Make an Auto Radiator Pressure Tester

by Phil B

258K6263

Intro: 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.)

STEP 1: My Homemade Tester

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.

STEP 2: Compatible Vehicles

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.

STEP 3: Tire Pressure Gauge

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.)

STEP 4: Tire Valve Stems

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.)

STEP 5: Plastic Tee

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

STEP 6: Brass Hobby Tubing

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.)

STEP 7: Cut Two Short Pieces

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.

STEP 8: Solder the Brass Tubes

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.

STEP 9: Stopper for the Radiator Neck

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.

STEP 10: Angle Iron

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.

STEP 11: Attaching the Stopper to the Radiator Neck

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.

STEP 12: Attach a Valve Stem

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

STEP 13: Attach the Gauge

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.

64 Comments

WoundedEgo 7 years ago

If my leak is from the radiator can I fix it or do I have to replace the radiator? I've heard both. Thanks.

Phil B 7 years ago

I am not well qualified to know. Newer radiators are plastic, at least in part. My impression is that those are not repairable. Older copper radiators are repairable. As you likely know, there are products designed to stop the leak in much the same way that your blood clots when you have a cut. That works with a small leak not under too great a pressure.

ErnestB12 8 years ago

just an idea insted of the wood get old cap and put hole in it and your valve steam .

Phil B 8 years ago

I thought long and hard about doing that, but the part that seals is spring loaded and "floats" independently from the piece that locks onto the radiator neck. I could not develop a way of remaking the part that seals on the radiator to make a pathway for air under pressure,

cody305 10 years ago

Now it's four years later and thought I would update you on my radiator leak problem. Since I couldn't find the leak, I replaced the 195 thermostat with a 180 and I removed the inner seal on the radiator cap. Now, when the coolant expands, it can go out to the reservoir without building up 14 pounds of pressure needed to lift the cap valve. The outer cap seal is still there, which allows it to pull the coolant back in when it cools off. Without any pressure in the radiator, it doesn't leak. Of course, I'm sure it would boil over if I ever drove in the mountains where the boiling temperature would be lower. So i guess there's more than one way to skin a cat. Interestingly, with the lower temperature, I thought my gas mileage would suffer, but i haven't noticed any difference in all these years...

don47130 11 years ago

hi. i`d like to add to this subject if i may.it`s a good ideal to take the spark plugs out and see if you leak is in the head gaskit. also i drain the oil and take out the oil drain plug. if water comes out in the oil pan then you could have a cracked engine block. and i would run the engine first to heat the block up. some leaks wont show up until the block expans.i have an 283 small block chevy.have plugs in the intake to tape i into with home made pressure test. also water pump plug to tape into. hope this helps.

Phil B 11 years ago

You make good points. It is to be hoped that most coolant loss problems are due to minor external leaks that do not involve tearing down an engine to rebuild them. But, I did once trade a car a little early because I discovered traces of engine oil floating on the coolant inside the radiator.

chrisnotap 12 years ago

I made one like this years ago except I used a discarded rad cap, tore the guts out of it, and you end up with just the metal part with a hole in the middle. Now take your rubber and push the valve into the centre of it. Bingo, your done. It has no gauge on it but with a few pumps with a tire pump it will find a leak. Small, easy, compact. I made 2, one for honda and toyota with the small rad cap and one for north american with the bigger cap. Keep at it, good idea, great minds think alike!!

firegal35 12 years ago

ok so i have a leak somewhere and the each time i put coolant in radiator i can see it leaking out as fast as i put it in......the leak is coming from the driver side n i can see it leaking from under car so my question is would ur invention help me find leak?

juggalo_assassin183 11 years ago

this is beyond a pressure tester it sounds like you blew a hose or your radiator

chrisnotap 12 years ago

if it's coming out that fast you don't need any tester you just have to use your hands to feel or get your head in a position to see it. Look from all angles. Disconnect things and get them out of your way to access a better view, you will find it.

