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Why is my Onan Generator "Clunking"?

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My 8kw Onan Diesel generator has been working fine, when I push the pre-heat switch it goes "tickety-tickety" and after about 15 seconds fires right up.

This week while running it, it started to emit more of a "clunking" sound, very rythmic and regular (and pretty loud). I shut it down, today checked the oil level (clean and full) and when I pushed the Preheat button again to try staring, this time it gave off the "clunking" sound instead of the quiet purr of the normal "tickety-tickety".

Any ideas on what might be the problem? I didn't leave the preheat button pushed long enough to try to start it for fear the clunking was ruining something.

Thanks in advance for any ideas / input

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That's a pretty expensive piece of machinery. If that sound is a rod knock, main bearing knock or a valve hitting a piston you could be in for a big fix. Hard to say... how many hours on the generator?


I'd get somewhere that you can get an educated opinion. You did the right thing shutting it down.

Edited by Stekay

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Took it to the Freightliner dealer tonight to have them check out the brakes (heading to the west coast in a couple weeks) and when they're done with it then taking it down the street to a Cummins/Onan dealer/repair shop - hopefully it won't be TOO expensive!


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Onan dealer (Fort's Industrial Engine, Mansfield, OH) called today and said it's a fuel pump. They've ordered it, it'll be in tomorrow and it'll be a little over $300 installed. i'll find out more tomorrow why a fuel pump would "clunk" - I'm guessing there must be some sort of linkage that came loose and/or broke and was flopping around. In any event, it'll be fixed and a lot less expensive than a new $4000-$7000 generator. :)

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