Onan QG 3600 LP running issues

[The_Dave]

I have a Onan QG 3600 LP in my 2017 Chateau 24HL. I am the second owner of this unit but the RV is very low mileage and the gen-set has approx 125 hours total Time, another words its like brand new.

It has been running perfectly until about a week ago. Now the unit will start but runs horribly. See attached video. It acts like its being fuel starved (Propane).

https://www.youtube.com/watch?v=mMaSyX4OpEY

I started with the basics, I refiled my propane tank...no change. I checked oil, it was good. Checked air filter and it was serviceable but I changed it any way. After doing some internet reading I decided to change the coach LP two stage regulator, no help.

The operation of the generator was deteriorating and finally it would run (poorly) and shut itself down. This resulted in a trouble code of 36, which is basically fuel starvation. I verified propane to the inlet nipple on the generator, so no issues there. Now I focused on the vacuum regulator valve, specifically the vent tube. I was able to get it out of the generator enclosure and I checked it for obstructions, nothing was obvious. Tried running the generator and no change. So I removed the vent hose from the vacuum regulator and now the generator would not even start. so I put the hose back one and tried again, same result, runs poorly.

At this point I decided to bag it for the day. So I decided not to reroute the vent hose (figuring I've not finished with it) and I coiled it up and put it in the compartment by the ECU. For the heck of it I tried it one last time before I installed the enclosure cover and the generator started and ran, perfectly. Ran it for 5 minutes with it powering my AC and no Issues! What I am finding is that if I move the vent line around while the generator is running I can induce the problem, yet its just a clear 14 inch piece of rubber hose!?!?!?

It seems that the vent system is extremely sensitive. The Generator will not start with the hose disconnected, and will run with the hose connected and held in a certain way.

Is there a recommended length for this hose? Could it be this sensitive? Could there be another issue (like maybe the propane tank internally) that could intermittently cause this issue?

Anyone with any experience on this issue, please comment with suggestions. I'd appreciate it!

[javelin]

I am surprised to see no responses to you so far. You did an excellent write up and video regarding your LP gen problem and I wish I was an LP expert in order to assist properly.
Based on your information it sounds like a LP starvation issue to me also other then when you prime to start. What is odd is that you can adjust the vent line (which I assume connects to the vent opening on the demand regulator on the top of the gen) into a certain position and the generator will start and run normally. Do you restrict the vent line in some way, or apply more side fitting pressure on the vent connection point? I wonder if some critter (spider etc) has entered the demand regulator vent opening via the vent line and is causing the demand regulator to be restricted and not work properly. It looks like a PIA to get at that demand regulator to investigate as a bonus. The fact the gen runs fine when the vent line is “adjusted” seems to confirm all else is working normally......but some investigation is require on the demand regulator I suspect (as a start).



Attaching a link to a similar LP gen problem (different model than yours) and all the advice provided......but in the end he changed the demand regulator and never posted if that fixed his issue. Not sure if that will help you or not, but there is some good info included. Hopefully some experts will respond to your post in the mean time.


https://www.smokstak.com/forum/threa...nt-run.131766/

[EA37TS]

A 5 year old generator with that low usage may need to see an Oman Service Center.

That generator has not been exercised any where near what Oman recommends.

When was the last generator service?

[The_Dave]

To be clear, the Gen set had several hours on it prior to this event. As matter of fact it started an ran perfectly today after about 4-6 attempts. I’m beginning to think it’s the fuel shutoff solenoid. It’s highly unlikely that the vapor regulator would operated intermittently, it’s a simple system with one moving part, the diaphragm. If the diaphragm was ruptured you’d think it would just be failed. The fuel SOV on the other hand is electro-mechanical. It does open when you select prime (I can hear it) but who’s to say it’s not cycling while the engine is running. Maybe a dirty connection, bad wire, something that is unstable while the Gen set is running? Still to be investigated. As far as taking it to a service center, there is nothing local and frankly from what I’ve read they won’t do any better than I could. I figure if I can fix a G650, I’ll run down this issue. The smart play is to leverage the collective experience.

No idea on last service but the oil is clean….

Any thoughts on the fuel SOV?

[javelin]

I guess my thoughts on the demand regulator were based solely on your comments about positioning the vent tube in a certain way would somehow make the gen work well.......so, just thought maybe something crawled inside and was perhaps restricting the rubber diaphragm from moving as it should to correctly meter the LP gas into the carb.


When it ran without issues the last time you tried, did you do anything differently (i.e., move the vent line again)? It is not impossible that the shut off gas valve is intermittently operating/opening partially and probably a lot cheaper to replace than an onan demand regulator (or just the diaphragm would be maybe). I am not sure how you can safely bypass the shut off valve to see if you get consistent gen operation. Maybe a portable LP tank with its own low pressure regulator would allow you to test a direct gas feed into the demand regulator input port just to see if the problem is the shut off gas valve? Not sure if a portable tank would start and run the gen with no load on it though.

[RM7]

Hi Dave,

I was relieved when I found out that I’m not the only with this issue. I have the exact same thing happening and currently am at the stage when the generator works for under a minute and shuts down.

Were you be able to resolve the problem? Was it the solenoid for the fuel, the regulator on the generator or something else?

Thank you very much!

[ducksface]

Observations of little value;
Put a ball valve in that vent line so you can adjust as needed.
Propane is 66%+- of gas combustion power, so altitude rules.
And
If this were a gas engine, your choke is stuck. I don't know if you have a choke system.