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Old 07-19-2020, 11:05 PM   #1
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Generator vapor locking?!

So I've been trying to track down this issue I have been having with my 5500 Cummings/Onan generator. It will die out intermittently. I finally tracked it down to it dies when you hit a very rough highway or harsh jolt from a dip on a bridge expansion joint. And I diagnosed it even further in that it does it only when hot outside 85+ degrees. I found that if you give the generator a good push/shake it will want to die or will die. It doesn't do it when it 85 or less. It has a complete tune up, filters, oil plugs etc. I'm going to clean the carb and maybe decaf in it. It has 370 hours on it. But it's not a runny no idling problem. I believe the gas in the carb is boiling and when it gets shaken hard it sucks the air from the boiling gas.
Has anyone run into this. I was going to install some type of vent on the outside generator door, but when I open the compartment it's not hot. When I open up the generator cover you can really feel the heat, which is why I'm think the fuel is boiling when it's hot outside. The generator will run with no issues when it's below 80, it never dies then!
I have an outdoor thermometer in one of my storage compartments for outside use, but I have seen compartment temps up near 115 degrees on very hot days on the road.
The generator is at the LR and I believe is just sucking in all the hot air from under the coach while on the road!

Anyone?
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Old 07-19-2020, 11:54 PM   #2
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Sounds like a carb float level issue.
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Old 07-20-2020, 07:46 PM   #3
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Every time I have read about this problem it's been a weak fuel pump. Do you have a way to measure the fuel pump pressure?

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Old 08-04-2020, 06:00 PM   #4
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OK. Well a little update. I replaced the fuel pump on Sunday. I hot wired the old pump and the thing was as noisy as I’ve ever heard one! The new one is nice and quiet. They are a pulsing type of pump, not a constant motor type. We will see how it does this weekend. Although I’m only driving about 2 hours, It’s supposed to be hot so we will see.
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Old 08-23-2020, 04:54 AM   #5
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Dodge Guy,


You probably got yours fixed by now but you're not too far off on the heat problem. It must be an artifact of the 'quieting' engineering attempt by sealing them up.


I think there is a problem with fuel boiling, but I didn't run into it while running. I think the whole generator got too hot on an over 100F day while running in my driveway. I was having trouble with my 110v supply and used the generator to run the AC. The carb is somewhat cooled as long as the genny runs but not at all after the fans and air stops.


I only left it idle about 3-5 minutes after I shut the AC down and the next time I went to use it, it surged like it was too lean. Like you say, I think the fuel boiled, but after I shut the genny off... which has the same sediment effect as not running it for a year and allowing the fuel to dry out in the bowl. Sea Foam didn't help a bit after that.


I finally had to take the carb apart to thoroughly clean it and it works great again now. The recommended $ alternative is carb replacement. Since then I've opened the side door for quicker cooling after extra hot runs.


I intend to eventually stick my bar-b-que thermometer in past the bristle seal with it closed up hot one of these days, but so far I noticed the carb bowl stays too hot to touch for over a half hour, even when left open. Could bake a pizza in there after shut-down from residual heat. No wonder there is a solenoid fuel stop built in... or I imagine the jets would be spurting raw fuel in a dead engine the whole time till the bowl ran dry.

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Old 08-23-2020, 12:29 PM   #6
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No wonder there is a solenoid fuel stop built in... or I imagine the jets would be spurting raw fuel in a dead engine the whole time till the bowl ran dry.
The solenoid is likely at the input to the bowl. The carb wouldn't work with one between the bowl and the jets.
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Old 08-23-2020, 04:02 PM   #7
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The solenoid is likely at the input to the bowl. The carb wouldn't work with one between the bowl and the jets.
Actually my Inverter Generator has a fuel shutoff solenoid at the bottom of the bowl that shuts fuel off to the main jet.

Also a common setup in older auto carbs to prevent dieseling when shut down.
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Old 08-23-2020, 04:21 PM   #8
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Also a common setup in older auto carbs to prevent dieseling when shut down.
What the anti dieseling valve did was shut off the butterflys completely when the ignition was turned off. The problem was the idle stop would keep the plates slightly open. With an anti diesleing valve the idle adjust was basically pushing on the anti dieseling valve.

As far as the genertor, I guess its possible.

