Onan 2500 LP Generator Acting Up (again)

Here is also my 2 cents. Mine starts but turns off as soon as I let go of the start button. It was doing intermittently for about a year. It became more frequent. I was going to check the oil pressure switch but I read somewhere mine does not have oil pressure switch. So I brought it to a local Onan repair facility. I was told magneto seemed to be bad and will cost me almost $1,000 to replace it. I asked about the oil pressure switch and the guy told me there is no oil pressure switch but there is oil level switch...??? I told him to disconnect the switch and test and I was told he did and it didn't run. I searched internet for a used 2500LP and it was around $1,000. Told them to change out magneto and also asked them to disconnect the oil level switch just to make sure I don't get stuck in remote area with bad oil level switch. I paid close to $1,000 for the job and took back my RV. Upon returning home and running for a few times over a few days, I reconnected the oil level switch just to see if it still runs. If it runs, then the magneto was bad, right? Alas! As soon as I connected the oil level switch, it was doing the same thing again! This proves the problem is the oil level switch, not the magneto!!! I was really disapponted at the outcome! I don't know what the shop was thinking when I asked if they tested the oil level switch. It could've been a $50 buck job, simply disconnecting the oil level switch, and now I wasted $1,000 and back with the same thing. I disconnected the switch and it starts every time now. I was thinking to challenge the shop but I remember him saying there could be other problems that will cost me another $1,000. After pondering for a few days, I decided to bite the bullet and not go back to the shop. If they can tell me a lie like this, I am sure they can do more. At least it runs now but I need to check the oil level once in a while. I read in many posts online with the same symptom. Try disconnecting the oil level switch first before you do anything.

Definitely the same type of problem. They seem to require a very specific airflow or they run terribly. Even an air filter that isn't visibly bad can cause problems, as can the vent tube below the unit and the door you are talking about.

While this most likely is not your problem, I am putting this out there in the hopes of helping someone else. When I purchased my 2009 Lance 845 the Onan RV QG 2500LP generator with 40 hours total run time would start very hard and would not run smooth. I pulled it out of the camper and started playing with and figured out if I shut the propane bottle almost all the way off, it would smooth right out. I tested the gas pressure and it was right where it was suppose to be. I checked out all of the fuel system and I could not come up with anything wrong. Then one day I was looking through the parts listing I noticed it showed a little door that covered the air filter access opening and that made me wonder, as mine was not there (up until that point I did not even know there was suppose to be a cover there) Anyway to test it out I covered the opening with duct tape and the generator started and ran perfect. Well evidently that door has to be there as the air filter area sits in the cooling air flow and the air flows acts as a sort of supercharger and leans the mixture down just enough to make it run properly. I was able to order a new door from Onan.

I wasted a ton of time trying to figure out that simple problem.

If the outside air tube is ok, change your air filter. Fixed this issue 100%

After starting place hand on mixer. How cold does it get? Some regulators respond sluggishly. So do the hand test there as well. A pinched regulator is an orifice and gas expansion consumes heat.