Introduction: Make an Auto Radiator Pressure Tester

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of Plastic Tee

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

Step 6: Brass Hobby Tubing

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.


WoundedEgo (author)2016-12-02

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 (author)WoundedEgo2016-12-02

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 (author)2016-02-20

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

Phil B (author)ErnestB122016-02-20

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 (author)2014-03-22

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 (author)2013-01-03

hi. i`d like to add to this subject if i`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 (author)don471302013-01-05

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 (author)2012-02-09

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 (author)chrisnotap2012-04-07

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?

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

chrisnotap (author)firegal352012-04-08

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 (author)firegal352012-04-07

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 (author)Phil B2012-04-07

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 (author)firegal352012-04-08

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 (author)chrisnotap2012-02-10

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 (author)2012-02-23

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 (author)YoungEng2012-02-23

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 (author)Phil B2012-02-24

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 (author)YoungEng2012-02-24

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 (author)2011-08-03

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.

i would take a garden hose cap drill a hole for a valve stem insert in the cap screw it on the flush kit you can pump it up with a bike pump with gauge or a 12v roadside pump or a Tire Inflator with Gauge and an air compresser thats what hit me when i read this plus my truck thats leaking already has the kit in the heater hose im going try it today ill let you guys know

stoneyone (author)stoneyone2012-01-21

it works i found my leak in lower left corner on the back side of the radiator im glade i found this site

Phil B (author)stoneyone2012-01-21

Thank you for reporting back to us. I am glad you found something useful here, and that it was one of mine. ;-)

Phil B (author)montanasoftware2011-08-03

Thank you for the idea. It could work quite well. I see the kit you mention is about $20 and would need to be left installed in the car. But, it could still save a home mechanic some money, especially if the flush and fill kit were installed on a car anyway.

montanasoftware (author)Phil B2011-08-03

Hi Phil: The kit actually sells for $3.25 at my local WalMart, and less than $4.00 at local auto parts stores. That includes the 3 different sized t-fittings, a cap to seal it off when not flushing, a large yellow diverter elbow that snaps into the radiator neck, and a female attachment that allows you to hook a male garden hose to the male threading on the T-fitting. It doesn't include the flush chemicals.

One thing ... i would want to solder up my own copper t-fitting, even if it cost more.  It would be a much more durable solution than the plastic T-fittings.  You can get the parts for all of it in the plumbing section of any hardware store ... even have raised lip edges on the t-fitting where it pushes into the hose.  You can get a metal cap in the garden hose section.  Then make the pressure tester from a *female* garden-hose-threaded brass barb (I got that wrong in the first post), and it would be a very rugged, jeep-worthy solution.

Also, Dorman Auto Parts makes just the plastic T-fittings and caps for less than 1.50 each.   The part numbers are:

- 47120 for 3/4" heater hose.
- 47121 for 5/8" heater hose
- 47122 for 1/2" heater hose

But that doesn't include the adapter that allows a male hose end to attach to the male T-fitting.  It mostly serves as a replacement or extension for the Prestone flush-n-fill kit (allowing you to use one flush-n-fill kit on more than one car.)

Anyway, I hope this helps someone.  And thanks again for sharing your knowledge with us.  You've given a lot of us something very good.  I've always known a pressure tester would come in handy, but couldn't justify the cost for occasional use.

Phil B (author)montanasoftware2011-08-04

Thank you for all of the good information. I had found the kit on the Internet for almost $20, and thought that is a common price.

I am thankful that I have not had a radiator leak problem since the 1980s or early 1990s. I will certainly keep your suggestions in mind for the day when I do.

mmeeker (author)2011-11-17

Thank you lots an lots. I used your idea and applied it to my pressure tester idea. So i modified your idea to make a cylinder pressure tester and also a leak down tester.

Phil B (author)mmeeker2011-11-17

I saw your Instructable. It was quite interesting. I had not thought about gauges reading up to 200 pounds of pressure. It was clever to utilize an old spark plug for connecting the tester to the engine. I did not realize you used my Instructable as a starting point. You did a good job and it was clever of you to make the adaptation. Thank you for your report. I am glad it worked for you.

Esmagamus (author)2011-03-20

On the Citroen ZX, at least, this can be done in a lot simpler way: the air purge on the highest part of the circuit is a little hole on the heater core plugged with an air valve nut. You just have to pump some air and watch the gauge on the pump.