Not that I use my generator much but I would prefer a way to just let the bowl run dry if I chose to. I need to put in a fuel valve inline with the bowl on my generator to allow this or just drain the bowl if it has one.
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Old 08-23-2020, 04:44 PM   #9
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What the anti dieseling valve did was shut off the butterflys completely when the ignition was turned off. The problem was the idle stop would keep the plates slightly open. With an anti diesleing valve the idle adjust was basically pushing on the anti dieseling valve.

As far as the genertor, I guess its possible.

Not that I use my generator much but I would prefer a way to just let the bowl run dry if I chose to. I need to put in a fuel valve inline with the bowl on my generator to allow this or just drain the bowl if it has one.
Depends on the vehicle mfr. Some used idle stop, some fuel shutoff in carb. Foreign mfr's were first to use the solenoid in the carb. Was a real simple setup. Just an armature with "rubber" seal on end. Cut off fuel instantly rather than waiting for bowl to run dry if shutoff was on carb inlet.
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Old 08-23-2020, 04:50 PM   #10
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Hello. sorry I forgot about this thread ( I had a couple on the generator issue) I hvaent really got it fixed. I did the trip with the new fuel pump and it ran great the entire 2 1/2 hours. However it died when making a left into the campground! it started right back up after it cranks a while. I did have the oil switch disconnected so its not an oil pressure/level issue.
My next fix will be to wrap the fuel line inside the generator compartment (behind the access panel) and the hose going into the generator with heat shield covering. and then I`m ordering a vent that will duct outside air from the side of the RV generator access door instead of from under the RV. So I`m hopeful this will cure it.
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Old 08-23-2020, 04:56 PM   #11
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Hello. sorry I forgot about this thread ( I had a couple on the generator issue) I hvaent really got it fixed. I did the trip with the new fuel pump and it ran great the entire 2 1/2 hours. However it died when making a left into the campground! it started right back up after it cranks a while. I did have the oil switch disconnected so its not an oil pressure/level issue.
My next fix will be to wrap the fuel line inside the generator compartment (behind the access panel) and the hose going into the generator with heat shield covering. and then I`m ordering a vent that will duct outside air from the side of the RV generator access door instead of from under the RV. So I`m hopeful this will cure it.
How full was your fuel tank at the time this happened. Most have generator fuel pickups that only suck fuel from the upper 2/3-3/4 of the tank so you can't run yourself out of fuel way out in the boondocks.

If tank was low the generator may have sucked air on the hard turn ant it took a while to purge it from the system.
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Old 08-23-2020, 05:55 PM   #12
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The solenoid is likely at the input to the bowl. The carb wouldn't work with one between the bowl and the jets.
TitanMike is correct that the valve is on the bottom of the main jet.

There have been some carbs that did shut off fuel input to the bowl, particularly on stationary engines or '80's+ motorcycles with a gravity fed fuel supply.

The reasoning is that if the needle and seat valve on the bowl leaks and overflows on the garage floor, or ground, it causes a fire hazard. I found it a nuisance in motorcycles and yard equipment because my favorite method to prevent the carb drying out (and then the needing cleaning), was to leave the manual fuel valve lever on so any carb evaporation was automatically refilled for several months. But I parked my small engine stuff either in yard sheds or outside. Gasoline leaks on motorcycles are more dangerous in garages where autos are started or heaters kick in for instance.

My understanding of the Onan solenoid valve original purpose is that it is there as 2-fold universal EPA, 1st to stop fuel flow when the gen shuts down and 2nd the needle/seat problem. For one, the massive armature causes quite a few flywheel-like crank revolutions after shutdown that would otherwise keep siphoning fuel. Secondly, if the gen series are also used as a stationary gen, and the fuel is gravity fed, it still eliminates the chance of bowl leak.

It is probable that TitanMike is correct in his assessment that the valve also prevents dieseling. I didn't know that, but it makes sense since dieseling is more likely in newer, leaner EPA engines. I too, am more familiar with the anti-diesel solenoids used to cut all idle air from gas carbs from the '80's.

I've also seen these type valves referred to as back-fire valves, but that can only work if the valve shuts fuel off during deceleration. Just as engines benefit from an accelerator pump because it takes a bit for fuel passages to start to re-flow. When an engine decelerates, the inertia keeps fuel passages flowing and wastes it after fuel is not needed. This excess fuel is the reason excess fuel in the exhaust pops off causing the back-fire sound. This problem has been a major advantage to the gains in modern fuel injection efficiency.

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Old 08-23-2020, 06:56 PM   #13
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TitanMike is correct that the valve is on the bottom of the main jet.
Yes...You guys made me want to look at these carbs and the fuel shutoff solenoid appears to be beneath the bowl.

