5.5 LP Onan has started to backfire and quit

[howardwheeler]

I have an Onan Marquis Gold 5.5HGJAB-1119C that is a vapor LP generator. It is 14 years old and has has been flawless in performance until now. All maintenance is done. But while it has now begun to run okay on no load (though I sometimes detect some occasional miss that’s hardly noticeable), as soon as you apply the load, say of one AC, it begins to falter, then begins to back fire loudly inside the enclosure. I have read the service manual but really don’t have a bead on what’s going on. I wonder if a less than adequate LP supply could cause this. The reason I ask is I’ve had some oil looking stuff in the lines coming from my regulator. Actually the lines going to the regulator from the tanks. I discovered this when I changed regulators a while back. I have a high capacity regulator. Any ideas? Is there any way a low supply of gas can cause backfiring? It does okay when the demand isn’t there, but when it needs more power it starts coughing pretty bad. That’s why I’m wondering if it has gas coming to it but not the volume it needs for a load.

[howardwheeler]

It could be entirely true that the liquid line in my home propane tank sucks oil from the bottom, but I read in numerous places that oil comes from overfilling. It seems either that it’s true or an urban legend because quite a few people say that’s the case.

[time2roll]

Fill yourself? Is this off a liquid feed from a large tank for your home or something? Could be that source tank has some oil settled to the bottom. Ask your propane supplier what can be done to check and adjust the situation. I doubt overfilling a bit from a clean supply would create this oil in the lines.

[howardwheeler]

Fixed! It was definitely a propane supply problem. I got a new regulator and put a different tank with no oil in it and the generator started working much better. But when I put it under the big load of two ACs it would again starve for fuel. So we took everything out—big job!—changed plugs even though they looked okay and search out everything we could think of. We took apart the low pressure regulator that’s inside the generator and it looked good. But then because everything was out of the way we could finally see the breather tube or whatever you call it that allows the low pressure regulator’s diaphragm to move freely. At the end, hidden behind a wiring harness and the propane feed line, was a small screen that was totally clogged. We blew it out and everything is working great!! The clogged breather wouldn’t let the diaphragm open all the way and therefore wouldn’t let the full supply of propane to the mixer. I was impressed by how clean everything was as we tore into the generator. Propane really does burn clean. This generator has electronic ignition so it didn’t have much to tune up. We are all happy now. Thanks. And I’ve learned that the oil comes from over filling my tanks. My fault. I fill from my own big propane tank and apparently leave the fill on too long. I try to stop as soon as it spits liquid out the little hole that the bleed screw controls but sometimes have left it too long. Lessons learned.

[wolfe10]

howardwheeler wrote:
Does anyone know where this oil comes from?

Longer hydrocarbon chain impurities in the propane. Not familiar with the 5.5, but on the 6.5 liquid LP Onan, the regulator is "laying on it's side" so the impurities collect in the regulator. Have had to clean out a light-viscosity grease like substance from the regulator.

But, this is on LIQUID LP. Is that what you have or is yours a VAPOR LP generator (look at your propane tank to see if the tap for the generator comes off the vapor tap (same as house regulator) or a separate liquid tap.

[howardwheeler]

I’m going to pull it and check out all ignition parts along with a complete tuneup. The only lines that have the oil in them appear to be the ones around the regulator, both input and going out. But the line going into the generator is clear. Does anyone know where this oil comes from?

[Chris_Bryant]

I would sure throw a set of plugs in to start.

[railtrailers]

My Generac 66LP (twin cylinder) was barking back thru the carb.
The magnetos (which fire each revolution) were both failing with very weak spark.
They could not fire the spark plugs at the top of the compression stroke.
But they did manage to fire occassionally the next revolution between the exhaust and intake strokes leading to the barking back.
Solution was two new magnetos and it fired right up!

[howardwheeler]

It is definitely backfiring from the carburetor/mixer, not the tailpipe. We’ve run it and confirmed.

[time2roll]

Backfire through the carburetor is usually too lean. Through the exhaust is too rich. If that oil is restricting flow it could cause a lean condition. +1 to start with a tune up.

[ScottG]

Valve adjustment?

[suprz]

MrWizard wrote:
all my experience with engines back firing
#1 timing
#2 exhaust valve not close
#3 unburned fuel in the exhaust, because the plug did not fire

do you get loud bangs from the exhaust ?

or engine that coughs from the intake and sound like it is choking ?

those are different symptoms for different problems

I agree! And let me add a engine that is overheating and is about to suck an exhaust valve! Had that happen on a riding lawnmower my neighbor had, mouse nest in the shroud... No air to the cylinder

[eHoefler]

Start with a tune up, classic sign of a bad condensor.

[howardwheeler]

I will check in the morning, but it is a loud bang like a backfire, but it seems to come more from inside the enclosure than from the tailpipe. Would valve opening and not closing be a symptom only on load? The idle really does seem pretty good, and, of course, it’s at full speed. But when the governor or whatever it is calls for more fuel to maintain rpms that’s when it begins to sputter and loudly backfire. I’ll have to check in the morning if it’s also come from the tailpipe. Thanks

[MrWizard]

all my experience with engines back firing
#1 timing
#2 exhaust valve not close
#3 unburned fuel in the exhaust, because the plug did not fire

do you get loud bangs from the exhaust ?

or engine that coughs from the intake and sound like it is choking ?

those are different symptoms for different problems