Fresh water tank won't stay full

[Stratman50th]

Greetings friends. New to me 2006 42' Magna. Took my first shakedown cruise this past weekend and I have a few things on my list but all in all, great time and I just love my "Bus".
This problem was going on before I left. I would hit the tank fill button on the silverleaf and the fresh water would fill. I shut it off and it vents over the side until it hits about 35% and then stops. I haven't been able to crawl underneath yet but it seems to be a hose type flow rather than a crack in the tank. It's coming out just forward of the drive axle and is just about equal distance from each side. Does anyone know of a valve or something that might be opened without my knowledge? The dump switch works as it should and the water dumps from the lowest point on the tank. So this is something nearly halfway up the tank volume.
Thanks, Don
PS: There's a couple other issues but I want to keep hunting for a solution before I post.

[mel s]

Originally Posted by Stratman50th

This problem was going on before I left. I would hit the tank fill button on the silverleaf and the fresh water would fill. I shut it off and it vents over the side until it hits about 35% and then stops. I haven't been able to crawl underneath yet but it seems to be a hose type flow rather than a crack in the tank.
Thanks, Don

Stratman50th
On some coaches, (mine for example), if the water tank is filled until water runs out of the overflow tube it will continue to "siphon out", (down to the 3/4 to 2/3 mark).
On my coach, and others with that problem, if the tank is not filled completely, it will not siphon.
BTW, running a few gallions out of the tank using the 12V water pump sometimes stops the siphoning.
Mel
'96 Safari

[Winepress]

I have the same issue. It's a pressure issue when the air in the top of the tank forces out the water.
My solution, Once filled I insert a small rubber stopper into the overflow tube. After I've used some water, I remove the plug. Simple, problem solved.


Try and live up to your dogs opinion of you.

[mel s]

Originally Posted by Winepress

I have the same issue. It's a pressure issue when the air in the top of the tank forces out the water.
My solution, Once filled I insert a small rubber stopper into the overflow tube. After I've used some water, I remove the plug. Simple, problem solved.

Winepress
Good solution
But always remember to remove the plug!
The tube we are calling the "overflow tube" is there to allow air to escape as the tank fills ...AND to allow makeup air into the tank as the pump draws water out.
Mel

[courtjester]

Originally Posted by Winepress

I have the same issue. It's a pressure issue when the air in the top of the tank forces out the water.
My solution, Once filled I insert a small rubber stopper into the overflow tube. After I've used some water, I remove the plug. Simple, problem solved.


Try and live up to your dogs opinion of you.

I used to do that, until the following...

I had installed the small plug..then promptly forgot. Later that day or the next, I filled up the water tank on my old monaco.

After a bit, I noticed water runing out the bottom of the coach. Light went on and I ran to shut the water off and pull the plug out...water still coming out the bottom. Hmmm...
Finally pulled the side panel off and holy smut..that tank had ballooned big enough so that it broke the shelf above it and pushed the black water tank up high enough so that it broke the vent line.
By the end of the day, the tank had leaked dry.
I ended up pulling the fresh water tank and found that it had split in a couple places on the bottom of the tank. I plastic welded it back togehter, fixed the shelf above it, repaired the vent to the waste tank and put everything back together.

Lets just say i'm not a big believer in using a plug for the fresh water tank vent any more.

[phays]

If you can get to the high point of the overflow tube you could put a tee in the line and put a short hose running vertically upward to break the siphon. I've heard of this happening on several different brands of motor homes and cannot believe the manufacturers didn't think about this problem.

[mel s]

Originally Posted by phays

If you can get to the high point of the overflow tube you could put a tee in the line and put a short hose running vertically upward to break the siphon. I've heard of this happening on several different brands of motor homes and cannot believe the manufacturers didn't think about this problem.

phays
IMO, the problem is not as much with the manufacturer as much as it is with the owner/operator.
Since I quit over filling the water tank, water never "siphons out".

(BTW, I'm not defending the manufacturers, they do a lot of things wrong, but I also know that many owners don't have a clue about how a lot of things work and/or how things should be done).

[Winepress]

Originally Posted by mel s

Winepress
Good solution
But always remember to remove the plug!
The tube we are calling the "overflow tube" is there to allow air to escape as the tank fills ...AND to allow makeup air into the tank as the pump draws water out.
Mel

The key word is "remember". You need to have a system where you don't depend on "remember". It's best to have a checklist for these kind of things. I use my phone timer when filling just to remind me. My M/H checklist is pretty extensive.

Checklists prevent those "Oops, what was that?" For example, when landing a plane with retractable gear, it's best to land with the gear down

[pumper9x9]

If it keeps siphoning you may have a bad check valve in the fresh water pump. Search around on this site to find more on that issue.

[Stratman50th]

Thank you all for your replies! Mel s, I think at this point you win the prize. so I've asked this same question in several places and I frequently get the "overflow-siphon" answer. Why would that be if I'm not topping the tank off? On this forum I find some awesome names... Kevin Waite, and Butch Williams, formerly of Country Coach. For a small yearly fee They provide unlimited tech and parts support for Country Coaches, and I will tell you it's worth every penny to me.
So the answer to this question was in fact the overfill tube. But it didn't explain "TO me" why it did it when the tank wasn't full. Answer: It was full. The tank sensors need to be re-calibrated. All of them. Mystery solved! I was advised to wait until my next trip. Start with all tanks empty and calibrate at that time.
Again, I do appreciate the assistance from everyone!
Don

[Stratman50th]

It's been almost 4 months since I posted this question. Had a lot of responses so I thought that I would update this for anyone who may be interested.
I just got the problem straightened out about a month ago. The tank is 104 gal. First clue, only took less than 10 minutes to go from 35% to 100%? Little quick. Then even less time to empty again. Second clue. The hose was not high pressure so something was up. <= (Meaning it was filling way too quick).
Ok, turns out the calibration was off on the tanks. The entire time I have been running around with a full fresh water tank. The water coming out was the overflow. And indeed, some did siphon out when it overflowed, but not 70% worth. Problem was as simple as tank calibration. Waited till I was at a campground and completely purged all tanks as best I could. Then with all empty, I recalibrated the sensors. Now everything works as it should. The readings are what is to be expected. Oh, the bus handles much different without that extra 1,000lb 0f water in there!
Thanks again everyone who responded!

[TonyDi]

If is a siphon that is causing the water loss, an easy fix is to drill a small hole in the top of the overflow line, this will keep a siphon from starting.

[mel s]

Originally Posted by TonyDi

If is a siphon that is causing the water loss, an easy fix is to drill a small hole in the top of the overflow line, this will keep a siphon from starting.

Stratman50th
My overflow line had a 1/8" hole drilled in it.
It is surprising how much water ran out of that small hole, and into my carpeted "basement", every time I filled the tank to capacity.
Be aware of that possibility before you drill one.
Mel
'96 Safari

[Smitty77]

OP - Thanks for coming back and closing the loop on what you found out. This helps others when the do searches for similar problems.

Best,
Smitty