Your idea is good, but I'd pressurize the circuit on another point that didn't require the cap to be removed. That way I could test the cap for leaks and I'd also make sure the pressure release system works as it should.

Phil B (author)Esmagamus2011-05-26

Thank you for your comment. I do not know how many models use the feature you described on the Citroen ZX. There may be more. Perhaps it would be possible to add a fitting like you describe on one of the hoses, particularly a heater hose.

RDTexas (author)2010-11-22

Go to your hardware store and buy a galvanized tee and two pipe nipples. Check the size of your heater hose if it is 3/4 ID buy 3/8 pipe. If it is 5/8 ID buy 1/4 inch. Screw the tees and nipples together. Buy one foot of heater hose and two hose clamps. Pull off a heater hose, shove the tee/nipples in one end and use the foot of heater hose to connect up to where you disconnected the hose. You can pressurize your system through the tee and look for leaks. Don't go over 10 psi.

Phil B (author)RDTexas2010-11-22

There is always the possibility that the leak is one of the heater hose connections you pull off to attach the tester, which would show an intact system and would not find the leak. And, my hoses often form an adhesive bond to the metal tubes from the heater core after a year or two. I am not sure I could pull a heater hose off very easily without damaging it, which would cause another leak source when re-attached. It also seems some sort of reducer would be necessary to go from the heater hose I.D. to the I.D of the hose used in the tester. Have you tried your suggestion? How did it work for you? Thank you for the comment.

rock3r4life (author)2010-05-27

you could add two bolts with the heads facing down to catch the lip of the radiator.

Phil B (author)rock3r4life2010-10-05

Sorry I missed your comment in May. I had considered hooked ends on threaded rods. Bolts with heads turned downward would be similar, so long as the heads did not slip out to the side and release as they were tightened.

learnbydoingstuff (author)2010-04-25

Here' s a stupid question, why pressurize at the radiator cap?  Why not pressurize at a heater hose at the thermostat bypass?  Sure its a little more involved and you might drip a little but it should be able to pressurize anywhere in the system you like, right?

Phil B (author)learnbydoingstuff2010-10-05

Somehow I missed your comment back in April. Pressurizing anywhere in the system would work just fine. I suppose I was thinking in the paradigm of traditional radiator testers that always attach at the radiator neck.

farna (author)2009-07-19

What about using an old radiator cap for the stopper? I'm thinking drill the rivet holding the spring in out, then use a large piece of rubber instead of wood for the plug. Maybe a piece of foam and a thin wood plug with flat rubber face. The plug would have to compress to hold pressure. Hmmm.... maybe leave the original spring in and just make a new face with the valve stem running through, holding the thing together. Like the commercial kit, youd need to make several plugs to match different size caps. You could use it on a sealed cooling system (with a pressure tank and no cap) also -- get an extra cap and epoxy a tube to it. Most of the pressure tanks have a small hose connected somewhere also. Would be easy enough to remove the hose from the tank, plug the loose end, then pressurize the tank through the hose barb.

Phil B (author)farna2009-07-19

I thought about using a radiator cap to connect to the radiator and there are problems to be solved. In the end I just made a wooden plug with a rubber surface and decided on a way to clamp it in place. If you get it to work, I would be interested in photos and a description. Thanks.

reversebias00 (author)Phil B2010-05-06

You're a cool dad. 

Phil B (author)reversebias002010-05-07

If you meant me, thank you.

cody305 (author)2010-03-20

I have a bicycle pump with a built-in pressure gage.  Looks like it would be easy to adapt it to attach to the overflow tube on the radiator neck, using a tire valve, some rubber hose, and a couple of hose clamps.   I could leave the standard radiator cap on, and pressurize through the reed valve in the radiator cap that allows coolant to be sucked back into the radiator when it cools down.  The pump I have is very leak-tight, so I don't have to worry about the pressure bleeding down through the pump, as could happen with many tire pumps I have used in the past.  I was going to try this, but then noticed my radiator was full to the brim, indicating that the leak stop compound I added a couple days ago had finally sealed the leak.  I will have to wait until next time to try this idea, but maybe someone else would like to try it and report the results here.  Thanks for a great article!!

