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Old 07-20-2018, 06:18 PM   #1
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Onan 5500 mysterious surging

Ok so first off been lurking a long while, many hours of reading. I’m stumped after reading and reading about Onan surging on here.
Onan 5500 Marquis Gold HGJAB 240 hours
New carb, new fuel filter, new fuel pump, new air filter, new oil and filter, fresh gas.
Now I’ve read and read about Onan surging, called Cummins, started throwing parts at it, and still, bad surging, I’m out of ideas except taking it to a certified Onan tech. It is *slightly* better with the AC on but not by much

Did I miss something in my forum search?
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Old 07-20-2018, 06:33 PM   #2
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All the parts and probably not the right one. Go to "flight systems" website. These are the people. There are troubleshooting guides there and they WILL have an answer. E-mail them if need be. My guess, without seeing the genny is the governor control box. Mine is a 1999 and it is electronically controlled, not mechanical. Flight systems will be able to guide you to troubleshoot it and then fix it properly. Onan techs are at best a second choice for troubleshooting.
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Old 07-20-2018, 06:40 PM   #3
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Yes, deff not the right ones as it still surges, but it sat for 3 years before I got it so stuff that needed to be replaced anyway. Thanks for the reply I’ll head over there now and check it out, this is a 2005.
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Old 07-21-2018, 07:25 AM   #4
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If flight systems can't help go to smokestak.com scroll down to the onan section. A lot of smart onan people. warning give them complete model and serial number.
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Old 07-22-2018, 01:23 AM   #5
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UPDATE: TODAY, I replaced (my) fuel pump and ran GREAT under load for 2-hours test with no issue; **** did still have to ROCK THE ALTITUDE ADJUSTMENT 15-20-times to settle down the SURGING when first started. ISSUE HAD BEEN that it would die after 20-30-minutes under load and not restart until it cooled down 15-30-min. Gen is an (20-y/o) 220-hrs ONAN Emerald Plus 4000 BGE, spec-P, Fuel Pump appears to have been original. This symptom is typical for what i have experienced w/ several vehicles. **** Two thoughts on WHY this ALTITUDE ROCKING helps (and does/ has helped with surging on mine for my 2-yrs ownership): 1) Rocking the altitude lever moves the needle tip in/out enough to allow some debris to pass (that was partially blocking main jet); 2) frees a sticky float (ONAN has a unique float arrangement from what little I have found on internet/ youtube); repair kits not available, and carbs listed at $300+ parts alone, I will continue to rock the altitude lever as long as it provides relief :-) and fuel additives (cleaners/ lubes) offer a possibility. HOPE THIS HELPS SOMEONE?
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Old 07-22-2018, 01:31 AM   #6
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I noted today that there is a light tan deposit coating in the steel tubing fuel line; guessing similar maybe in carb? Ethanol induced? Moisture induced? Is there algae anywhere? It ONLY takes a few drops of water in proper place in bottom of tank/ (or carb) to start growing algae. 20-years of possibilities x 365-days/year :-(
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Old 07-22-2018, 02:02 AM   #7
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Thanks for the advice everyone.
So Iíve been talking to the guys at flight systems. Ran the suction of the pump to a 1 gal gas can with a healthy dose of sea foam. It calmed down mostly. It would run smooth under load and every 20-30 seconds have a *slight* surge and come right back to running smooth. Ran that whole gallon of gas through and hooked it back up to the tank. BAM started surging again, next step is to pull the pick up out of the tank and clear any debris blocking the pickup.
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Old 07-22-2018, 02:59 AM   #8
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Thanks for the advice everyone.
So Iíve been talking to the guys at flight systems. Ran the suction of the pump to a 1 gal gas can with a healthy dose of sea foam. It calmed down mostly. It would run smooth under load and every 20-30 seconds have a *slight* surge and come right back to running smooth. Ran that whole gallon of gas through and hooked it back up to the tank. BAM started surging again, next step is to pull the pick up out of the tank and clear any debris blocking the pickup.
May have cracks in any "rubber" fuel line in the path from the main tank to the fuel pump. Will suck air .
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Old 07-22-2018, 11:42 PM   #9
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May have cracks in any "rubber" fuel line in the path from the main tank to the fuel pump. Will suck air .
Yes thank you, I inspected the line all the way back. Also installed a clear fuel filter at the gen to see what’s going on. No air bubbles noticed at all when priming or running. I did not get to pulling the pick up today and we are leaving for a cross country trip tomorrow. I have all my tools and if it comes down to it I’ll pull it on the road, otherwise, it will have to make due until we get back from the trip.

