Onan Generator Hesitating or Stuttering

[Saithies]

I just acquired this genset. Its an Onan Cummins HDKAT 7.5kw Diesel Generator. Its got a lot of corrosion due to rain but it runs ok for the most part. I had to replace the battery charger and replace the ends of the wires on the transformer due to corrosion just eating through the connectors. Its charging the battery great now. My problem isn't a huge one but an annoying one, that I don't want to cause other problems or get worse. I was told the genset was fully serviced before I got it, but it did sit for a while (unknown length of time) un-used and I doubt the fuel was treated in any way. I'm not sure if it was actually serviced or not but as I said the genset starts and runs well other than an occasional stutter or hesitation about once every minute or two. With no load its barely noticeable while standing next to the genset except the puff of black smoke emitted from the exhaust. Under 90% load it stutters a lot less often but when it does it's hard enough to shake the trailer and the lights dim. It is not a load kicking on or off. The only variable loads are the air conditioners and we don't run them often. The fridge is non mechanical, it runs off of a large peltier unit, its load is barely noticed by the genset. Could this be due to a dirty fuel or air filter? I read somewhere I may need to adjust for altitude but that may have been on a gasoline set. I'm not sure if that's something I can adjust on this set. I don't want to do too much without some guidance. Thanks in advance for any help provided.

[Saithies]

Thanks so much for the quick repsonses! I have to do a few things out of town today but I'll check these things out as soon as I can and I'll post back with the results and if I need further help. Thanks again!

[Strabo]

My suggestion,

Oil change and filter
Air filter
Add seafoam stabilizer

Then just run it as much as you can, it will smooth out, mine did.

[lj2654]

When I first bought our 2001 Beaver coach the owner told me the genset was recently serviced. I pulled the air filter soon after and discovered the air filter was packed full of sand and was quite heavy....glad I checked it out.

[DrewE]

There would not be any altitude compensation for a diesel engine, at least not any somewhat standard design. The altitude compensation adjusts the mixture for variations in the air pressure at altitude, since a gas engine should run at the same fuel mixture and varying throttle settings. A diesel, on the other hand, has no throttle valve (it's always wide open), and the amount of fuel (and hence mixture) is varied as needed.

I would probably suspect stickiness or play or other similar problems in the governor linkage first. It certainly would not hurt to check basic things in the fuel system as well (fuel filter, leaks in fuel suction lines, shut-off solenoid valve, etc.) I rather doubt a dirty air filter would cause stumbling under mainly low load conditions, but it's also a good thing to verify regardless.

Disclaimer: I am in no way a diesel mechanic, nor a generator expert, so don't take my advice as absolute gospel truth.

[doxiemom11]

Ours is a gas powered Onan, but a dirty air filter gave us those symptoms and then when generator got hot, it was starved for air and would die. Sounds like yours is near to dying when it stutters under load and shakes everything. Ours had supposedly been serviced by a dealership when we bought it used -- air filter looked like the original and like it had never been changed. Ran great after changing that filter.

[Chris_Bryant]

I would definitely start by changing the fuel filter and checking the air filter