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Old 10-25-2014, 08:55 AM   #1
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Ghost in the Machine

Input needed. I'm having a strange issue with my generator. It starts and will run perfectly under load for hours at a time. It will run perfectly for several days of normal usage. Then, without warning, it will lose RPM, try to recover and then quit. I can restart it immediately and it will run for hours and even days before it repeats its swan song again. It is an Onan 8 KW.

I have checked all the fuel line connections for leaks, changed all fuel filters, immediately checked the oil pressure and temperature gauges and all seems normal. Any ideas about how to exorcise the ghost?

Thanks, Howard
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Old 10-25-2014, 09:45 AM   #2
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Call ghost busters?

About every other time I start mine it quites within 5 seconds. Always starts and runs normally after the 2nd start.
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Old 10-25-2014, 10:28 AM   #3
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Try blowing out the fuel line. There could be something clogging the intake.
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Old 10-25-2014, 12:19 PM   #4
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What model gennie? Does it have an electric fuel pump? Sometimes those things flake out in strange ways.
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Old 10-25-2014, 12:39 PM   #5
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Quote:
Originally Posted by hmason View Post
Input needed. I'm having a strange issue with my generator. It starts and will run perfectly under load for hours at a time. It will run perfectly for several days of normal usage. Then, without warning, it will lose RPM, try to recover and then quit. I can restart it immediately and it will run for hours and even days before it repeats its swan song again. It is an Onan 8 KW.
You didn't say what model you have, but I had a similar problem with my MDJE 7.5 Onan at one time. We fought it for a long time until I had an Onan mechanic look at it. He quickly determined the over temp sensor switch was defective. It's an oval shaped button held against the engine head with a screw in either end. When cold it worked fine, but at 180 degrees it became sensitive to vibration and would shut down under certain loads.
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Old 10-25-2014, 12:41 PM   #6
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Did you change the on-engine fuel filter? Your experience is a repeat of mine (8kw Onan).

New filter, then fuel pump.
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Old 10-25-2014, 12:43 PM   #7
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It's an Onan MDKD. I was thinking it coul be one of the sesors. I will check that out. The fuel pump is just a year old. It seems fine. Ran it and it pumps fuel like mad.
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Old 10-25-2014, 12:44 PM   #8
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Filters have been changed. Primary and secondary.
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Old 10-25-2014, 01:42 PM   #9
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Doubt it is a sensor. If one of those was wacky, it would either run or not. Would not cause it to bog down, then stall.

Even a new fuel pump can act up, but that is rare.
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Old 10-25-2014, 01:58 PM   #10
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It's an Onan MDKD. I was thinking it coul be one of the sesors. I will check that out. The fuel pump is just a year old. It seems fine. Ran it and it pumps fuel like mad.
Looking at your manual, I see the sensors are in series with the fuel actuator solenoid like mine. Look for a frayed wire, bad connector or sensor which could trip the fuel solenoid.

Here is your manual in case you don't have it:

http://www.cumminsonan.com/www/pdf/manuals/981-0502.pdf
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Old 10-25-2014, 03:08 PM   #11
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Going by your first post, in that it loses RPM, then tries to recover, then quits.

Its the tries to recover part that is getting my attention.

If it was a protective shutdown, IE a sensor failure then it would just shut down, not try to recover first. Sounds to me like something is causing a RPM slowdown, then after that you're getting a protection system shutdown. Could be wrong, but thats what it sounds like from your description.

I'd chase this as what is causing it to loose RPM intermittantly.

Any chance you're hitting it with too much load? Something coming on that it cant take care of, like a AC unit, or a Water heater, or a combonation of a already large load, then a large motor starting load?
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Old 10-25-2014, 07:35 PM   #12
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It's a fuel issue, not a sensor. Check that you don't have an air leak or a restriction in a line, maybe a screen is plugged and the line is collapsing? What does the fuel supply line look like? Is it unique or does it share with an engine?
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Old 10-26-2014, 03:20 AM   #13
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I had a sailboat that had a westerbeke engine in it . It would run fine for 4 or 5 hrs but then it would die . It was a cobweb in the tank vent . Took a while to figure out .
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Old 10-26-2014, 08:27 AM   #14
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I'm in the bad connector and/or intermittent sensor camp as first suspects and sudden heavy load as next suspect. Not to say that the latter couldn't be part and parcel to a fuel problem, but if it was purely a fuel issue, it wouldn't run for days with no problem, it would get flaky as normal loads came on and off.
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