Also, I put 91 octane in the Toy Hauler tank, because the tank will be used in the future for the dirtbike/boat and requires 91, not sure if this would have any effect on the generator but this trip we are not taking any toys so will run 87 for the rest of the fill ups.
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Old 07-23-2018, 08:20 PM   #10
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Two thoughts, 1) if sucking air in fuel line, it would eventually run low in float bowl and die, so not the issue: 2) TRY syphoning a FUEL SAMPLE FROM (BOTTOM) of MAIN TANK to a 1-quart jar, and let it sit, see if water settles out? This is not a 2-cycle, it is a 4-cycle pulling gas from a float bowl, so if you have water in main, you may have (some) water in float bowl, too. 3) (IF by chance you have a model w/ a horizontal float bowl DRAIN SCREW below the Altitude Adjust, I would want to drain, retain, inspect that fuel sample also for water.); often a float bowl can retain 1/2-teaspoon of water, which would explain the occasional surge w/ connected to the other tank. Your other tank might also have had a small amount of water. If you determine that you do have water, add a bottle of HEET to main tank, or syphon off UNTIL you are confident that it is water free. (CLEAN SAMPLES SYPHONED)
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Old 07-24-2018, 04:31 AM   #11
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Two thoughts, 1) if sucking air in fuel line, it would eventually run low in float bowl and die, so not the issue: 2) TRY syphoning a FUEL SAMPLE FROM (BOTTOM) of MAIN TANK to a 1-quart jar, and let it sit, see if water settles out? This is not a 2-cycle, it is a 4-cycle pulling gas from a float bowl, so if you have water in main, you may have (some) water in float bowl, too. 3) (IF by chance you have a model w/ a horizontal float bowl DRAIN SCREW below the Altitude Adjust, I would want to drain, retain, inspect that fuel sample also for water.); often a float bowl can retain 1/2-teaspoon of water, which would explain the occasional surge w/ connected to the other tank. Your other tank might also have had a small amount of water. If you determine that you do have water, add a bottle of HEET to main tank, or syphon off UNTIL you are confident that it is water free. (CLEAN SAMPLES SYPHONED)
Since generator pickup tubes are set at about 1/4 tank height, sitting off the bottom, they shouldn't ever suck up a water/gas mixture from the bottom of a gas tank. If there was enough water to effect the generator, the main engine would never run.
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Old 07-24-2018, 07:38 PM   #12
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How about the fuel tank not venting? Pull gas cap and see if it helps
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Old 07-24-2018, 11:38 PM   #13
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Thanks guys,
So hereís some additional info. I drained the float bowl while I was originally troubleshooting a few times, no water, good gas. I had about a half fuel tank (itís a toy hauler so I think the fuel pick up is all the way down?) I filled the tank all the way up, towed the trailer 500 miles and then the generator fired right up in 115 degree heat and zero surging for an hour. Started it again a little later and it surged again. So it seems to be getting a little better at least. Too hot out to work on it more for now, as it is still functioning for my needs.

Oh and I tried with the gas cap off to no avail.
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Old 07-25-2018, 12:55 AM   #14
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reL: 1/4-tank-pickup, While I agree, a great thought for a stationary engine/ fuel tank, but we have a mobil tank that stirs up the water, ethanol that often allows water to MIX w/ the gasoline and (I read) a problem called "phase-separation" that allows water to later separate from the alcohol/ gas... (My guess) is WHEN the volume of water exceeds the capacity of the ethanol to mix?... and, from experiments I have seen, allows the water to come off the bottom and mix throughout the entire tank while being shook/ stirred.
I tore down a riding mower carb 3-4-yrs ago, water evident just the size of a match head or two, but had also grown the prettiest GREEN algae in the float bowl and anti-backfire solenoid that had it locked down completely SHUT/ unmovable/ unrepairable. :-( :-( :-(
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