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Old 07-05-2022, 11:23 AM   #1
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Main water tank issues

Morning.
I have a 1997 350 with the factory 3 water tanks. The main is metal and I believe 85 gallons The other 2 are plastic and 50 gal each. When I use the fresh water I empty the 2 50 gallon tanks however the large 85 gallon does not draw. All the shut off valves are lined up next to each other in the engine room and they are all full on. what else can I look at?
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Old 07-05-2022, 02:29 PM   #2
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You say all tanks are on but you can't draw the 85G tank. Have you tried shutting the two empty 50G tanks then trying?
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Old 07-05-2022, 04:21 PM   #3
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Quote:
Originally Posted by jclays View Post
Morning.
I have .... 3 water tanks. The main is .... 85 gallons The other 2 are.... 50 gal each. When I use the fresh water I empty the 2 x 50 gallon tanks however the large 85 gallon does not draw. All the shut off valves are lined up next to each other in the engine room and they are all full on. what else can I look at?
Once one tank is empty and the pump starts sucking air, it's all over. Depending on the location of the Tee's, the pump may simply be pulling from the 2x50 more efficiently. Such as if the 85 is lower. It could also be that the vent line for the 85 gallon is blocked.

Shut off the 2 x50 tanks and re-prime the system. The 85 gallon should start getting drawn from. If not, check the vent line.
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Old 07-10-2022, 02:16 PM   #4
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Once one tank is empty and the pump starts sucking air, it's all over. Depending on the location of the Tee's, the pump may simply be pulling from the 2x50 more efficiently. Such as if the 85 is lower. It could also be that the vent line for the 85 gallon is blocked.

Shut off the 2 x50 tanks and re-prime the system. The 85 gallon should start getting drawn from. If not, check the vent line.
When you say re-prime does that only involve turning the pump on?
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Old 07-12-2022, 08:42 AM   #5
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Just curious, I have a 1997 MS350 and only have the two plastic 50 gal tanks. Was the 85 gal tank an option? Where is the 85 gal tank located? I have a 40 gal holding tank just ahead of the ER, but I don't see where I could put an extra 85 gal tank.
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Old 07-12-2022, 05:46 PM   #6
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my holding tank is in the engine room between the engines. The 85 gallon is in the area behind the stairs leading to the bedrooms your holding tank might be where my water tank is.
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Old 07-12-2022, 06:16 PM   #7
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Oops maybe its a holding tank in which case I strongly suggest not using it for drinking water. Shoot, where is my smiley face option?
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Old 07-12-2022, 06:38 PM   #8
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Ah, I have a single Cat 3116 so that accounts for the different placement of tanks. Yes, my holding tank is where your 85 gal tank is located. That also brings some interesting issues. The 2 plastic tanks located in the lazarette are at a higher level than the 85 gal tank so there's no way that 85 gal tank will draw with the plastic tanks. How can you fill the 85 gal tank? Is there one filler on the Stbd transom that fills all 3 tanks? If so, the 85 gal tank would not empty until the 2 plastic tanks empty because the whole tank is below the bottom of the plastic tanks. There has to be some unique plumbing here that is not being explained and I can't figure out just thinking about it. Personally, I would prefer that tank be a fuel tank rather than extra water.
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Old 07-12-2022, 06:56 PM   #9
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I just checked my MS350 manual that shows the tank configurations for the twin engine install. It calls out the single tank as a 67 gal and has a separate water fill on the stbd side. Funny, instead of the 2 50 gal plastic tanks, it shows a 40 gal on port side and 20 gal on stbd side. But then in the Fresh Water System section it says the middle tank is 80 gal. It then shows separate feeds from each tank to the pump. So, in theory you should be able to shut off the two rear tanks and draw off the 80 gal tank. But if you have all 3 valves open, the two rear tanks would have to drain first before you would start drawing on the 80 gal tank due to it's lower location. Is there any way to determine how full the 80 gal tank is? You can see into the plastic tanks to read the levels.

Also, the manual shows, for single engine configuration, that the middle water tank can be installed where my holding tank is and the holding tank would be off to the stbd of that water tank.
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Old 07-14-2022, 06:17 PM   #10
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Yes you are correct I have 2 plastic 40 gallon tanks astern and an 80 gallon metal tank under the stairs. I have a fill astern that fills the 2-40 gallon tanks, and a fill on the starboard side for the 80 gallon tank. When the 2-40 emptied the pump ran dry and did not start pumping the 80 gallon tank. I tried filling it and it over flowed at the fill. Tapping it it sounded full. But it doesnt pump out.
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Old 07-14-2022, 06:33 PM   #11
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Sounds like an obstruction in the feed line to the pump. Could be a valve or?? I would disconnect the feed line from that tank at the pump and try to get water to the pump. Itís also possible/probable the pump is above the middle tank and needs suction to get that water out. Then definitely the valves to the 40 gal tanks must be closed. I know my pump is located on the ER front firewall and is above that tank. I would think others with that tank configuration would have the same issues.
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Old 07-23-2022, 12:42 AM   #12
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If, as has been said, your two small tanks are above the larger one, then once they empty the pump will draw air so of course it won't draw from the larger tank. You must shut off the valves from the smaller (higher) tanks and only suck from the larger lower tank. Or what am I missing?
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Old 07-25-2022, 12:29 PM   #13
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I found a clogged vent line from the 80 gallon tank. I shut off the valves to the 2 rear plastic tanks and with some coaxing the pump started drawing from the front 80 gallon tank.
this weekend past I had all valves (3) open. I went to the local islands for a cruise. I used up the rest of the water in the 2 rear tanks and sucked air and lost the prime to the lower front 80 gallon tank. I shut off the 2 water valves to the rear tanks and still could not get the pump back to prime and take from the front tank.
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Old 08-01-2022, 11:35 AM   #14
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I followed the water line and it rises quite a bit from the water tank location to the pump. I will re-route the line and see how much I can reduce the rise to the pump and maybe this will help with the priming.
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Old 08-02-2022, 09:07 AM   #15
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My pump is located on the ER front bulkhead which is where I suspect yours is also located. It is at or just below the bottom of the rear tanks so makes it easy to draw. The 80gal tank is definitely below the pump. If you still think the vent is obstructed, I suggest disconnecting the feed line to the pump from the 80 tank and using compressed air in the vent until a good flow comes out the feed line. You could then shut that off and you should be able to hold the prime to the valve. I would put some water in the 40 gal tanks and make sure the pump is primed then switch them off and the 80 on to see if you have a prime and good flow. Just a suggested method to get your prime back.
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Old 08-02-2022, 09:12 AM   #16
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Another operational idea is when you suspect you will use more than 80 gal on an outing, start drawing from the 80gal metal tank first. If it goes dry, you will always be able to get a prime when you switch to the 40gal tanks.
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