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Old 06-13-2021, 04:45 AM   #1
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Brand: Thor Motor Coach
Model: Omni RB34
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THOR #20891
Omni F550 Fuel Usage

I did a search, but couldn't find this specific discussion...

F550 two diesel fuel tanks. My RB34 works as outlined in manual, and as discussed on this forum: Starting full, uses front tank for approx 1/4 then begins draw from rear tanks. Fully draws from rear tank before switching back to front tank. Generator draws from front tank.

I can't confirm the "Fully draws from rear tank" because I haven't drawn it down that low. I usually refill both tanks before rear tank is empty. But, with the front tank hovering between 1/2 to 3/4, and the rear tank gauge dropping below 1/2, I assume everything is working as advertised.

Here's my surprise: Driving down the highway with the generator running to keep the A/C on (dogs & grandchild in the back), the front tank continues to drop well ahead of the rear tank. Twice I've found myself near empty in the front tank while the rear is still approx 1/2 full. The "Distance to Empty" range was showing 200+ miles, then suddenly drops with an alarm flashes "10 miles to Empty"!

So, Is this how it works?

I was expecting that enough fuel, for both truck and generator, would be pumped from rear tank into front tank to fully draw down rear tank before front tank ran empty.

Also, things seems to get a little wonkie after I've used the generator at a Rest Stop with the truck turned off. After heading back down the highway (generator off), the draw from the rear tank seems to stop and only the front tank is used. (This happened once, so I'm not ready to insist it always happens).

Anyway, I've come to always show the two separate fuel gauges on my dash, not trusting the blended single gauge. And, if I use the generator, my highway range is less than I thought I would get, because there will be unused fuel in the rear tank.

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Old 06-13-2021, 06:54 AM   #2
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You are correct that the front fuel tank will drop to about 3/4 full before the rear tank starts to drop, it doesn't transfer to the rear tank the rear tank gravity feeds to the front, when the rear tank is empty you will then start to see the front tank start to drop again.
The generator and the big engine both draw fuel from the front tank.
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Old 06-13-2021, 02:39 PM   #3
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Its a model year specific set-up. On mine (2017 chassis), the engine draws from the front tank...as it drops (never below 3/4 full) it will draw fuel from the rear tank into the front tank to keep it full. My generator runs from the rear tank. If the rear tank drops down to 5-8 gallons, the generator will shut down. The engine up front never draws directly from the rear tank. This is how the chassis is designed by Ford. Call your local commercial Ford dealer and run this by the service department for more clarification on your specific year model and VIN. Both tanks also have an auxiliary port to connect another fuel driven device. When I find the time, Ford has told me they can connect dual feed lines to my generator from both tanks (from these auxiliary ports) to my generator with a manual switch over valve whereby I can run generator from either tank. This will allow me to keep the generator running even if the rear tank gets too low. I don't see getting in this situation as I always fill tanks at the end of day, prior to camping, but you never know...the Colonial Pipeline issue was an example of having the manual ability to control 100% of my fuel to the generator if necessary. The extra 28 gallons up front (not available to my generator now) would run it for an additional 50+ hours in an emergency. Additionally, I always carry 4 each, 5 gallon Jerry Jugs of diesel with me on the car hauler trailer with a portable fuel transfer pump that operates on a single "D" cell battery.
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Old 06-14-2021, 08:00 PM   #4
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Thanks for the confirming info.

On the 2021 Omni RB34, Is the rear diesel tank gravity fed or pumped to the front tank?
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Old 06-14-2021, 08:27 PM   #5
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Generator specs say half gallon per hour. My experience seems to suggest much more. Unless, running the generator while driving shuts down the transfer of fuel from rear to front tanks.

Anyone else have experience running the generator while driving?
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Old 06-14-2021, 08:41 PM   #6
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Post #4: It pumps electronically
Post #5: Fuel transfer system is separate from generator usage. You can run it while underway...has no effect on the transfer system. Mine will use about 3/4+ gallon per hour with both A/C's running...about 1/2 gallon per hour running only one A/C.
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Old 06-14-2021, 08:46 PM   #7
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Thanks,

Do we know the pump volume capacity?

