I just replaced mine at 750 hours because I wasn't gonna put 2,000 into a 4,000 generator.
I've had to replace my Onan:
several fuel pumps.
belt
bent rocker arms
worn out push rods
intake manifold gasket
ignition coils
carbureator
Now needs new HEADS!
I totally believe what the mechanic said. I have kept up all the maintenace on my Onan Marquis Gold, yet there is ALWAYS a problem. I have a stand-alone Gilette Generator, and a Honda 2000. On those 2 units I rarely do much of anything. They are totally reliable. If I had a choice to get away from Onan, I WOULD. But ONAN has the RV market. I don't want to pack around a plug in generator so I'm stuck with UNRELIABLE ONAN. I always hear about how rough/tough well built Onan generators are. Not been my experience after 15 years of RV-ing with it. I depend on my DASH A/C over the Onan! Never replaced one item on my Gilette or Honda 2000 generator. not one part! Onan, many parts!