When & How To Change The Coolant In An Onan Quiet Diesel 7500 Generator

If you’ve ever wondered, “Can I change the coolant on my RV’s Onan generator myself?” the answer is yes, you can! We’ve created a comprehensive, step-by-step tutorial on performing a coolant change on our Onan diesel generator as a DIY maintenance project. The tutorial includes a list of necessary items and a detailed set of steps, accompanied by photos and a video, to guide you through the process.

Before we begin the tutorial, we want to note that we performed this task on our 2005 Onan 7500 Quiet Diesel generator. However, it is a very popular model, and other Onan generator models will likely follow a similar process. Therefore, be sure to check your owner’s manual for any specific instructions related to your Onan generator coolant change, as other models or model years may have a different recommended schedule and/or procedure.

Without further ado, here are the steps for changing the coolant on 7500 Onan generators.

How Often Should You Service the Coolant On an Onan 7.5 kilowatt QuietDiesel Generator?

You can refer to your generator’s owner’s manual to check the service intervals for its cooling system. We did our Onan Quiet Diesel 7500 coolant change every two years, flushing the system and replacing the antifreeze and radiator pressure cap. Every other time (every four years), we also replaced the thermostat. Unfortunately, the thermostat can only be replaced if you can access the top of the generator’s engine. So if your RV’s generator isn’t on a slide-out, you may be unable to do that yourself (it can require dropping the generator down off its mounts to reach), although the rest of the job can be done without a generator slide.

So, the schedule for our former 2005 Onan QD7500 HDKAJ generator is:

• Coolant Flush & Change + Cap Replacement: Every 2 years (or every 1,000 hours, whichever comes first)
• Thermostat Replacement: Every 5 years (or 1,000 hours, whichever comes first). We performed this every four years with the coolant, as the parts aren’t that expensive, and it’s easier to do it all at once (and track it for our records).

This is what our 2005 generator required; however, please refer to the owner’s manual for your specific generator, as other models or model years may have different recommended schedules and/or procedures.

What Items Do I Need to Change the Coolant In an Onan 7500 Quiet Diesel Generator?

We’ll provide a list below. However, first, we would like to note that we use a regular automotive radiator flush, anti-freeze, and a replacement pressure cap. All of these items can be purchased at most auto parts stores (but we’ll provide links to get them online). We purchased our new thermostat and gasket directly from Onan (we’ll also provide links to these).

We also used pure antifreeze and mixed it 50/50 with de-ionized or distilled water, rather than buying the pre-mixed 50/50 coolant. It doesn’t matter which you choose. Regular tap water can be used, but ONLY if it is low in minerals. For such an important application, we don’t mind spending an extra dollar or two on distilled/bottled water (although we do use regular tap water for the flushing and rinsing steps).

PRO TIP & UPDATE: Since we first bought our rig and made the video below, Fleetguard Compleat “OAT” extended-life coolant has become available. This red-colored pre-mixed coolant can halve the frequency with which you need to flush and replace your generator’s coolant. We used it in both our genset and engine. Check your owner’s manual to confirm, but you should be able to take advantage of this awesome, updated product.

Fleet Cool ES Complete Oat Antifreeze/Coolant, Concentrate, 1 Gallon, 2-Pack. bitterment Agent Mixed in

• EXTENDED LIFE: Concentrate coolant designed for life-of-the-engine protection up to 1,000,000 miles of service
• UNIVERSAL COMPATIBILITY: Works with all diesel and gas engines, fully compatible with other coolant types for hassle-free maintenance

Buy on Amazon Price & availability info updated 2026-01-30 at 03:30.

Note that we’ll be including a replacement thermostat and thermostat gasket; however, this part of the job can only be done if your generator is on a slide.

Following is a list (with links) of all of the items you’ll need to complete the tasks:

• Coolant (choose the right one based on your current coolant):
  • Ethylene Glycol (Green) 50/50 Pre-Mix 
  • Fleetguard Compleat OAT 
• Radiator Flush 
• 13psi Replacement Radiator Cap 
• Metric Socket Set (includes 10mm needed) 
• Funnel 
• Transmission fluid funnel 
• Disposable Nitrile Gloves 
• Replacement Thermostat
• Replacement Thermostat Gasket
• Oil Drain Pan 
• Distilled Water (typically available at your local grocery store)
• A mat for lying on the ground and catching any coolant drips
• A rag (or two… or three… 😂)
• Empty containers for used coolant (empty gallon milk or water bottles work well)
• Pliers
• A length of clear plastic tubing  (may be needed if your coolant overflow tank remains full of fluid and needs to be siphoned)

How to Do a Coolant Change In an Onan Quiet Diesel 7500 Generator

Before we start, we want to note that the two most common challenges with this task are:

1. Getting all of the coolant drained out of the system
2. And then getting all of the air back out when refilling

Onan published a service bulletin (Onan Coolant Service Bulletin 981-1049) to address this issue, in which they recommend exercising care when filling the system by avoiding the pouring of water, flush, or coolant into the vent or overflow tubes. While the entire process can be somewhat time-consuming, the method we’re about to demonstrate was highly successful for us, and we never encountered a single issue getting our genset back up and running afterward.