I’m trying to understand why, when using the generator while driving, I run out of fuel in the front tank while the rear tank has plenty.
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Old 06-14-2021, 08:57 PM   #8
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Capacity varies depending on driving demands. Sound like your pump is not working. Have you gone through your fuse/breaker boxes to see if one has tripped/blown. That pumping action is totally automatic. If you have generator running while driving, it would just pump a little more often to keep front tank full. Again...my guess is pump issues.
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Old 06-14-2021, 09:06 PM   #9
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Yes, might be a pump issue. However, it works fine without the generator running...
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Old 06-14-2021, 09:14 PM   #10
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In that case, I would contact Thor to see if they may have miss-wired something. Somehow, the generator seems to be cutting off power to the pump. That's a really weird situation. I've seen every possible issue come across this forum multiple times but never this one. The fuel system/pump on diesels creates a constant flow, returning unused fuel back to the tank as it uses what it needs. I wonder if...your generator is drawing from the front tank but returning fuel back to the rear tank?? The "draw line" and the "return line" are two separate lines. If so, this still doesn't explain why the rear tank don't continue to pump fuel to the front tank, aside from any generator action. Really a stumper!!
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Old 06-14-2021, 09:28 PM   #11
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I’m stumped too. Thanks for thinking it through with me, and agreeing that it shouldn’t work this way.

My Thor authorized dealer says it’s a Ford issue. I’m afraid Ford will say it’s a Thor issue, since Thor installed the generator.

My dealer says they had it inspected by Ford service during the delivery inspection and all was ok. Dealer suggests I take it to same Ford service to explain my experience. If they can’t fine any issue, there is a major Ford Truck service center in Portland that is supposed to know Ford RVs.

Or, I learn to live with this “feature”.
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Old 06-15-2021, 02:52 AM   #12
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I have driven ours with the generator running several times with both coach a/c on and have not encountered the issue you're describing. I have also had the rear tank empty and was still able to run the generator, I just recently ran it with rear tank empty to help draw down the fuel level in the front tank so I could remove the fuel filler and reposition it, I went out a drove the coach with the generator running till front tank was down to 1/2.
This on a 2020 XG32, did they change the fuel system after 20? possibly, but I doubt it.
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Old 06-15-2021, 04:22 PM   #13
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It isn't a Ford issue but they have a better chance of finding the problem. The system was factory installed prior to whoever connected the generator into that system. Could also be a ground issue coming from the generator when running, causing the fuel gauge to show empty even when it isn't. If it were empty, she would shut down, as all F550's with dual tanks run off the front tank.
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Old 06-16-2021, 01:07 PM   #14
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I'm still on my first tanks from the dealer's fill up. Sitting with just under 3/4 front and just under 1/2 rear. I'm taking it out today and we'll get about another 160 including the return. I'll fill it up on the way back in. The miles remaining figure must derive off the front tank reading or else mine is still confused after sitting on a dealer's lot for a few months. So far only some minor issues, other than the bumper, and using it close by exercises everything so any warranty issues can be fixed before the late summer/fall trip.

My "toad" stuff is arriving so next week I'll be working those things plus I'll order the crossfire. I wish RecPro could get the fabric we want in stock so I could order and instal them.
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Old 06-16-2021, 05:27 PM   #15
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"Mileage To Empty" is for both tanks combined. At least mine is that way. With both full I start out with about 650 miles to empty. There is a built in reserve in their calculations.
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Old 06-17-2021, 01:00 AM   #16
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Quote:
Originally Posted by The Gritz Carlton View Post
"Mileage To Empty" is for both tanks combined. At least mine is that way. With both full I start out with about 650 miles to empty. There is a built in reserve in their calculations.
Same on mine also.
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Old 06-19-2021, 06:19 PM   #17
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F-550 Fuel Usage

My SK-35 has a fuel tank selector switch. I can select what tank to run on. I always start running on the larger rear tank til I get to a quarter left than switch to the front witch I consider my reserve. Gen runs on either tank til it reaches a quarter then shuts down the gen as a safety issue. I have never heard of such an issue that you are having. Wow what a hassle.
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Old 06-19-2021, 10:39 PM   #18
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2021 sv34

We just drive and enjoy it…never even thought about which tank we are pulling fuel from…Been all over the the southeast in our SV34 and never an issue. Try not to get “wrapped around the axel”. It has two tanks?
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Old 11-12-2021, 06:41 PM   #19
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Update:

Problem has occurred twice now without using the generator. My RV dealer, in consultation with Thor, says it’s a Ford problem. Problem is now local Ford dealer service, who had been working with my RV dealer, says they can no longer work on RVs due to their Workers Comp Insurance. So, I’m in search for a Ford truck service center that will take RVs on warranty.

Update: Found a Ford Truck Service Center in Portland. Have an appointment in December.
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Old 11-14-2021, 12:27 AM   #20
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