’s always good to hear we’ve been helpful. Our generator has gotten a little older since this video was made several years ago, so it doesn’t look QUITE as new now. But it still does look pretty good for over 13 years old. Whenever we do a thorough wash of the RV, I pull out the generator slide, and give it a good cleaning and power washing along with everything else. We even touched up and painted a couple of small rusty spots last year. Being anal retentive does have its advantages. LOL

  2. Can overfilling the coolant reservoir cause the generator not to run? Mine has been running fine and I topped the reservoir off yesterday and it hasn’t started and run since?

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      Hi Steve. Are you saying that your generator worked fine, and then the only change you made was to add more coolant into the reservoir to top it off and now the generator won’t start? We’ve never topped ours all the way up, but know of no mechanism that would even sense that it was overfilled and therefore prevent it from starting. What make and model of generator do you have?

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          Author

          If it worked fine until you added more coolant, you could try siphoning out from the overflow tank until the level is back down below the full line, but we have never seen anything that indicates that an Onan generator should not run just because the overflow tank is filled up.

  3. Not a problem servicing my generator. Could not get more than 50% of the fluid back into it. Finally found service bulletin supplement 981-1049 on how to clear an air blockage from the vent tube. Would like to show a couple of pictures on how this is done but can’t find any way to attach them to this.

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      Author
  4. Great vid as usual.
    I would remove the old thermostat at the beginning, and reinstall just the goose neck. In doing so flushing and rinsing will be more effective as there will be greater flow, and a greatly shortened wait time during each phase. With this method, the new thermostat is installed just before the fill of the fresh/new coolant mixture.

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