Be sure to catch and contain all coolant and flush liquid and dispose of them properly.

Access the Radiator Cap

The radiator cap is located under a small plate beside the coolant and oil fills.

The radiator cap beneath the plate should slide right out, and you can remove the plate.

Remove the Radiator Cap

With the generator cold and your hands protected with nitrile gloves, pull up the cap and filler tube and hold it straight up as high as possible. Then, push down on the cap and turn it counterclockwise to remove it like any standard radiator cap. You may need to push down on the cap again to twist it off. Have your rag handy to catch any coolant dripping from the cap.

Remove the Coolant Drain Plug

With your pliers handy, identify the coolant drain plug under the generator. It should be clearly marked.

Place your catch/drain pan under the cap and use your pliers to loosen it. Once loose, it’s easy to finish opening the drain plug by hand, allowing the coolant to drain into your catch pan. Once all the coolant has drained out, replace the cap and tighten it back down.

If Needed, Siphon Coolant From the Overflow Tank

This step may not be necessary, but it’s common for the overflow tank to remain full of coolant even after the generator drains. If this is the case for you, you’ll want to use your clear plastic tubing to siphon out the fluid from the overflow tank now. (We’ve had to siphon the overflow tank every time we’ve changed our generator coolant.)

Flush the Radiator

Before beginning the radiator flush process, we want to note that it’s important to avoid getting fluid into the vent and overflow tubes as much as possible. The middle tube shown here is the vent tube, and it’s particularly important to avoid getting any fluids into that tube.

You can use a long-neck funnel with a flexible tube at the end to avoid getting fluid where it shouldn’t go. Slip the tube/neck of the funnel down into the fill tube to isolate any fluids to that tube only. We used a clean transmission fluid funnel  for this purpose, which works great.

Pour the entire contents of the bottle of radiator flush into the fill tube, and fill the radiator the rest of the way with clean water through the same tube. Pour slowly to give air time to escape through the vent tube. When the water reaches the top of the fill tube, replace your pressure cap and secure it firmly.

Fill the Expansion Tank with Clean Water

After emptying the expansion tank, again by siphoning if necessary, refill it with fresh water using a clean funnel. Add water until the level is between the low and full levels on the side of the translucent tank. Then put the cap back onto the overflow tank.

Start the Generator and Bring It Up to Operating Temperature

With the cap back on the overflow tank, start the generator (you can use the start switch right there on the face of the genset). Once the generator has run for a couple of minutes and has come down off fast idle, put a load on it and let it run for a bit to bring it up to full operating temperature. We liked to do this by turning on our electric water heater element and one of our air conditioners. We left them running for about 10 minutes to get the generator up to full operating temperature.

Turn Off the Generator After Ramping Down the Load

Turn off the generator by first ramping down the load by turning off any appliances. Allow the generator to cool down at idle for at least 2 minutes before shutting it down.

Allow the Generator to Cool Down

After shutting down the generator, wait a while to allow it to cool down before taking the next step. This might be a good time to take a lunch break to allow it to cool enough to avoid scalding you, because if you don’t let it cool first, you’ll have to be EXTREMELY careful draining the flush.

Drain the Flush

Pour the fluid from your catch pan into a spare plastic container, and place the empty catch pan under the coolant drain. Remove the drain cap again, allowing all of the flush to drain out from the radiator. Carefully removing the radiator pressure cap can help the flush drain (again, AFTER the genny has cooled). Once it’s completely drained, replace the drain cap.

If the expansion tank didn’t empty when you removed the drain cap (again, likely), siphon it using your clear plastic tubing.

Refill the Entire Cooling System With Fresh Water

Now that you’ve used radiator flush to flush out the coolant, you’ll want to use clean water to flush out the flush! Refill the radiator and the expansion tank with fresh water. Replace the pressure cap and the expansion tank cap.

Start the Generator Again

Re-start the generator, and once it has warmed up enough to come off fast idle, put a load on it again, just as you did before. Then allow the generator to run under load (again, by turning on the water heater, air conditioner(s), etc) for about ten minutes.

Turn Off the Generator After Ramping Down the Load

After the generator has run for about ten minutes, ramp down the load by turning off all loads, and let it cool down at idle for about two minutes before turning it off. Now you’ll need to let the generator cool down again for safety.

Remove the Pressure Cap After the Generator Has Cooled

Once the generator has cooled down completely, remove the pressure cap again.

Drain the System Again

Again, pour the fluid from your catch pan into a spare plastic container and place the empty catch pan under the coolant drain. Remove the drain cap again and allow the fluid to drain into the catch pan. Siphon the expansion tank again, as needed.

