VW Vanagan, testing pressure cap

Testing the coolant pressure cap 

on a water cooled VW Vanagan

WATER COOLED VANAGANS (1984-1990) can drive you crazy when the coolant system malfunctions as the resulting system peculiarities are not easy to diagnose.  However, one thing you always know: If you pressure release cap is not working (almost perfectly) you are going to have all kinds of coolant problems, too numerous to detail here.  Let’s just say if the pressure is to high you’ll get leaks and fluid loss, and if it’s to low the system won’t cool efficiently. 

coolant pressure cap, getting ready to test

After buying a new pressure cap that didn’t seem to work properly I resorted back to my parts box and found four more old caps of various vintages.  One came on the van used, one was a new replacement three years  ago, one I’d just bought, , one clearly didn’t hold pressure, and the last one was an extra of unknown origin and function in my parts box.  I started switching them in and out for short trips and discovered that I got all kinds of different engine behavior with the different caps, indicated by leaks and illogical coolant reservoir levels.  I therefore needed a way to measure the pressure of the caps myself.  Of course VW service shopswill do this for a fee, but I wanted the flexibility of testing the caps on the van once I knew what they were. 

 Here is my custom pressure cap tester:  Find an old Vanagan (or other VW  pressure tank) for this project.  The vanagan pressure cap must fit the  screw-on filler cap hole and screw in tight.  Into one of the side hose holes adapt a water pressure gauge that goes to 30 lbs. On the other hole adapt a garden hose connector outside of a water valve, preferably 1/2'”.

water flowing out of the cap nozzle at a determined pressure

Test the cap as follows:  Hitch up and turn on the garden hose with the valve off,  screw in the cap to be tested, slowly release the valve watching the nozzle on the cap. It will probably dribble a bit for a while and then start full release at some pressure.  Note the pressure when the dribble starts and when the full stream starts,,, those are both important functions.

gauge working, water flowing, check pressure

 When the engine cools, the coolant contracts and pulls coolant back into the engine.  There is no tester for this return function,,, simply notice if coolant sets at a lower level in the overflow tank when the engine is cold then when it is hot. If that happens the return feature of your cap is functioning adequately for our purposes. Four of my five used caps perform this important function so most caps return coolant to the system when cooling.

cap working, gauge indicates pressure, water valve shown

Water not flowing at 25 pounds, cap clearly malfunctioning

The water can start dribbling before 14 pounds and in fact, It’s my  opinion that it should start to dribble at 10 pounds or so.  Keep turning up the water and the full stream should commence out the cap spout before 17 pounds.  If it goes higher for a full stream, the cap will probably cause problems in the form of leaks.  Discard all caps that don’t’ hold pressure or that hold pressure to a release point above 17 pounds.  If your van is old you want lower release pressure, but still some pressure.  If you engine is rebuilt, you can go as high as 17 pounds for release but not higher.  Test all new caps before you install  then (I found new caps that malfunction).  If you can’t get a cap that works correctly, buy some used ones from a wrecker yard, test them and install the best one.  As you can see, it took me 5 caps to find one that works on my van.

I dont' have a picture of a cap that opens at a pressure below 14 pounds but you get the idea.  Our van (L'l put-put) has run perfectly since I installed the "mikehilerfiredclay certified cap".  The beauty of this tester is it tests water pressure for cold water.  I dont' like the official VW tester as it doesn't test the pressure the cap starts to dribble.  

Hope you liked this post, it was fun to put the pressure tester together with spare parts laying around (had to buy the 30 lb water gauge).  If  you need help putting the tester together, just read the post again and talk to your mechanic.