Phil B (author)cody3052010-03-20


Thank you for your interest and your comment.  It sounds like your bicycle pump adaptation should work.  Watching the gauge for declining pressure may not be necessary.  Sometimes you will hear a hiss or see bubbles.  Do not worry.   Leaks are of such a nature that you will have another one day, even if you have none now. 

cody305 (author)Phil B2010-03-20


I had written a longer comment (my first one here) but when I tried to send it, it burped and lost the whole thing.  I should have copied it to the clipboard before I sent it just in case it bombed out.  Anyway, in the longer comment, I mentioned how my service manual suggested doing a pressure test, and while searching for a pressure tester, i found your article, and signed up here.  This looks like a great website, and has lots of things my wife would be interested in too.  When I get some extra time (I have to replace our water heater that just sprung a leak tomorrow) I plan to try the bicycle pump method just for fun (and to be ready next time I have a leak), and I will report the results here, maybe with a photo or two.  Until next time....

Phil B (author)cody3052010-03-21


Sometimes strange things happen to comments here.  While offensive comments are often seen and removed by the moderators, comments also disappear all by themselves once in a while, only to reappear days later.  Who knows? 

I wondered how you found my Instructable on the radiator tester. 

This is a great site.  I wish they had a category for "workshop."  When posting, "life," "home," "ride," and "technical" just do not describe some of my projects; but that is what I need to use.  I am not sure it makes a difference, either.

Some of those posting are young high school kids who think pranks are cool and only want better spitwads or stink bombs.  I prefer useful how-to information that solves a problem inexpensively.  Currently I have 127 things I have posted.  Some are better than others.  Those I think are significant often do not gather much attention, while something I was almost embarrassed to publish excites thousands.  Go figure.

Instructables has a "be nice" policy on comments and most are very kind in their comments or they ask a simple question.  Some can be very sarcastic.  These usually have posted nothing of their own.  Often someone makes a comment to something I posted that expands my idea in a way that makes it even more useful to all of us.  I have made several friends here whom I will likely never meet in person.  Some of these live on other continents, but we have similar interests.

I hope you continue to enjoy the site and will post some things.  Remember most users simply appreciate the honest effort you made, even if they are not likely to use the idea.  But, it is great when someone posts a photo of their execution of your idea.

cody305 (author)Phil B2010-04-05


I finally had a chance to try the bicycle pump in the overflow tube idea.  Imagine my disappointment when I found out that it wouldn't work.  It would have worked, except for one problem.   The radiator cap has an inner seal and an outer seal.  The inner seal keeps the fluid in the radiator until it gets hot enough to overflow.  It is usually set to relieve at somewhere around 15 PSI or a little higher, I think.  The outer seal around the rim of the radiator cap is to prevent leakage in or out of the "coolant recovery system" which includes the hose and the coolant reservoir.

I discovered that the outer seal relieves at a much lower pressure than the inner seal.  Thus, each time I pumped air into the "coolant recovery system", although some of it would go into the radiator through the center "vacuum return valve", most of it would leak out around the cap with a loud hissing sound.  Thus, I could only get around 2 or 3 PSI pressure into the radiator, which didn't register on the pressure gage because the "vacuum return valve" would close at the end of the pump stroke, which would isolate the pump gage from the pressure in the radiator, and the pressure outside of the valve would quickly bleed down through the outer seal.

I only knew that I had gotten pressure into the radiator because when I removed the cap, there was a mild "whoosh" of air coming out.  I repeated it a few times to make sure that's what was happening.  So, although the pump with built-in gauge might work ok, I need a different location to attach it to the cooling system.  I might try cutting a heater hose and installing a "tee" connector in it, with a tire valve on the branch from the tee.  If I actually do it, I will post the results here. 

Phil B (author)cody3052010-04-06


Thank you for the report.  I hope it works for you eventually.

rimar2000 (author)2009-11-24

Phil, if I had had this gizmo (or the idea of doing it) would not have sold my car so cheap some years ago. It had a leak in the radiator of heating, my mechanic took months to detect.

Phil B (author)rimar20002009-11-24


Thank you for looking at this.  I regret that it was not available to you to help in solving the problem on your previous car.  It still may be a help to your one day.   

2nup350 (author)2009-07-24

about the fuel pressure guage idea, i made one from an old locking tire air fill i got at Harbor freight. it has a lever that locks it onto the tire schrader, I took the hose and locking mechanism off and adapted a mechanical fuel pressure guage from a junkyard car onto the hose end with Napa brass hardware. total cost of 10 bucks or so new, 3.79 for me since i had stuff already

Phil B (author)2nup3502009-08-01

Thanks for sharing your idea.

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Bio: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying ... More »
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