Replace Your Thermostat

If your generator is on a slide and your thermostat is due for replacement, now’s the time to remove the top of the generator to access the thermostat. Before opening the top of the generator, turn off the chassis batteries to ensure there’s no power going to the generator.

Once the cover is removed, take the opportunity to inspect the hoses inside the generator, checking them for cracks or other signs of aging. If they appear to be aging, now would be a great time to replace them.

The thermostat is located toward the back of the generator. You’ll need to remove two bolts (one on each side of the thermostat housing) to access it.

Remove the Old Thermostat and Gasket

Loosen the two bolts and lift the housing off of the thermostat. Remove the old thermostat’s gasket, and then remove the thermostat itself. (It will simply lift out without tools).

Install the New Thermostat and New Gasket

After using a clean cloth to wipe away any excess liquid around the seat of the old gasket, drop your new thermostat into place. Then place your new gasket on the seat and reinstall the gasket housing, tightening down the two bolts.

Replace the Generator Cover

Replace the generator cover and tighten down the screws to secure it.

Replace the Coolant Drain Cap

Replace your coolant drain cap (for the final time!), and tighten it firmly using your pliers.

Pour Fresh Antifreeze into the Coolant Fill Tube

Slowly and carefully pour the new antifreeze (pre-mixed or mixed 50/50 by you) into the coolant fill, being very careful not to get any fluid into the vent tube or overflow tube. Hold the filler tube as high in the air as you can safely stretch it, making it easier to fill as completely as possible. Fill your expansion tank with coolant to at least the cold/minimum line.

Turn Your Chassis Batteries Back On

Before restarting the generator, ensure that your chassis battery is turned back on. If you forget this step, you may have a quick heart flutter as you wonder why your generator won’t start! 🤦️

Start the Generator and Release the Air From the System

While holding the filler tube as high as possible, with the pressure cap still off, start the generator. Don’t worry about the cap being off — the coolant won’t splash out (and even if a little bit drips, it’s not hot yet, so no worries).

While continuing to hold up the fill tube, run the generator for a few minutes until it warms up and comes down off fast idle. Have a helper turn on a load inside the RV (again, the electric water heater element or an A/C unit or two). Run the generator under a load for a few minutes. This process is what releases the air from the system.

Turn Off the Generator After Ramping Down the Load

Have your helper turn off the loads inside the RV, wait a minute or two, and then shut down the generator.

Top Off the Coolant

After you’ve released the air from the system, you’ll likely see (by looking in the filler tube) that the fluid level has gone down. This is a completely normal part of the process of purging air from the system. Top off the coolant at this time by adding fluid slowly until it’s back at the top of the filler tube.

Install Your New Pressure Cap

Install your new pressure cap and then slide the fitting on the plate back over the top of the cap.

Reinstall the Plate

Slide the pressure cap and the plate back into place and tighten down the bolt that secures the plate.

Check the Coolant Level In the Expansion Tank

Check the coolant level in the expansion tank to ensure it’s above the minimum cold line. We generally add coolant to the expansion tank until it’s halfway between the cold/minimum and hot/maximum lines.

Start the Generator and Check for Leaks

Start the generator again and look under the generator to check for any leaking from the drain plug.

Run the Generator on High Load

Run the generator on high load for about 45 minutes to an hour. This helps any remaining air work its way out of the system. However, the generator may occasionally overheat and shut down during this process if there’s still air in the system (in our 20+ years of full-time RVing, we never had this happen). If it does happen to your generator, continue topping off the coolant through the filler tube and the expansion tank as needed, always ensuring that you never open the filler cap while the generator is hot.

It’s possible that it could take a couple of days of starting and running the generator and checking the coolant level, topping it off as necessary, so that you ultimately get all of the air out and the coolant as full as possible.

Check the Coolant Level In 24 Hours

Even if the generator doesn’t shut down during the above process, you’ll want to return and inspect the coolant level in the filler tube 24 hours later. If the coolant isn’t all the way to the top of the filler tube, you’ll know that more air has worked its way to the top of the system.

Top off the coolant level, bringing it back to the top of the filler tube’s neck. Check the coolant level in the expansion tank at this time as well, making sure it’s between the minimum and maximum lines. Check again to ensure there are no leaks under the thermostat or near the drain cap.

After you’ve run the generator a few more times over the next several days, continue the process of checking the coolant levels until you find the coolant all the way up to the filler neck and the expansion tank full to at least the minimum cold level.

Now your Onan RV generator is all set with this maintenance task for another two years (or longer if you’re using extended-life OAT coolant).

Video Tutorial

For a visual guide to this entire job from start to finish, follow along with us as we do the job on our former rig in the following video:

If you’re having issues with your Onan generator, check out our post on motorhome generator troubleshooting. And if it’s time for a generator oil change, follow along with our guide to your Onan diesel generator oil change.

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