Make an Auto Radiator Pressure Tester

Pictured is a commercial radiator pressure tester. They are relatively expensive for the DIY home mechanic. But, there are times when it would be very handy to have a radiator pressure tester for finding leaks leading to coolant loss and possible engine overheating out on the highway. (The photo is from Google images.)

When we had one youngster in college, another in high school, and my wife was working we had four cars in the driveway for a while. We had another child out in her own apartment, but she always brought her car to dad when it needed to be fixed. It was not uncommon for one of these cars to have a mysterious coolant system leak. I made this pressure tester so I could locate and fix such leaks quickly.

My pressure tester works on radiators with a neck and cap on the radiator. I have not used it on closed radiators that are filled through a coolant recovery bottle. It might be possible to adapt it for fitting onto the fill opening in a coolant recovery bottle, but I have not had to try it yet.

I selected and bought a dial indicator tire pressure gauge identical to this one. Its outside diameter at the end fits nicely inside a piece of 5/16 inch I.D. fuel line hose.  This gauge holds pressure until the brass release button on the side is pressed.  You will want to watch the gauge to see how fast it leaks down when the tester is in use.  I removed a valve core from the end of the gauge before attaching it.  Another option is to use a gauge that does not have a pressure release button.  (The photo is from Google images.)

Buy a package of tire valve stems. I used a sanding drum to remove the expanded section where the stem attaches to the steel rim. See the yellow lines. You will need two valve stems. (The photo is from Google Images, but edited by me.)

You will need a 5/16 inch plastic tee. (The image is from Google Images.)

When I made my pressure tester I could not find a tee like I needed, so I decided to make my own from 5/16 inch brass hobby tubing. (The photo is from Google Images.)

I cut two pieces of brass tubing a bit longer than an inch each. I coped one to fit over the other in a "T." I drilled a small hole in the middle of the piece not coped so air can flow through the tee in all directions. See the black spot.

Place the coped end over the small hole and solder the tubes together. Check for any air leaks in your solder joint. I was concerned that my solder joint would not have enough physical strength to stand up to normal use, but this joint has survived quite a number of years.

Cut three pieces of 5/16 inch I.D rubber fuel line hose. One piece should be about 10 inches long. The two others can be 2 - 3 inches long each. The hose on the left side of the photo is the 10 inch piece. It will go to the radiator neck stopper. See step 9. Clamp the hose pieces with small hose clamps.

Cut or turn a cylinder of wood to fit inside the neck on your radiator. Use epoxy to glue smooth rubber to one side of the cylinder. Drill a hole through the wooden cylinder and epoxy a valve stem in the hole. It probably does not matter much, but I set it so the threaded end is pointing toward the upper part of the photo away from the rubber on the bottom of the cylinder. Remove the valve core from this valve stem.

I used a piece of angle iron as a retainer for clamping the stopper in the radiator's neck. The angle iron is about 3 inches long. Drill a hole as shown. Make it large enough to fit over the end of the valve stem in the wood cylinder. After placing the angle iron on the valve stem, attach and clamp the rubber hose as seen in step 9.

Let your radiator cool before opening it to avoid serious burns and boil overs. Smear some grease on the rubber of the stopper to assist in sealing the tester's attachment. To attach the stopper to the neck I used two very small "C" clamps, one on each side of the neck. The red dots mark the positions of the two clamps. The neck of the radiator is not very strong and you do not want to distort or break it. On this automobile the neck is made of a tough plastic. I have not used my tester on a neck of plastic. All you need is just enough clamping force to withstand 15 pounds per square inch of air pressure without leakage.

I always wanted to find a better system for attaching my tester to the radiator neck, but never developed anything better.

Insert a valve stem into the end of one of the shorter pieces of fuel line hose and clamp it.

The tire gauge will attach to the other shorter piece of fuel line hose. Clamp with small hose clamps.

You have now finished your radiator pressure tester. It should look like the photo in step 1.

Attach the stopper to the neck of the radiator. See step 11. Attach a bicycle pump to the valve stem. See step 12. Your car's engine should not be running. Pump the system up until the gauge reads 15 pounds. Often you can hear a hiss where the system is leaking. You may see evidence of moisture appearing where the leak is located. You will also notice that the pressure reading on the gauge leaks down slowly after a few minutes.

Sometimes the pressure reading does not leak down, but you know your system has a leak. In those cases, chances are the leak is coming from your radiator cap. If the cap is a few years old, replace it and see if your loss of coolant has ceased.

I have used this tester several times and it was always a big help.