Monday, December 4, 2023


Ask Dave: Should I use a pressurized leak finder to find leak in RV’s sealed underbelly?

Dear Dave,
My 5th wheel is winterized. I used compressed air and blew out the water in the lines but I did not get the cover on for winter. I live in the Northwest, so rain and snow are a sure thing. I’ve found that my RV’s sealed underbelly is collecting a small amount of water. Is this a leak in the sealed underbelly? I turned to YouTube for ideas on how to look for leaks and found a video on how a dealership in PA places a positive 2-3 psi on the inside of the trailer then soaps the outside looking for leaks. Have you ever heard of such a process? Do you know if this is a good or bad idea? Your thoughts, please. Thank you. —Mary

Dear Mary,
The process you are describing is probably from a machine known as Sealtech. This is a machine that draws air from an outside source such as a roof vent or a side window and pressurizes the inside of the rig. The theory behind it is that a slight pressure inside the rig would cause air to escape out of any leaks or cracks around the rig. Spraying the outside with a soapy water solution would create bubbles where the air was escaping, similar to finding a nail or leak in a tire. The pressure is adjustable as larger rigs would require slightly more pressure to fill the rig and smaller ones would not want the high pressure as it could push out a slide room.

The most accurate way to find leak in sealed underbelly

We conducted a series of videos years ago at a local dealership and it was amazing how many leaks we found on a new unit. This method is the most accurate leak detector, as some of the leaks can actually come into the rig in an area away from where the leak shows up on the inside. Leaks are one of the most difficult issues to find as water can travel along hollow tubes used in the frame and other paths. The Sealtech machine finds the outside entrance source other than the old fashion way of soaking the outside of the unit down and searching the inside for moisture.

According to the locator on the Sealtech site, there are several locations in the Northwest from Eugene, OR, all the way up to Seattle, WA.

However, before you take it into a dealership, keep in mind the leak in your sealed underbelly could just be a corrugated piece of plastic covering the tanks and/or drain lines and using the Sealtech machine would not identify the leak. You would need to remove the material and visually inspect what might be leaking. It could be something as simple as water running off the roof along the sidewall or back wall. You might find a small crack or hole in the material running along the flat material to another crack or opening. I would take a look at what the bottom layer of your rig is constructed with first. If it’s a hard basement-type construction, then find a service center.

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club.

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Dave Solberg
Dave Solberg
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.



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Ron Yanuszewski (@guest_170890)
1 year ago

If its winterized properly it’s obviously not a plumbing leak

Larry (@guest_170576)
1 year ago

Rule out the black tank: buy red food coloring, pour into tank and fill with water; do same with gray tank with a different color and see if the leak has either of these colors in the drip. If not, then likely the water tank leaks. You could use this method to individually check the hot and cold water network of piping.

Jesse Crouse (@guest_170444)
1 year ago

Had this done by the dealer we bought it from and they found and fixed all leaks. We live in south east Pa. about 20 miles outside Philly.

Bob M (@guest_170417)
1 year ago

If it’s fresh water and it’s hooked up to the city water outlet with the water on. I’d think if there was a leak you’d see it drip out. if your fresh water tank was full, I also think it would drip out. As for putting air into the RV to check for leaks with using slight pressure inside the RV and spraying (Joy dishwashing soap) with a paint sprayer over the RV. For how cheaply the RV’s are made. I’d be afraid you can cause more leaks/damage than it’s worth. Plus with the pressurized RV it’t be a job and difficult to sal things like your doors and windows. If not sealed pressure can’t build up. I’ve tested thousands of equipment like rv’s with the test you mentioned. But the equipment was stronger and we used a manometer attached to air with a reading engineering said was acceptable. I know some RV dealers do the test you mentioned. But I never seen them done.

martin a (@guest_170405)
1 year ago

Propane lines run in that area also

martin a (@guest_170404)
1 year ago

Our Bigfoot has several penetrations in the floor to the tank area as the furnace blows heated air around the tanks, and the drains and fill lines as well as electrical wiring run under the floor in that area. Pressurizing the trailer would likely find many air leaks in the under belly. It is an area that is hard to seal along the frame etc.

Bill (@guest_170388)
1 year ago

The German-trainedmaster mechanic would fill our car with a non toxic ‘smoke’ and then turn the heater on high to find leaks around windows and doors .. it worked like a charm.

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