Forum Discussion

Explorer

Jul 04, 2018

5.5 LP Onan has started to backfire and quit

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.

14 Replies

suprz
Explorer
Jul 04, 2018
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
Explorer II
Jul 04, 2018
Start with a tune up, classic sign of a bad condensor.
howardwheeler
Explorer
Jul 04, 2018
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
Moderator
Jul 04, 2018
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