Monday, December 5, 2022


Ask Dave: What’s the proper procedure for winterizing an Onan genset?


Dear Dave,
Some health issues and the price of fuel are keeping me close to home this winter, so I’m in the process of winterizing the RV. I checked with Onan on procedures for the 5.5 Marquis Gold generator and was told to run it under load every 2 to 4 weeks. I was going to pull the batteries and store in the basement with a trickle charger but that would necessitate reinstallation each time. It gets pretty cold in Upstate NY, so I didn’t want to leave them in place or reinstall them in tons of snow. It’s stored outside next to the garage and I have both 120 v outlets and 30 amp RV outlet available. Are there any other options I can consider?  —Bud, 2005 Itasca Suncruiser

Dear Bud,
Over the years there have been several “versions” of what should be done to store or winterize an onboard generator. The biggest issue is letting gas sit in the carburetor bowl, in which case it will varnish and eventually not start. The second is the rubber components in the carburetor and other areas can get hard. The reason to start it periodically is to keep the seals soft and run it under load to make sure the varnish does not accumulate. I have heard from Onan reps to do this once a month, not every two weeks.

Here is the recommendation from the Onan Owner’s Manual of the 2015 Thor Challenger with a 5000 KW genset.

Storing the generator set

Proper storage is essential for preserving top generator set performance and reliability. If the generator set is not going to be exercised on a regular basis and not be used for more than 120 days, the generator set should be prepared for storage.

  1. If the generator set is a gasoline model, perform the following steps:
    a. Fill the fuel tank with fresh fuel and add a fuel preservative (such as OnaFresh), following the instructions on the container label.
    b. Run the generator set for about 30 minutes plus 2 minutes per foot of fuel line at approximately ½ rated power to fill the fuel lines with fresh fuel and preservative.
  2. Change the engine oil and attach a tag indicating the oil grade viscosity.
  3. Remove the air filter and restart the generator set. While the generator is running spray an engine fogger (such as OnaGard) into the carburetor, following the instructions on the container label.
  4. Stop the generator set.
  5. Disconnect all appliances.
  6. Disconnect the battery cables, negative (-) first, from the starting battery and store the battery according to the battery manufacturer’s recommendations.
  7. Cap the exhaust tail pipe to keep out dirt, moisture, rodents, and other foreign materials.
  8. Close the fuel supply valve (if so equipped).
  9. Position the line circuit breaker to Off.
  10. If the generator set is an LPG model, check the local ordinances if the vehicle is going to be garaged.
  11. If the generator set is a gasoline model, perform the following steps to drain the carburetor float bowl:
    a. Let the engine cool down thoroughly.
    b. Place paper towels under the drain to absorb all the drainage (less than 1/2 cup).
    c. Open the drain valve by turning counter-clockwise with a screwdriver.
    d. When draining is done, close the drain valve, turning clockwise with a screwdriver.

The manual doesn’t say to run the generator every 2-4 weeks

Nowhere in the manual does it say anything about running the generator every 2-4 weeks on load. It also says to not use a fuel under 87 octane and nothing more than 10% ethanol. I also have an Emerald Plus Series Service Manual that states the same thing, but also recommends taking the spark plug out and inserting a teaspoon of oil.

I have heard the same recommendation about “exercising” a genset every month under full load to break up the varnish or, as the manual states, “carbon,” but I cannot find it written anywhere. However, I believe it is a good idea, as most owners don’t run the genset as much as they think and then there has been an issue with varnish. But I also believe the average owner does not go through the previous steps when storing their unit, so maybe it is a recommendation just to make sure there is some type of action.

 You might also enjoy this from Dave 

I can’t exercise my generator as usual. What should I do?

Dear Dave,
We store our RV inside over the winter and do not have access to it for 4 months. We are wondering what problems we might expect from our generator. Normally, we try to start it every 30–60 days and put a load on it for 15–20 minutes. But with the new inside storage we do not have that option. Your suggestions? —Bradley

Read Dave’s answer.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here


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1 month ago

The article did not mention anything on storing diesel generators! Wondering what the exact procedure would be? I religiously run my diesel gen for 2 hours under about 1/2 load once a month! Hmm maybe not so long with this supposedly diesel shortage!

1 month ago

Install a cutoff valve in the fuel line. Turn fuel off and run the Genset until the carb bowl is empty.
Just what I do.

1 month ago

Hi Dave. Thanks for all your tips. A suggestion would be to circle the item you are referring to, on your pics. Thanks again!

1 month ago

I have seen a number of recommendations for half to full load genset exercising that give the reason as to cause the windings in the generator to warm up enough to drive out moisture and thereby protect the generator portion of the genset.

Makes more sense than ‘varnish’ in the engine! Don’t know if this is correct either! Both sound like OWT’s, but I recently trashed a failed 5kw construction genset (motor ran poorly and no output at all).

Unit was out of warranty by about 5 minutes and had only been used twice in 2 years. Autopsy revealed aluminum oxide corrosion inside the generator had spread to the copper windings and was jamming the armature (wrong term) as well. My guess was friction had tore up the insulation on the windings and shorted out the whole unit.

So, moisture & condensation did appear to be the cause of death.

Bill T
1 month ago

Hi Dave. How does running the genset “under load” reduce varnishing and lubricate the seals that simply running it at idle for 30 minutes doesn’t? It’s the same carb and seals is it not? Thanks.

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