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Old 06-11-2018, 04:05 PM   #1
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Black Tank Flush Problem

My 2017 Forester 3171DS has a black tank flush which, up till now, has worked perfectly. Today, I hooked up the hose and turned the water on, and NOTHING. I couldnít hear any water rushing into the tank at all. I disconnected and tried again, and nothing. There is pressure in the line, but no water running into the black tank, or anywhere so far as I can tell. No water running out the bottom of the Motorhome, no water inside, no water anywhere. Itís like the flush is clogged or something. I ran a snake through the line about three feet and it his solid, perhaps the bottom of the tank? Suggestions please. Click image for larger version

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Old 06-11-2018, 05:10 PM   #2
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How full is your black water tank?
Is the dump valve open?
What's the water pressure?
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Old 06-11-2018, 05:23 PM   #3
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Your supply water runs first to an air gap-type backflow preventer., then to the flush head if it was installed by the factory or by somebody who knew what they were doing. There could be something wrong with the air gap, so I would check there first because it is probably more accessible than checking the connection at the tank.

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Old 06-11-2018, 05:40 PM   #4
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The end of the flusher might be full of crud, so hook up water and wait 5 minutes to see if it clears up, it happen to me last year.
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Old 06-11-2018, 06:48 PM   #5
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Quote:
Originally Posted by myxkp View Post
How full is your black water tank?
Is the dump valve open?
What's the water pressure?
Yes, the dump valve is open. The water pressure is low. The Black tank was full. I emptied it and then was trying to flush the tank.
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Old 06-11-2018, 07:19 PM   #6
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Do you have a source for higher water pressure?
Maybe compressed air similar to winterizing faucets - that would allow putting a static charge against the blockage without having to stay and monitor the water input.
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Old 06-12-2018, 03:45 PM   #7
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Had same problem on 3011, filled black tank with water and let soak.
Re-try the flush
Luckily it was not the preventer, in my case.
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Old 06-12-2018, 04:13 PM   #8
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Quote:
Originally Posted by deanjp1 View Post
Had same problem on 3011, filled black tank with water and let soak.
Re-try the flush
Luckily it was not the preventer, in my case.


Thank you. I filled it and will let it set overnight. Will try it in the morning when we break up camp.
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Old 06-12-2018, 04:13 PM   #9
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Quote:
Originally Posted by myxkp View Post
Do you have a source for higher water pressure?
Maybe compressed air similar to winterizing faucets - that would allow putting a static charge against the blockage without having to stay and monitor the water input.


I will try that when I get home.
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Old 06-12-2018, 04:21 PM   #10
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Originally Posted by nomad297 View Post
Your supply water runs first to an air gap-type backflow preventer., then to the flush head if it was installed by the factory or by somebody who knew what they were doing. There could be something wrong with the air gap, so I would check there first because it is probably more accessible than checking the connection at the tank.

Bruce
What he said.

Look for a back flow preventer like in the pic below:



They are often located under the bathroom sink. It will be high up in the cabinet, right under the countertop. Looking at your unit on-line, I'm voting for under your bathroom sink.

The usual complaint is that they leak, but it could be stuck closed somehow.
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Old 06-12-2018, 05:06 PM   #11
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I have found the back flow preventer and will check it out when I get home. Leaving in the morning. Thanks for your help.
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Old 06-12-2018, 06:01 PM   #12
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Quote:
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Yes, the dump valve is open. The water pressure is low. The Black tank was full. I emptied it and then was trying to flush the tank.
What I do when I'm too lazy to hook up my black flush is lift the sewer line up out of the drain after the black is drained. I hold the hose above tank level and stick the water hose down the sewer line to back fill the black tank. Most hoses flow at 10 gallons a minute or more. I stand there for 3 minutes or more until I figure I have back filled the black tank enough then I'll lay the line down in the drain again.

If your tank is just so gooped up that the water isn't blowing out of the flush nozzle my process May loosen it up for you.

Good luck. Tom
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Old 06-12-2018, 06:07 PM   #13
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If you are hitting the road without solving your problem be sure to put 5 gallons of water in the black tank and throw a good size bottle of cheap dishwashing liquid in with it.

That usually has mine pretty clean before I get home and might relieve whatever the problem is affecting your black tank flush if it is a problem inside the tank.
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Old 06-12-2018, 08:41 PM   #14
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Quote:
Originally Posted by Tom48 View Post
If you are hitting the road without solving your problem be sure to put 5 gallons of water in the black tank and throw a good size bottle of cheap dishwashing liquid in with it.

That usually has mine pretty clean before I get home and might relieve whatever the problem is affecting your black tank flush if it is a problem inside the tank.


Thank you, Tom, for both suggestions. I have filled the Black tank and letting it soak overnight. After I drain it in the morning, I will try to flush it. If it still doesnít work, I will use your system and flush it, and then try the dish liquid in the water on the way home. I have around 350 miles tomorrow. I will see how it goes then. Thanks again.
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Old 06-12-2018, 09:52 PM   #15
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Another one of my lazy methods for black tank flush. I have a clear 45 so that I can watch how clean the water is coming out. I pick up the hose above the tank level after the black tank is empty and then open my grey tank causing the grey water to flush back into the black tank. Then I close the gray valve and lay the hose down while it drains and again watch the black tank flow through the 45 clear. Since I carry 70 gray and 40 black I can backflush that black tank several times and still have some gray water left for a final hose cleaning at the end. I do that when I don't have ready access to a water hose at the dump station and don't want to get mine out.
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Old 06-13-2018, 02:22 PM   #16
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Thanks Bruce

Quote:
Originally Posted by nomad297 View Post
Your supply water runs first to an air gap-type backflow preventer., then to the flush head if it was installed by the factory or by somebody who knew what they were doing. There could be something wrong with the air gap, so I would check there first because it is probably more accessible than checking the connection at the tank.

Bruce
I was messing with my water pump the other day and saw a hose going up from the tank flush connection, up behind the wall out of sight with a return hose coming down and couldn't for the life of me figure out where it was going. The air gap explains it. Thanks. Learn something all the time from these forums.
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Old 07-13-2018, 05:14 PM   #17
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The black unit reference in the picture (#10 ) is a check valve / vacuum breaker to prevent black water from backing up into flush line and/or water being trapped in line and freezing.
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Old 07-13-2018, 06:54 PM   #18
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Thanks to all for your help. I ordered a new check valve from Amazon and installed it. That solved the problem, but the new check valve is noisy. I never heard the old one, but can definitely hear this one. I can, however, hear the water running into the black tank. I will try it a few times. If it doesnít calm down I will order a brass one. Thanks again.
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Old 07-13-2018, 07:14 PM   #19
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Thanks to all for your help. I ordered a new check valve from Amazon and installed it. That solved the problem, but the new check valve is noisy. I never heard the old one, but can definitely hear this one. I can, however, hear the water running into the black tank. I will try it a few times. If it doesnít calm down I will order a brass one. Thanks again.
Actually, I'd rather hear mine while I'm using it than to have it block all passage of water.
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Old 07-13-2018, 08:12 PM   #20
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Actually, I'd rather hear mine while I'm using it than to have it block all passage of water.


Thatís true, if it is working correctly.
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