my bathroom sink burps

[ford_truck_guy]

So, I have this issue - the bathroom sink and shower both drain into the same grey tank. When the tank valve is closed, I take a shower and the sink will burp up water... It's like a vent is blocked?? And YES, the tank is EMPTY.. There is a screw on vent cap under the sink, would that be the one that needs to be replaced? It is stamped Sewer Vent.. Perhaps a roof cap vent? 

[ford_truck_guy]

So,

I changed the vent under the sink and upgraded to the Stupor valve... I am ASSUMING that original valve was bad as when I removed it, it sounded like loose pieces inside somewhere..

I went onto the roof, pulled both caps off and took my fiberglass plow markers that are 1/4" diameter and 48" long.. I fastened 3 together and marked the depth that I THOUGHT it should go..

I lowered it into each pipe from the roof and each went all the way down to the bottom where I heard a plastic sound as I raised a few inches and dropped...

BOTH pipes were sticking out from the roof approximately the same length as the other..

I am thinking it was the vent under the sink... It will be a while before I will be out camping again to try it out.

Joe

[Krusty]

I would bet that the vent pipe runs into the top of the tank through a rubber grommet that seals it, and the pipe has dropped down into the tank too far and could be resting on the bottom of the tank, or just low enough that the bottom end of it is under water.

[ford_truck_guy]

Thats what I am checking next...  Figured that would be my Saturday chore. 

[ford_truck_guy]

** UPDATE ** 

Well, not really. Didn't go on the roof, ran out of time. The valve I purchased for under the sink did not fit .... BUT, I ran a couple tests, filled sink up several times with the valve open, and several times with the valve closed.. The sink drained like a whirlpool each time..  What I did notice was if I kept the sink drain stopper closed, all would be OK until the tank reached the 3/4 or more level, thats when it would blow a burp of water up the sink.

[MFL]

Signed up again, to give ya a thought. I think your vent is blocked, or your vent pipe is too low in the tank. Air admittance valve? Maybe, but I don't think so? My vent to roof for my galley tank, became partially plugged once, which caused that tank to drain really slow. I removed roof cap, run garden hose down, using water pressure, which opened it up, from the obvious bug nesting, and has drained normal ever since.

Does your shower/sink tank drain normal speed, when you open the dump valve?

Hope you get problem solved, have a great day!

Jerry

[ford_truck_guy]

Thanks Jerry, I will be back at the RV tomorrow and will mess around... When the valves are OPEN, all is ok. I have not noticed any slowing of sink water but will look at it all Wednesday. I did buy a new Air Admittance valve today to take with.

[StirCrazy]

the admittance valve under the sink would be a cheep option to try it and wouldn't hurt to change if the trailer is older anyways, they are famous for failing.  they are for letting air in and disrupting a vaccume that forms when water is rushing buy one pipe and no air is following, but you said it is when you are having a shower, so air should be following the water down.  if you have hookups try leave your grey water dump open while you have a shower and see if it does the same thing, if it goes away then yes look at the actual tank vent to make sure nothing has built a nest in it.  I usaly run a garden hose down it while its running if I suspect it is plugged, so far I have never found a plugged one...

[ford_truck_guy]

I did buy a new one today and will take it with me tomorrow night when I go back to the RV. The water seams to drain fine when the vale is open. I will take the hose up with me to the roof as well.

[opnspaces]

I don't think the screw on vent cap is the issue because it's a one way only allowing air into the tank and preventing it from escaping the tank. It sounds like you have a problem with the vent that goes up to the roof. Most likely you're going to have to get up on the roof and remove the cap over the vent. Test the shower with the cap removed, it's possible that the vent pipe was left too long and is blocked closed by the vent cap. If no change in the behavior the pipe might be plugged. If you have a willing helper have them crawl under the rv and listen while you drop a few ice cubes down the pipe and see if they hit the bottom. Alternatively a heavy fishing weight and some heavy test line can be lowered down the vent and see if you hear it hitting the bottom of the tank.

[ford_truck_guy]

This all just started recently so the pipe seams to be ok I guess. gonna run up there with a hose on Wednesday and see if that helps.. I did buy a new Air Admittance Valve to take with, they are cheap enough.