Holding tank issue. Waste sensor problem.

[Cdn20valve]

I have a threaded fireboy/xintex tank level sensor in my poly waste tank. after getting a bad smell from the boat, I narrowed it down to a loose sensor, it was unthreaded and the oring was cut. I replaced the oring, and tightened it down. Two weeks later same thing. Popped out again. After tightening it down numerous times, i’ve narrowed it down to the pump out. During the pump out, it Somehow loosens. I have not seen it actually happen, however it was tight before the pump out, and afterwards, it was completely loose.

i am a pipe fitter by trade, and know what tight is. This time I added Teflon tape to help make a better seal. Also, the tank fitting is not cracked.

I thought it to be a plugged vent. So I checked that and it is not plugged. What else could it be?

 

[alldodge]

Don't know, but do the threads look good?
Maybe use some thread sealer like permatex aviation sealer

 

[Cdn20valve]

Threads look good. I think I fixed it. Won’t know till I get the bit back to the pump out station. When I checked the vent hose I disconnected from tank and it was clear. However I never checked the vent elbow on the actual tank, Tonight I unscrewed it, and heard vacuum, looked inside and it was clogged with gunk. Cleaned it, put it back together and I’m pretty sure this should fix the issue. Surprised the tank didn’t collapse/split from the pressure.

 

[Cdn20valve]

Threads look good. I think I fixed it. Won’t know till I get the bit back to the pump out station. When I checked the vent hose I disconnected from tank and it was clear. However I never checked the vent elbow on the actual tank, Tonight I unscrewed it, and heard vacuum, looked inside and it was clogged with gunk. Cleaned it, put it back together and I’m pretty sure this should fix the issue. Surprised the tank didn’t collapse/split from the pressure.

Update: and this is a good one.

The problem was still there. It was not vent related.

Solution: When under vaccum the tank collapses a bit. the sensor is threaded into the center top(weakest part) of the tank. The tank is 11" tall. The sensor is 10" tall. When under vacuum, the top of the tank sucked down over 2". which would push up on the sensor, and force it through the threads.

I cut down the sensor tube 2". reinstalled, and solved. i would have never guessed it. these poly tanks flex allot under vacuum.