Changing The Oil In An Onan Diesel Generator: How & When to Do It

Each spring, we serviced the generator in our motorhome as part of routine spring cleaning. And the Onan diesel generator oil change is so easy that it makes a great DIY project for almost anyone. In this post, we outline the step-by-step process for changing the oil in an Onan 7.5 kW diesel generator, allowing you to follow along and incorporate it into your DIY generator maintenance schedule.

How Often Should You Change the Oil In an RV Generator?

Changing the oil in your rig’s generator is part of regular Onan RV generator maintenance. In our old rig, the oil and oil filter needed to be changed every year, along with cleaning the spark arrestor. Some years also required air and fuel filter replacement, as well as coolant system service.

Typically, generator service intervals are based on either time or the number of hours the generator has operated. We generally didn’t use our generator enough to need service more than once a year. But, if you use your generator frequently and put a lot of hours on it, you may need to change the oil and filter more often, especially if you use your RV year-round. However, in general, the oil in an RV generator should be changed at least once a year. Be sure to check your owner’s manual for the recommended maintenance intervals for the specific year, make, and model of generator in your RV.

How to Change the Oil & Filter In an Onan Diesel Generator

To change the oil and filter in an Onan RV generator, you’ll take the steps we’ve laid out below. Note that our generator is an Onan 7.5kW Quiet Diesel RV Generator, but this process applies to many makes & models of Onan RV generators as well.

What You’ll Need

• Oil Drain Pan 
• Replacement Onan Air Filter 
• Replacement Oil   (rated for diesel engine use)
• Replacement Onan Oil Filter 
• Oil Filter Wrench 
• Socket Wrench 
• 10mm Socket with 3/8" Drive 
• Rag(s) 

You may also want something to lie on when you’re under the RV (like a large, flattened piece of cardboard).

Note that you can buy Onan Generator Maintenance Kits that include all the supplies needed for various genset models. Choose the one you need for your particular generator.

Identify the Location of Generator Items

First, let’s identify the areas of the generator we’ll need to access for this job. You can refer to the descriptive photo below as a visual guide:

• Oil Fill and Dipstick: You’ll find the oil fill and dipstick right on top of the generator, where they’re easily accessed.
• Oil Drain Plug: Heading under the RV, you’ll find the oil drain plug right in the center. On our generator, the plug requires a 3/8″ (or 10mm) socket to remove and replace.
• Oil Filter: The oil filter is located behind a small metal access door. On our generator, the door is gold in color. You’ll need to pinch together the two releases to swing the door open, where you’ll find the oil filter.

Loosen the Oil Filter

Before draining the oil, you’ll want to warm it up to help it run smoothly and completely out of the generator. But before you do that, we recommend using your oil filter wrench to loosen the oil filter slightly (just enough to be sure it’s loose). That way, if the oil filter is installed very tight, you won’t be fighting with it in a hot engine, possibly burning yourself.

Once you’ve slightly loosened the oil filter, hand tighten it back down prior to turning on the engine to be sure no oil comes out while it’s warming up.

Turn on the Engine

Start the generator to warm up the engine. In most cases, you should be able to start the engine using the start button on the front of the genny itself rather than going inside the RV to use any dash-mounted start switch. Either way, hold the control button until it starts.

NOTE: It’s perfectly normal for a cold diesel genset to take several seconds to start while the glow plugs preheat. Allow the generator to warm up for a few minutes until it comes down off of fast idle.

Since the goal is to warm the oil in the genset, we put a load on it for about five minutes. To do this, you can turn on the electric side of your water heater, for example.

Once the genset has warmed up, turn off all loads and let the generator return to idle. Allow the engine to cool down for approximately two minutes, then turn off the generator. The oil has now been warmed up and will run out of the engine more quickly and thoroughly, taking contaminants with it.

Loosen the Oil Fill Cap

Loosening the cap will allow the oil to run out faster in the next step. This faster rush of oil will help drain contaminants more thoroughly.

Loosen & Remove the Oil Drain Plug

Of course, you’ll want to ensure that your oil drain pan is in place before removing the oil drain plug. When you remove it, use caution to avoid getting burned by hot oil. This is where wearing protective nitrile work gloves is particularly handy.

Let the oil drain as thoroughly as possible, which will likely take several minutes. It’s not possible to remove every drop of oil, but once the stream of oil has slowed to a drip, you should be good to go.

