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Author Topic: Onan 8.0 Quiet Diesel Glow Plugs  (Read 236 times)

Kim (skyking4ar2) Bertram

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Onan 8.0 Quiet Diesel Glow Plugs
« on: August 31, 2017, 05:26:38 PM »
Our Onan 8.0 QD generator seems to take longer and longer to start, from what I recall as 3 to 5 seconds to now closer to 15 plus seconds, regardless of ambient temps. I suspect one or more glow plugs is tired, although this generator has less than 200 hours on it, but it is some five years old.

Unfortunately, we don't have the slide out style generator mount, and it looks like getting to, and changing, the glow plugs, is going to be a "drop the generator" style repair.

Can anyone confirm my fears? And I am expecting that is least a two labor job, or more?

Kim
Kim & Christi Bertram
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2014 Jeep Grand Cherokee Diesel
Wanderlust RV Park, Eureka Springs, AR

SargeW

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Re: Onan 8.0 Quiet Diesel Glow Plugs
« Reply #1 on: August 31, 2017, 06:17:15 PM »
Ouch. I bet just dropping the gen would take most of that 2 hours. Have you tried running some diesel fuel system cleaner through it? The fuel lines in the gen are quite a bit smaller than in the motor, and just a bit of debris can slow the flow considerably.  Run a tank of fuel system cleaner through and let the gen run for at least 2 hours under load. It won't cost much, and may save a bunch. 
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Kim (skyking4ar2) Bertram

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Re: Onan 8.0 Quiet Diesel Glow Plugs
« Reply #2 on: September 01, 2017, 10:28:02 AM »
I am not opposed to the fuel cleaner solution at all, but that's going to go into my 100 gallon fuel tank and take its sweet time, right? The only problem I have is starting it, and once it fires it runs like a champ. Oil and fuel filters are changed annually but that doesn't mean there's not some trash elsewhere to your point.

Where's a slide out generator when you need it? I'm disappointed there isn't a more convenient way to service the unit but when was service convenience a top priority with the builders, right?
Kim & Christi Bertram
SKP 106183
FMCA 420913
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2012 Thor Damon Tuscany 42RQ
2014 Jeep Grand Cherokee Diesel
Wanderlust RV Park, Eureka Springs, AR

gravesdiesel

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Re: Onan 8.0 Quiet Diesel Glow Plugs
« Reply #3 on: September 01, 2017, 10:40:24 AM »
I would imagine it is a glow timer issue more than a glow plug issue.  Maybe the timer for the glow plugs is not even activating the glow plugs at all?  Could also be loss of power to the glow timer or glow plug circuit/relay.  You probably have a Kubota 3 cylinder engine in it, correct?  I have a miller welding machine and a Kubota riding mower with the D722 Kubota diesel engine.  Neither have ever had the glow plugs changed and they still fire up perfectly.  Mower is a 1999 model and welder is a 2009 model.  If it were a fuel restriction, the issue would be when it is under full load.
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gravesdiesel

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Re: Onan 8.0 Quiet Diesel Glow Plugs
« Reply #4 on: September 01, 2017, 10:42:50 AM »
To test the glow plugs, remove the harness from each of them and measure resistance from the center pin to ground.  They are either open if bad or will have continuity if not, like an incandescent light bulb or a fuse.  There is no 'in between".
2016 KZ Spree 262 RKS
2003 Dodge 3500 4 door flatbed 4x4 diesel, 6 speed
1996 Dodge 3500 extra cab flatbed 4x4 diesel 5 speed
2006 Arctic Cat TRV diesel 4x4 ATV
(2) 1981 Yamaha G1 2 cycle golf carts
Many other diesels on the farm!

Kim (skyking4ar2) Bertram

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Re: Onan 8.0 Quiet Diesel Glow Plugs
« Reply #5 on: September 01, 2017, 02:18:55 PM »
To test the glow plugs, remove the harness from each of them and measure resistance from the center pin to ground.

Yes, it is the Kubota version, and no question about how to test. But there's the rub...you cannot remove the top cover of the generator while it's in the coach to test.
Kim & Christi Bertram
SKP 106183
FMCA 420913
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2012 Thor Damon Tuscany 42RQ
2014 Jeep Grand Cherokee Diesel
Wanderlust RV Park, Eureka Springs, AR

Gary RV_Wizard

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Re: Onan 8.0 Quiet Diesel Glow Plugs
« Reply #6 on: September 01, 2017, 06:26:24 PM »
I'll agree with Gravesdiesel re the glow plugs and its pre-heat circuit. My '04 coach with the same genset had 1900 hours on it and the glow plugs were still fine.

Just to be sure we are on the same page, your problem is that you have to hold down the start button and wait 15 or so seconds  of click-click-click before the genset starter cranks?
Gary
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Kim (skyking4ar2) Bertram

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Re: Onan 8.0 Quiet Diesel Glow Plugs
« Reply #7 on: September 01, 2017, 06:39:40 PM »
That's correct, but regardless of what needs to be tested, access to test is the primary issue.
Kim & Christi Bertram
SKP 106183
FMCA 420913
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2012 Thor Damon Tuscany 42RQ
2014 Jeep Grand Cherokee Diesel
Wanderlust RV Park, Eureka Springs, AR

Willandgiselarv

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Re: Onan 8.0 Quiet Diesel Glow Plugs
« Reply #8 on: September 02, 2017, 02:28:21 PM »
So if you are pulling genset off to repair might as well replace the belts although 200 hours is low.
I had 700 hours on my Onan when I had the temp sensor go bad and mine was fused in the housing. It will take a couple of hours to remove genset. I don't have a generator slide as well but on mine they jacked the front end up and removed it that way. All they had to do was remove a center beam the generator sat on.
It is indeed 2 hours for a competent shop one hour to remove and it will depend on the body of your RV and 1 hour to put back. If you are close to northern California there is a shop that just specialise in generators. After all the work was done mine seem to start much quicker and ran quieter.
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Utclmjmpr

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Re: Onan 8.0 Quiet Diesel Glow Plugs
« Reply #9 on: September 04, 2017, 06:22:13 PM »
  My 7.5 has 2650 hours on it and started to take longer as yours,,I remove both positive and negative cables at the rear of the gen (input) and cleaned them well, then did the same at the battery bank.. All went back to normal.>>>Dan
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