Back to your issue...out of curiosity, did you ever check the float and make sure it was adjusted correctly? That would also point to a problem if you were going around a corner.

Sure aren't many videos on rebuilding these carbs. People must just remove/replace these things instead of actually rebuilding.
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Old 08-23-2020, 08:41 PM   #14
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No. I haven`t had a chance to get into the carb.

as for the fuel issue. It will die at anything from full to less than a 1/4 tank. I actually had the tank down to 15 gallons and the generator still running without an issue on I94 in WI!
The only thing I can relate anything to is a very harsh jolt to the MH or a hard dip to the left.

I will probably check the fuel float before I head down to FL in Oct.
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Old 08-28-2020, 06:15 PM   #15
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Generator vapor locking?!

I bought a 2013 new Georgetown XL back in 2012 and had this issue with my generator the whole 4 years I owned it, dies and hard to restart in HOT weather, sometimes couldnít run both A/C units while driving, hit a bump and generator would kick out, had to turn off a/c units, let the transfer switch kick back in and turn a/c back on, or it would die and had to restart. NOTHING I did to resolve this issue ever did anything, I replaced transfer switch, fuel pump, complete new carburetor, I was told the issue may be with a loose connection on the roof air units, in the heat, if you hit a bump, that connector looses a momentary contact and basically like switching the a/c off and back on, the load on the generator would be to much in the heat. ( parked with generator running I had very few issues in the heat running both a/c units ) I traded the unit off for a Berkshire with a diesel generator and I have no more issues. I didnít get to play with the plugs / connectors on the A/C units - I was gonna disconnect the wiring and inspect pins and apply di-electric grease to them, didnít get that far. ( it was a onan 5500 quiet gen. )
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Old 08-29-2020, 08:34 AM   #16
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I bought a 2013 new Georgetown XL back in 2012 and had this issue with my generator the whole 4 years I owned it, dies and hard to restart in HOT weather, sometimes couldnít run both A/C units while driving, hit a bump and generator would kick out, had to turn off a/c units, let the transfer switch kick back in and turn a/c back on, or it would die and had to restart. NOTHING I did to resolve this issue ever did anything, I replaced transfer switch, fuel pump, complete new carburetor, I was told the issue may be with a loose connection on the roof air units, in the heat, if you hit a bump, that connector looses a momentary contact and basically like switching the a/c off and back on, the load on the generator would be to much in the heat. ( parked with generator running I had very few issues in the heat running both a/c units ) I traded the unit off for a Berkshire with a diesel generator and I have no more issues. I didnít get to play with the plugs / connectors on the A/C units - I was gonna disconnect the wiring and inspect pins and apply di-electric grease to them, didnít get that far. ( it was a onan 5500 quiet gen. )
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Old 08-30-2020, 12:43 AM   #17
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Yes...You guys made me want to look at these carbs and the fuel shutoff solenoid appears to be beneath the bowl.

Back to your issue...out of curiosity, did you ever check the float and make sure it was adjusted correctly? That would also point to a problem if you were going around a corner.

Sure aren't many videos on rebuilding these carbs. People must just remove/replace these things instead of actually rebuilding.
The only video that knew what it was talking about on a 4000 was:
https://youtu.be/Ph_C6XF2TCU by Jeff Berry. Take care on the audio volume because he talks soft, then starts his not-so-quiet Onan Microquiet 4000 at 40 seconds so the viewer can hear what a lean fuel surge sounds like.

All the other videos usually just recommended running sea Foam and such through, which didn't help my clogged 'leaness 'surging, and for what is mostly an engineering problem error, most blamed ethanol added to the fuel for gum-up (including Onan). Onan actually recommends perpetually treating the entire 55 gallon RV tank with their expensive fuel preservative.

I have a friend who burns nothing but non-alcohol premium in a ga$-hog MH because he bought into the ethanol demon propoganda . I've never had any trouble with alcohol blended fuels and as a matter of fact, alcohol is exactly what I add when I do have trouble with water in the fuel. Alcohol blended fuels deal a little better with ordinary condensation than those without, because the alcohol absorbs reasonable amounts of excess water and the 'mix' then passes through the system unnoticed. On the other hand, I had a gasoline freeze-up in the gas line when I was young and dumb because pure water collected in a low spot. A bottle of Heet will fix it right up after only a few minutes wait.

Best to keep fuel tanks full when stored, to deter condensation.

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