Water in diesel tank

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Jul 2, 2012
Some of the more experienced RV'rs have probably come across this. We just brought our Class A for a maiden run last weekend and 3 miles from home I filled up the 80 gal tank. Dummy me forgot the gas cap and two days later we got about 1 inch of rain. I'm pretty sure some water got in the tank, not sure how much. What should I do? I'm new to diesels so any help is greatly appreciated. I haven't run it since and don't want to until I know what's best.

Thanks in advance,




Jan 13, 2005
Depending on how much rain got in, there are several options:

  • Don't worry about it, and let the on-board diesel filter with water separator deal with it. Be sure to drain the water from the filter frequently.
  • Buy an additive such as Diesel 911 and add to the tank.
  • If your tank has a drain, then drain fuel until water no longer comes out (recommended by a number of diesel sites).
  • Have the tank pumped out, fuel repeatedly run through a water separator, and pumped back into the tank.


I'd look carefully at the tank opening and decide how likely it is that water got in. I might even, with the cap on of course, have water from a garden hose "rain" down on that part of the MH and see if the water runs off the wall into the tank (onto the cap in this case) or not. If it does get a direct shot, then I'd either drain or polish (run thru filter media repeatedly). If it doesn't take a direct hit I'd use some additive and use up the fuel in the tank and maybe a few more tankfuls as soon as possible to insure there is no problem.


(please note: the run several tankfuls through, is a suggestion made to make sure the better half allows me to take the rig out some more before winter. I've put several lines between my sig and this part so you can print out the advice and show it to her........

Gary RV_Wizard

Site Team
Feb 2, 2005
Looking to buy a new home
I'm with Tom's suggestions. I would not go beyond steps #1 or #2 unless there were indications of a sure problem.  You have a water separator in the filtering system, and you probably have a "water in fuel" indicator on the dash somewhere as well.


Well-known member
Mar 1, 2005
Goodyear, AZ
Buy a couple of extra fuel filters and learn how to change them. That could handle the problem if you did have water.
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