Phil B 12 years ago

A radiator pressure tester is for detecting subtle leaks. Your leak is not at all subtle and should be easy to find at that rate of leakage. My guess is that a hose has either come off of its fitting or is split open, or there is a catastrophic opening in the radiator. Do you have a protective cover under your engine (splash pan) that restricts your view of the hoses and radiator? You might need to remove it for a good view. Anyway, your leak should be easy to find without pressure testing equipment. Thank you for looking and for commenting. I wish you well in correcting the problem on your car.

firegal35 12 years ago

Ok well my radiator n hoses r all new
There is no cover underneath so I have a good
Visual..... The leak is coming from one of the
heater hoses that connects to the throttle
Body, those hoses n clamps are all new but
There is still a leak....HELP

Phil B 12 years ago

With computer things there is a saying that you always look at the last thing to happen. I have always found that to be a good strategy with automotive problems, too. If that heater hose is very, very new; there may be something done poorly during the installation, or there may be a defect in the hose. When the engine is cold to avoid burns I would get a helper who can pour water into the radiator slowly while I felt along the suspect hose. There can be a problem that seems improbable because it is not visible looking down from the top of the engine. I once had an intermittent problem like that with a pleated air tube feeding into the air cleaner. There was a crack that made a big opening inside one of the folds. It was baffling and not possible to see the crack from above, but when I (at someone's suggestion) removed the tube and looked inside the folds I could see it very plainly.

Phil B 12 years ago

Thank you for including the photos. Yours is an excellent idea. Finding a way to pump up a radiator system without buying a commercial tester seemed a simple enough idea. Someone else had to think of it, too. Thank you for commenting.

YoungEng 12 years ago

I was very excited when I found this site when searching around for a place to buy or have my cooling system tested. The design is straight forward and won't take much to build at all. Not to mention the money you will save! I became curious about my cooling system when we started on this unit in my autos class, and when I got home and decided it would be a smart idea to check my coolant level since I had not in over 10,000 miles (learned my lesson) and found my over flow tank to be extremely below the "add" line. Needless to say I waited until the engine was cool and topped in off with coolant mix. I had been having problems with the heat not warming up during the fridges Wisconsin winter. Figured this might have to do a little something with in. I'm not aware of any leaking coolant but figured before I do a complete flush of the system I should check the pressure. After reading through the comments I decided whether or not my school has a tester or not (have to ask my teacher) I am going to build one for myself. Being a big bike racer and having a spare pump with a Schroeder valve and built in gauge I figured this would be my base build pump. This will allow me to save a little cash on a dial and t fitting. I also am going to attempt to use one of the other posters ideas and make the radiator fitting out of a large rubber stopper with a hollow bolt stud fixed with two pairs of flat washers and nuts. Allowing the rubber stopper to expand out such as his did creating a tight seal while not risking bending or damaging the filler neck. Since I'm out of town until Sunday I'll have to wait to tackle this one, but will attempt to post pictures once I figure out how. Thanks!

Phil B 12 years ago

Thank you for looking. I am glad you can use this Instructable. Fitting the tester to your system is a little different from what I did back when radiators were filled at the radiator rather than the coolant overflow bottle, but several people who commented came up with a good solution to that problem, too.

I like good practical Instructables that solve real problems and save people trouble and money. Not all that you find here is practical or solves a real problem. I hope you enjoy the site.

It will be good to see pictures of the testers you build. I hope it saves you some trouble and some money.

YoungEng 12 years ago

I should still be able to test at the radiator cap correct? I have access to both the radiator cap and overflow tank.

Phil B 12 years ago

I would try to test at the radiator cap. The overflow tank may be isolated by a pressure disc on the bottom part of the radiator cap.

montanasoftware 12 years ago

Hi Phil: Thanks a million for this writeup.

Here's an idea: Prestone makes a flush kit that installs a permanent T-fitting into your heater hose. It accepts a standard, male-thread garden hose. Install that T-fitting, and you have a permanent access point for flushing and pressure testing.

You could make a pressure-tester by using a solid brass piece with male garden hose threads, and a barb on the other end for inserting into hose. Build your pressure equipment onto that hose, and you've got a pressure tester with an easy access point. Take care, and thanks again.
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