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Old 03-29-2015, 01:59 PM   #1
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Tiffin Owners Club
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Onan 7500 Fuel Filter Change

I was planning to have the shop change my generator fuel filter because it conks out after running just a couple of minutes. However, I'm concerned that the propulsion engine might not start because we have a cold front coming through. I don't have power at the storage lot, and would normally start the genny and plug in the block heater for an hour or so.

Changing the filter looks fairly easy. I have a filter and flare wrenches. I read somewhere (probably on iRV2, but I can't find the post now) that the line from the fuel tank has to be plugged when the line is removed from the the filter or the line will continue to drain the tank. What is used to plug the line? A small cork? I wouldn't think a rubber stopper would be good due to exposure to the diesel fluid.


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Old 03-29-2015, 02:12 PM   #2
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You aren't going to have it off that long. Just put a pan down.

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Old 03-29-2015, 02:21 PM   #3
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Another tip.....tie a rag around the bottom of the fuel line.....it will stop the fuel from running down your arm when you loosen the line nuts. It's and easy change, 2 line nuts and center bolt holding the filter on.
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Old 03-29-2015, 02:24 PM   #4
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A pencil or golf tee works. A rubber stopper will last long enough to change a filter.
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Old 03-29-2015, 03:29 PM   #5
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FWIW, only using the engine heater for an hour or so isn't doing much, it takes about 4 hours for the heater to have effect on engine temperature. Most engine block or coolant heaters are 1000W.
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Old 03-29-2015, 08:35 PM   #6
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The golf tee is good or the tip of a tube of silicone. Make sure you use the flare wrenches or you'll round the nut. They're two different sizes. Rather than tie the rag around the fuel line, tie a rag around both wrists. This will keep the fuel from running down your arms.

Lastly, never use a trouble light when changing fuel filters!!!
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Old 03-29-2015, 08:55 PM   #7
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I just pinch the fuel line with a clamp to stop the flow
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Old 03-29-2015, 09:01 PM   #8
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The OP is talking about having flair wrenches. Makes me think steel fuel lines. But there could be hose, somewhere down the line.
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Old 03-30-2015, 12:31 PM   #9
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I've changed mine with a rag for the leaked fuel. I didn't use a device to stop the flow - didn't even think of that. Just got the new filter hooked up quickly. Wasn't a real messy problem.
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Old 03-30-2015, 03:06 PM   #10
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I have a 40' Tiffin Bus 2003, 7.5 Onan. The gen has two fuel filters in the system, one is an in line filter and the other is inside the gen compartment. I changed mine recently, only had the bus for two years and the filters were original, had never been changed. There were no flares in the system only tubing and hose clamps. I bought 8' of fuel line and changed both feed and return lines, the feed line had split. The inline filter is a universal diesel filter, the filter in the gen still had the lock tite on the threads. I had to cut down a pair of vice grips to get it loose, could not get a tubing wrench on it.
You can get the fuel filters from NAPA, primary 3031 secondary 3063, Wix 33063
Oil filters Fram PH3593A Wix 51064
Air filter Wix 49697
Not a lot of fuel, remember to hold the prime switch for a few minutes to get the air out of the system before trying to start the Gen.
Good Luck!
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Old 03-30-2015, 04:37 PM   #11
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Your Onan 7500 must have more room than mine - I have a devil of a time getting my fingers and the wrenches up in that little cubby hole to tightened down the metal fuel lines. Have even seriously considered adding an inline filter outside the genset.

The oil filter is up in that hole too and even though its a spin-on it too can be a real PITA to get off/on. Barely room for a filter wrench. I find the AC PF30 easier to get in/out than the Onan - it's longer but skinnier.
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Old 03-31-2015, 07:41 AM   #12
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Ray - An hour seems to do the trick for me, although I'm not talking really cold weather. I was leaving for Florida in January, temp about 40 degrees, and it would spin but wouldn't start. Started the generator, plugged in the block heater, and went to get a bite. An hour later it started right up, and the "Wait to Start" light didn't even come on.

A little off-topic - does anyone know what the "Engine Heat" switch on the driver's control panel does, if anything? I know it doesn't operate the block heater - maybe glow plugs?

Jamie - Interesting that you had the original filter. I had to change mine twice last year. I'm thinking that the extra inline filter might be worth its weight in gold. Has anyone added an aftermarket filter to the generator fuel system?
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Old 03-31-2015, 08:33 AM   #13
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I figured that the inline filter was original due to the fact that it had undercoat on it as did the bottom of the bus. Yes I am glad that I had the inline filter, it is outside of the gen, just follow the fuel line. If you had to change your filter twice in the same year you may have gotten some bad fuel or fuel with water in it.
I don't have a lot of room in the filter area, but enough to do the job. The original Onan filter inside the cubby hole still had lock tite on the threads, I am sure it had never been changed.

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