4000 hours on an Onan QD

If it were me, I would take it the Onan dealer and have them give it a though check out and than act on their recommendations. More than likely it has many hours left before major work is required.

When I responded to your question on hours on another forum, I neglected to find out what year the 10kw Onan was. Reason I ask...on some of the earlier 10kw quiet diesels, there were two belts and when owners were replacing them, the recommendation was to put on the pulley kit to eliminate the two belts and instead have just one belt. When I had my onan 10kw serviced this past year, I found the new kit had been installed and was not necessary to upgrade with the kit. My unit had 2500 hrs on it and I had a new water pump, new belt, new brushes and new radiator installed (due to previous service center tightening drain plug too tight and cracking the plastic causing a hairline crack). So..be very careful with the torque if you have the coolant changed. Otherwise, my unit showed very little wear on the belt, water pump or brushes. Tech told me I could have gone many more hours before the service but like you, I wanted a baseline and figured since they had it out for the radiator, I would just go ahead and have it done. Tech also told me that upwards of 10,000 hrs on the Onan 10kw quiet diesel was not unusual before a possible overhaul and that he had seen a few with 15000 on them before rebuild. I really like the 10kw as it will give you 220V unlike the 7500. This allowed my unit to come with the stacked bosch 220V dyer which make laundry day very similar to home as far a time to wash goes. I would check for the fan kit though and see if it has been installed.

If it's properly maintained, there is no reason to not expect 50+ thousand hours on that genset. We have larger gensets on the tugboat I work on, and we don't even think about overhauls until they have 30,000 hours...and expect to get in excess of 100K before any talk of replacement.
Granted, these are larger commercial units, but if you maintain and follow the recommendations for the engine and electrical end, there really is no reason you shouldn't get extended life from yours too.

I agree, if you don't know enough of the history of this particular engine and generator, you may want to have it "serviced" by Onan, to see if it's running up to their specifications....and if there are any things that they might recommend being done with it. (such as belt change mentioned above)

Good luck.

I would definitely change the water temp sensor also.

My thoughts are the fact that it ran 24/7 in the infield at race tracks is actually a good thing. It'll have had fewer warm-up and cool down cycles, and down time.

A friend worked on a dredge years ago, and there was a genny on the deck that had run continuously for YEARS. They would change the oil by draining and filling without shutting it down.

It was an Allis Chalmers diesel, and it was worn out to the point it wouldn't have restarted due to worn out rings, but she sat there going like the Eveready Bunny. By about his 4th year on the rig, the thing started sounding funny and a few hours later blew sparks and parts thru its stack and died.

That's an extreme case, but sitting for weeks, (or months) operating for shorter run times with acidified oil (from sitting) would be harder than one that just runs and runs and gets use.

If it isn't blowing smoke or rattling (besides the usual diesel hammer) the rings should be good, so just put on new belts, flush the coolant system well, add anti cylinder pitting additive, new fuel filters and maybe install a new hours meter to begin at zero again. At least pull the pump and look at the impeller, but I suppose just putting a new one back one would make more sense there.