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Old 09-21-2015, 05:07 PM   #1
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Onan 5500 Marquis GenSet spark plugs

I would like to hear from anyone thas has changed the spark plugs on a Onan 5500 Marquis GenSet.
I found them but can't pull the cap off of them. Is there a special tool?
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Old 09-21-2015, 05:15 PM   #2
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Mine pulled off but they were sticky. Twisting and pulling helped.
This was years ago but I didn't realize there were 2 plugs in the 5500 until they were both changed. The back one was harder to access using socket wrenches and swivels.
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Old 09-21-2015, 08:12 PM   #3
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Originally Posted by abarkl View Post
Mine pulled off but they were sticky. Twisting and pulling helped.
This was years ago but I didn't realize there were 2 plugs in the 5500 until they were both changed. The back one was harder to access using socket wrenches and swivels.
Thanks for replying. A friend just listened to it. When the start button is pushed (either from the drivers seat or at the gen itself) It starts and sounds like its running normally but it shuts off when the start button is released. He said its not the spark plugs because it wouldn't start if the plugs were bad.
So, I wish a onan shop was near by.
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Old 09-21-2015, 08:20 PM   #4
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Originally Posted by tim fm ct View Post
I would like to hear from anyone thas has changed the spark plugs on a Onan 5500 Marquis GenSet.
I found them but can't pull the cap off of them. Is there a special tool?
Just changed mine this afternoon. I removed the valve covers to do it. Then used a long screw driver to get leverage to pull the plug boot straight out. Worked fine.
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Old 09-21-2015, 08:25 PM   #5
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Originally Posted by tim fm ct View Post
Thanks for replying. A friend just listened to it. When the start button is pushed (either from the drivers seat or at the gen itself) It starts and sounds like its running normally but it shuts off when the start button is released. He said its not the spark plugs because it wouldn't start if the plugs were bad.
So, I wish a onan shop was near by.
If it doesn't run after releasing the start button then it really didn't start in the first place. I had 418 hours on my plugs and they looked like they needed changing. It took 15+ seconds of cranking to get it started before changing the plugs. Now it starts in like 1 second.
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Old 09-21-2015, 08:33 PM   #6
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If it doesn't run after releasing the start button then it really didn't start in the first place. I had 418 hours on my plugs and they looked like they needed changing. It took 15+ seconds of cranking to get it started before changing the plugs. Now it starts in like 1 second.
Thanks for your rapid reply. Do you think that a mechanically experienced person would think that is it running while the start button is pressed?
So, changing the plugs should be my first option? BTW; it has about 240 hours on it. The is a 2005.
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Old 09-21-2015, 08:50 PM   #7
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I never had the fail to run issue with the 5500 I had for 10 years. Got a 7500 diesel now, but the solution to the problem you describe is in these forums if you search for it.

My 5500 had a fuel pump failure, and I had to replace the carb twice after letting it sit for too long between uses. Completely different symptoms from what you are describing. It would either not start at all, or died after several minutes of surging.
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Old 09-21-2015, 09:03 PM   #8
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We went from spark plugs to no start ?. Actually it starts but doesn't continue to run.

Check the oil and make sure it is on the full mark. If low it has a safety shutdown.

You can also download a troubleshooting guide for it. Then it's a step by step procedure.

Good luck.
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Old 09-21-2015, 10:57 PM   #9
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Originally Posted by tim fm ct View Post
Thanks for replying. A friend just listened to it. When the start button is pushed (either from the drivers seat or at the gen itself) It starts and sounds like its running normally but it shuts off when the start button is released. He said its not the spark plugs because it wouldn't start if the plugs were bad.
So, I wish a onan shop was near by.
I might suspect low oil pressure or faulty oil pressure sending unit. The low oil pressure interlock is bypassed when the start switch is depressed. Releasing the start button brings the low oil pressure circuit back into play.
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Old 09-22-2015, 05:48 AM   #10
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We went from spark plugs to no start ?. Actually it starts but doesn't continue to run.

Check the oil and make sure it is on the full mark. If low it has a safety shutdown.

You can also download a troubleshooting guide for it. Then it's a step by step procedure.

Good luck.
Thank you..... where would I find the trouble shooting guide for it? Is it in the owners manual?
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Old 09-22-2015, 05:50 AM   #11
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I might suspect low oil pressure or faulty oil pressure sending unit. The low oil pressure interlock is bypassed when the start switch is depressed. Releasing the start button brings the low oil pressure circuit back into play.
Thank for ur reply. Is changing that a do it yourself task? Will the part be hard to find?
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Old 09-22-2015, 09:29 AM   #12
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Onan 5500 Marquis GenSet spark plugs

Had a similar situation as mine would crank and run for about 10 seconds and then die. It would flash one of two codes, 36- unexplained engine shut down, or 15- electrical need overload. This was strange since the breakers were turned off at the time and there was no load on the genset. If it is firing, it is most likely not the plugs but a fuel issue. You should be able to hear the fuel pump when you hit the prime button and the filter is prior to the pump so that meant the fuel restriction was in the carb itself. I removed the carb and bowl. The bottom of the bowl was full of green slime. The main "jet" is in the bottom of the bowl where the altitude adjustment needle is located. I dis assembled this part and cleaned it with carb cleaner, a toothpick, and some fine wire. I reassembled the carb and voila, it run but surged. I had to adjust the altitude needle to 5000 ft to eliminate the surge and after running overnight I readjusted to sea level and it ran fine. The green slime is ethanol gas which is almost impossible to avoid. I plan to install a in line shutoff valve prior to the filter so I can shut the gas off and run the carb bowl dry anytime it going to be sitting for a while. I don't have a Onan dealer within a 100 miles but any small engine shop should be able to help you with the engine problem if you ask them. Even though I've never seen an Onan engine on a lawnmower, the basic engine construction and design is the same.
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