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Old 05-21-2013, 08:37 AM   #1
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Diesel gen-set exercise- WHY

Can anyone please tell me the technical reason why a DIESEL generator is supposed to be run for a short period of time every month? I know that is what our owners manual says. I understand why a gas generator must be run so often is the fact that the gas [ethenal] will rapidly evaporate and "Clog up" the fuel deliver system.

Obviously the diesel fuel will not evaporate in a month, or maybe in years, so why run it? Compounding the case for no diesel evaporation is the fact that the diesel gen-set is obviously fuel injected and not exposed to atmospheric reactions. We can store the diesel RV main engine for a whole year without having to run it. What's the big deal with the diesel generator? Is this just a carry over from the gas generator instructions.

I met a quality engineer from Onan at a social event and asked him about this exercise deal and he jumped on the gas evaportion right away. I pressed on the diesel evaporation and he would not commit and would only say "The written instructions say to exercise the generator."

My rig is stored for the 10 next months in Arizona. I live in Minnesota and have made arrangememt for a friend in Arizona to drive over and start the generator everyother month or so, but I really wonder if that is necessary. No one has ever talked about anything except fuel evaperation as being the reason to exercise the Gen-Set. Is there some other reason?
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Old 05-21-2013, 09:03 AM   #2
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Running your diesel generator once a month is not to exercise the diesel engine.

It is to heat up the generator windings to drive out any moisture that may have accumulated there. Moisture and electricity do not mix well.

If you're in AZ, where the humidity is near zero, you probably will not have a moisture problem.

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Old 05-21-2013, 09:08 AM   #3
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Originally Posted by Tom N View Post
Running your diesel generator once a month is not to exercise the diesel engine.

It is to heat up the generator windings to drive out any moisture that may have accumulated there. Moisture and electricity do not mix well.

This seems to be the primary reason that is stated. It's also good to move fresh fuel through the pump and injectors, keeping them clean and lubed. After a hot shut-down, the injectors "hot soak" from the heat in the engine, and fuel breakdown increases. Regular running is also good for all moving parts in the engine.

If you can't do it, at least do an oil change before storing, so that the engine rests with a minimum of contamination and a maximum of available anti-corrosion additives.
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Old 05-21-2013, 09:13 AM   #4
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The main reason to exercise the generator is in this case not to run the engine but rather to keep a film from developing on the slip rings. When it sets for long periods the copper rings can corrode slightly. The corrosion (or film) acts as an insulator between the brushes and the rings. Depending on where the generator is stored the corrosion may totally insulate the brushes from the rings. The result would be no current output.

Having said that you have the motorhome stored in an area with possibly the least amount of pollutants in the air. No acid rain, no salt spray, just hot dry air. The likelyhood of corrosion is very small.
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Old 05-21-2013, 09:17 AM   #5
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It is also suggested that the "exercise" be done at half load. So the process would include running the AC which is good for them / it and whatever else you might chose to put power to, heaters in winter, refrigerator, battery charger, etc. This let's you know everything is working - no surprises 10 months from now. No load isn't really "exercise". Kind of like my claim that lifting the fork to my mouth is exercise.
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Old 05-21-2013, 09:56 AM   #6
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Pine Tree,
Unless your friend has to drive for 8 hours to get to where your motor home is, what's the big deal about running it once a month? I'm not trying to be sarcastic or, even argumentative. Leaving ANY engine sit for extended periods of time is hard on them. There's all kinds of parts in there that need "exercise". Seals need to be utilized in order to keep their resiliency, metal parts can, even in the interior parts of engines can develop tiny bits corrosion and rust, if any moisture is left for extended periods of time.

The fuel pump needs to be activated for its internal pieces to remain flexible. No, there's no diesel fuel evaporation issues but, it should be "moved" to keep fresh fuel in the lines, around seals and parts that are actually lubed by diesel fuel.

And this is only the diesel side. Then, as has been noted, there's the electrical side. All those things listed above need exercise too. The cost of repairs due to "neglect" can get high in some cases. Just ask the ones who've had to replace the infamous "Temp sensor" that, even when the gen has been ran at somewhat frequent times, can get lodged in its place and become seriously hard to remove.

So, it's entire up to you partner, a little time dedicated to, well, let's just call it "preventative maintenance" could go a long way in terms of preservation of EXPENSIVE components. It's up to you. Your choice.
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Old 05-21-2013, 12:54 PM   #7
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Exercising also gets the oil circulated on the main and rod bearings. I do mine every 3 months or so.
I knew someone in Houston that bought an MGA that has sat for 8 years, never started. He added gas, battery and it started right up. So he took it for a 2 hour drive and it threw a rod. Upon tear down he found a slight rust build up on the top side of both the main and rod bearings.

Any engine needs to be run or cranked occasionally to refresh the oil on all the internal bearing surfaces.
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