Grey water tank won't drain completely

[epennal]

New RVer here. My Grey water tank will only empty down to the 2/3rd level. Not completely clogged, obviously, but why would it stop draining at that point? I don't think it's a sensor issue because it does seem to stop short of completely emptying, plus it doesn't take long to fill back up to the full mark.

[MNtundraRet]

I would find a grease remover at a RV store, or supermarket. Let it sit for a few days and then drive around to loosen thing up. The vent may not be the problem. 

[Katdaddy]

Sounds like youe sensors are dirty. Buy the cheapest liquid dish washer detergent you can find, before next trip add 5 gallons of hot water and a cup of the detergent. The action of riding down the road will clean the sensors. Also helps if the gray tank starts to smell.

[Olcoot]

Glad you said you are a New RVer. We gave up trusting the gages years ago. Mostly there as a make you feel good thing. Mine read full after emptying and I knew they were empty. Simplest thing to do with ether gray or black tanks... key word "simplest". Dump 5 to 10 gals of water, half bottle of dawn into each tank. Hookup and drive about 5 miles OR do just before your next trip and at your first over-night dump. And don't be surprised your sensor still reads 2/3rd. The sky is not falling, just cheap sensors. 

[Cybergrunt]

Ask around and see if anyone near you has a digital camera inspection camera. If not, cheap models at Harbor Freight go for less than $100. Kind of handy for more than RV use.

[bluwtr49]

Remember also that if you sensor is acting up it may say full when actually much less. That would make dumping the tank seem faster if you dump when it indicates at or near full.

I would be in favor of a bad sensor first off.

[Executive45]

1/3 of your grey tank has a big hard mess of grease, hair, dried food and garbage across the bottom of it. You'll need to have the tank removed and replaced. Figure about $3k for the job!...:W

Really, follow DOTLDaddy's suggestions above and you'll know for sure if it's a sensor or something similar to above. Anything else is simply speculation. Filling the tank to capacity is the easiest way to trouble shoot.

If you're a glass half-full half-empty kind of guy, you could reverse the procedure. That is, once your tank if FULL, empty it into buckets and measure how much comes out..:)...Dennis

[valhalla360]

And the tank monster will form from the mass and take over the earth...no, you aren't going to fill 1/3 of your grey tank with grease and hair. Even 1 inch on the bottom would be a lot. 

As others have suggested, filling with a measured quantity of water after dumping will tell you if it's empty or not.

[rgatijnet1]

A plumbing snake run up through the drain valve should move any obstruction out of the way. It might not get it out of the tank but it would let you know if there is an obstruction.

[JudL]

From the bottom going out to the sewer or from the top through the vent on the roof?

[Old-Biscuit]

epennal wrote:
New RVer here. My Grey water tank will only empty down to the 2/3rd level. Not completely clogged, obviously, but why would it stop draining at that point? I don't think it's a sensor issue because it does seem to stop short of completely emptying, plus it doesn't take long to fill back up to the full mark.

New RVr........IS this a NEW RV also?

2 possibilities:

A) Dirty sensor. As suggested empty grey tank then when it has completely stop running any water out....run water quickly down into grey and watch for increased draining.

B) NEW RV......workman didn't retrieve the 'cut-out' disc from when they cut drain/vent/dump line holes into tank.
Disc floats around until you dump.gets pulled down to drain hole and blocks it off. Then drain valve closed and disc floats again until next time.

When grey tank stops draining....close drain valve, wait a few minutes, rock RV to slosh grey tank then open drain valve...does it start/stop flowing again??

Cut-out disc left behind....NEW RV.........DEALERS PROBLEM cause you will have to disconnect drain line/drain valve and hand fish for cut-out disc.

[rgatijnet1]

epennal wrote:
Thanks for the quick replies, everyone. I'm never sure of anything, when it comes to the tank senors. Lol
My impression is, it takes less time to drain than it used to. It's like, "Wow. Done already?" That, plus it only takes a day to reach Full again (normal water usage), rather than the 3 days it used to take, after emptying the tank. It's a 52 gallon tank. Just doesn't make sense.
I've left the valve open for several days, and it never goes below 2/3rds full. It doesn't go back to full until I close the valve. I'm hoping the next time we drive it, maybe something will work itself loose and the problem will go away.

I have heard of a piece of plastic, left when they cut the hole in the tank for plumbing, was blocking the drain valve. If that was the case, I doubt that you would get anything to drain from the tank, or it would drain very slow, but still empty completely.

[epennal]

Thanks for the quick replies, everyone. I'm never sure of anything, when it comes to the tank senors. Lol
My impression is, it takes less time to drain than it used to. It's like, "Wow. Done already?" That, plus it only takes a day to reach Full again (normal water usage), rather than the 3 days it used to take, after emptying the tank. It's a 52 gallon tank. Just doesn't make sense.
I've left the valve open for several days, and it never goes below 2/3rds full. It doesn't go back to full until I close the valve. I'm hoping the next time we drive it, maybe something will work itself loose and the problem will go away.

[valhalla360]

So what happened when you tried to fill using a bucket to measure? 

If you left it open for 3 days, even if partially blocked, any water should have been able to dribble out if it was a partial blockage. 

[DOTLDaddy · Accepted solution]

This is what I would do if I wanted to check how much water it takes to fill up the grey tank:

1. Drain grey water tank until no water comes out of sewer outlet
2. Close grey water valve
3. Get a bucket with a known fill mark, say 3 gallons
4. Fill bucket to 3 gallon mark and pour down your shower drain
5. Repeat as many times as it takes until shower starts to back up
6. If it took 10 buckets, then you have 30 gallons capacity
7. If it takes, say, 5 buckets until full, 15 gallons, then ya probably got a problem

I am assuming your grey tank should hold at least 30 gallons, BTW.