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Old 03-01-2016, 04:24 PM   #1
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What's wrong with my Genny? (Kind of long)

It's an Onan Marquis Gold 5500. 2000 model with 257 hours.

We were just down at Glamis and it was pretty hot. After riding we came back and fired up the Genny so we could run the AC. Well, it ran fine for a while (maybe a half hour), then cut out. And it wouldn't start. Initially I thought it was low fuel, but quickly ruled that out.

So I left the remote switch alone, went down to the unit, and tried the switch in the compartment. On prime the pump was audible and the fuel line vibrated. Usually! Sometimes after priming for a while it wouldn't. My assumption was that the motor was primed. Eventually it would start again, but only run for 5-10 minutes then quit again. This continued until the sun was long past overhead and it started cooling off a little. THEN, it would start and run uninterrupted.

I started thinking maybe it was vapor lock as it was really warm in the compartment, but hey, it's an air-cooled motor! Then I thought there might be a thermal switch, but can't find it in the electrical diagram (not that I'm good at reading those!) So I opened the compartment and took off the side cover. The next day it was wash, rinse, repeat. Once the ambient was cooler, it ran just fine.

Prior to this trip I changed the oil, oil filter, air filter. On the way home I started reading the manual and learned how to use the Genny diagnostic. I got an Engine Stop Code No. 36. I ruled out #1, #2, #3 and #6 on the corrective actions. #4 says secure the spark plug leads. Seems to me that would stop the motor from starting. Ya think? #5 is replace the spark plugs. Really? They aren't fouled, and the service part says every 450 hours anyway. It runs when it's not so hot outside. #7 says check for mechanical damage. Thanx Doc. What a smart diagnostic checklist!!

I should add that EVERYONE else' Genny operated flawlessly. So what say you all (ya'll from where I come from)? Where do I start?

Thanx.......Dubya
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Old 03-01-2016, 04:59 PM   #2
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Mine dropped dead one day. I took off the fuel line to the carb and hit the prime button. Little dribble of gas.

Blew back thru fuel filter, fuel pump and line to tank. Used a straw because I couldn't get my mouth down there.

Hit the prime and taa-daa, lots of gas. That was in the spring. I used it thru the summer right up until fall.

Must have been some stuff in the fuel pump check valves.
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Old 03-01-2016, 06:00 PM   #3
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How bout changing the fuel filter?
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Old 03-01-2016, 06:18 PM   #4
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Yeah, Bill, I thought about that. Doesn't explain the hot-cool thing, but it has been in there a long time, so I'm gonna do that along with the plugs. It just doesn't make sense.
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Old 03-01-2016, 06:26 PM   #5
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I just has a similar problem here in the recent AZ heat. I had the exact same issues (ran for 30 to 45 mins then quit, would not run until cool again...etc) as you describe with code 36. Took it too the dealer who then took it to an Onan Service Center who said all was fine, then decided to trace the fuel line from the tank. Then they found a fouled fuel line bracket that was around the hose / nipple that was allowing air into the fuel line. They repaired the bracket then did a service (plugs, oil , oil / air filter) and claimed all is running to spec. Just got it back Friday. I still have not had a chance to test it.

Mine kind of sounded like it was starving for fuel and seemed pretty hot when this was happening
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Old 03-01-2016, 06:31 PM   #6
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Onan Service Center who said all was fine, then decided to trace the fuel line from the tank. Then they found a fouled fuel line bracket that was around the hose / nipple that was allowing air into the fuel line. They repaired the bracket then did a service (plugs, oil , oil / air filter) and claimed all is running to spec.
Which would make more sense and also act like vapor lock. Was it hot when they tested it? Just wondering how they could make that statement if the ambient wasn't bad. Hmmmmmm.
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Old 03-01-2016, 06:57 PM   #7
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Which would make more sense and also act like vapor lock. Was it hot when they tested it? Just wondering how they could make that statement if the ambient wasn't bad. Hmmmmmm.
It was hot when the dealer tested it before they took it too the Onan guys. The dealer got code 36 with 85 degrees (in February!!). The day the Onan guys had it, it was in the low 80's. I have no idea how they determined all was OK. Hoping the Onan Cert will prove right

Are you near Phx? I can give you the Onan Service info
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Old 03-01-2016, 07:09 PM   #8
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Thanx, no. I was off roading down in the desert at Glamis. I'm home, up near the Sacramento area. I'll check the hose run from the genny to the tank. It isn't that long. Thanx for your input.
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Old 03-01-2016, 09:07 PM   #9
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I had that problem with mine last year. Turned out to be the fuel pump. It would heat up, generator quit. After cooling a few minutes, it would start again, heat up, stall. Your situation sounds exactly like what I had. Helpful hint. Don't buy an Onan pump, (not that there's anything wrong with them), but I found mine on Amazon for half the price. Aftermarket, but exact fit. Sorry, but I don't remember the name or part number. While you're at it, replace the fuel filter, and check to see if you have two of them. One is on the line entering the pump, and on some models, there is a second filter on the carb. The one on the carb has been eliminated by Onan, you can get a new fitting that replaces it.
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Old 03-01-2016, 09:12 PM   #10
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I had that problem with mine last year. Turned out to be the fuel pump. It would heat up, generator quit. After cooling a few minutes, it would start again, heat up, stall. Your situation sounds exactly like what I had. Helpful hint. Don't buy an Onan pump, (not that there's anything wrong with them), but I found mine on Amazon for half the price. Aftermarket, but exact fit. Sorry, but I don't remember the name or part number. While you're at it, replace the fuel filter, and check to see if you have two of them. One is on the line entering the pump, and on some models, there is a second filter on the carb. The one on the carb has been eliminated by Onan, you can get a new fitting that replaces it.
LINADR, I thought and thought and thought about this. I always thought the fuel pump was mechanical, but then with the priming function I soon realized it was either 2 different pumps, or it was electrical. Electrical makes sense as faulty electronics fail at higher temps. Thanx for this post!

PS. Not that it matters, but what kind of coin am I talking about for a pump?
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Old 03-01-2016, 09:27 PM   #11
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Well, lookie what I found from 2012. I do believe the mystery is solved!!!


Onan shutting down - Fuel pump or? Need advice
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Old 03-02-2016, 11:18 AM   #12
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Fuel pump easy to check, pull hose off carb and see if it is pumping during prime. More and likely a bad computer board, throw bag ice on board when it is hot and see if it starts again.
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Old 03-02-2016, 03:16 PM   #13
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Sad to report, I still have the same problem AFTER being at the Onan Service Center


Not the fuel clamp after all. Betting the fuel pump as many have suggested. Dang it all to you know where!
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Old 03-02-2016, 05:26 PM   #14
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Now Onan is saying they need to add a booster fuel pump. Original fuel pump is supposedly fine. Onan rep states that the 5500 generator with >=32 feet of fuel line needs a booster fuel pump to accompany the existing pump. Anyone heard of this?
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