Replace the Oil Drain Plug

Once the oil flow has slowed to a drip, you can replace the drain plug, being careful to start it by hand and confirm it’s not cross-threaded. Note that, unlike a car, the generator’s plug does not have a crush washer.

Hand-tighten the oil drain plug, then snug it down with your socket wrench. Be careful not to tighten it too much. The most common mistake people make here is overtightening the drain plug, which can strip the threads.

Wipe off any excess oil on and around the drain plug.

Remove the Oil Filter

Since you pre-loosened the oil filter before warming up the genset, you should be able to easily remove the filter by hand. Of course, be sure to have your oil drain pan underneath because the filter is full of oil.

NOTE: You can reverse steps 5 & 6 if you want to. Having the oil drain plug and the oil filter removed at the same time will allow the oil to run out of the drain pan and the filter at the same time. We don’t do that though, because trying to position the oil drain pan under the drain bolt and the filter mount simultaneously makes it more likely that we’ll spill oil on the ground.

Clean the Seat of the Filter Housing

Using a clean rag, carefully clean the seat where the oil filter screws onto the filter housing.

Lightly Coat the Gasket of the New Oil Filter

Use your finger to put a light coating of fresh oil on the gasket of your new oil filter. This ensures that it will install smoothly onto the filter mount without damaging the gasket.

Install the New Oil Filter

Install the new filter. Turn it until it just touches the block, and then turn it only another 1/2 or 3/4 of a turn. Important: Never use an oil filter wrench to tighten an oil filter. The wrench is for filter removal only.

Use a rag to wipe up any excess oil around the filter. Leave the access panel open for now, so you can check for leaks after adding the fresh oil.

Add Fresh Oil

Using a clean funnel, add fresh oil through the oil fill cap. Note that the oil fill and the dipstick opening are one and the same.

PRO TIP: Add about 1/2 quart less oil than the generator’s stated oil capacity. That’s because adding more oil as needed is much easier than removing some if you overfill. And running the generator 1/2 quart low for a few minutes while you check and adjust the level won’t harm it.

Start the Generator

Run the generator for about 5 minutes to warm the engine and new oil. Warm oil will run down into the bottom of the block more completely, ensuring a more accurate reading as you do your final check & adjusting of the level.

Check the Oil Level

Shut the generator down and wait at least a minute for the oil to run down into the bottom of the engine block. Then use the dipstick to check the oil level, and add more as needed. Be sure to fill only enough to get to the “full” line and no more (see our earlier tip about not overfilling).

Add oil and re-check the dipstick as needed until the level is correct at the “full” line on the stick.

Check for Any Leaks

Check for leaks near the oil filter and oil drain fill plug. If you don’t find a leak anywhere, close the filter access door (gold metal plate), and make sure it locks securely into place.

Check the Oil Level One Last Time

We like to check the oil one final time to confirm that the level is correct as we conclude the job. Oil is the lifeblood of any engine, and double- (or triple-) checking is well worth the small effort.

PROPERLY Dispose of Your Used Oil

Be sure to dispose of the old oil properly, which can often be done where you purchased it, or at a recycling center. Never, ever under any circumstances should used oil be thrown in the trash or allowed to run onto the ground.

And that’s all there is to an Onan diesel generator oil change!

Watch The Video Version

For a complete visual tutorial of this job, you can check out our video below, in which we change our generator’s oil & oil filter, air filter, and spark arrestor.

Remember to Exercise Your RV Generator

The single most significant cause of generator trouble is a lack of regular use. This is why “exercising” a generator is so important. Sometimes we were hooked up for fairly extended periods and didn’t need the genset, or we were boondocking in a very sunny location where our solar panels and lithium batteries provided all the power we needed. Either way, we made sure to exercise our genny regularly.

All genset owners should run their generator at least once a month for about 1-2 hours under load. For us, this meant firing it up even if it wasn’t needed for power or battery charging and turning on both air conditioners (or heat pumps). We would also heat a tank of hot water or plug in our portable space heater.

Exercising a generator is an essential part of RV maintenance. Doing it helps ensure that all internal parts stay properly lubricated, fresh fuel is circulated through the system, and the engine reaches its operating temperature, which helps burn off moisture that can lead to rust. Over the years, we have had more than one service technician tell us that the generators they see with the most problems are the ones with the fewest hours on them.

If you’re having trouble with your RV generator, be sure to check out our post on motorhome generator troubleshooting.

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