Coolant pressure tests

Ok, simple question -

When doing a pressure test on your cooling system, how long should it hold pressure?

The gauge from Autozone says the cap should maintain within the range for the pressure rating (ie, a 16 psi cap should maintain in the 15-17 range during the test) for 30 seconds. For the rest of the cooling system, it says 2 minutes.

Other sources say the gauge should not move at all in the test time, and I’ve seen up to 10 minutes for caps and 20 minutes for cooling systems.

I’ve never used these before - and here are the results I get:

Taurus - cap drops from 16 psi to 8 psi in about 6 seconds (this is why I rented the thing). The coolant system will drop from 16 psi to 15 psi in about 35 minutes if the engine is slightly warm. If completely cool, it goes 16 psi to 15 psi in as little as 10-15 minutes.

On the Camry, well, the thing won’t even connect to the radiator. And the cap? From its pressure rating (885 (?) kPa) to 0 in about 1 second. The test pump itself is clearly leaking - the cap simply isn’t fitting it well.

It is safe to assume any “rental tool” is NOT going to be in very good shape…The test is only as good as the tool…There was a time when ALL radiator filler necks were of a STANDARD size. Today, there must be dozens of different configurations, making pressure testing all but impossible EXCEPT FOR THE DEALERS SERVICE DEPARTMENT, who will have the specialized tools and equipment needed to work on the vehicles they sell…

All things considered, your Taurus looks fine, except the cap is suspect…

That’s what I was figuring - if it held pressure that much longer than the instructions said, then the leak was likely with the test equipment or the connection.

I’ve lost a minimal amount of coolant (maybe a tad over 1/4 cup over 3 months), but that could easily be explained if the cap was faulty. When I ran it with the cap off to check for bubbles (there were none), the coolant heated up quickly and put off some good steamy mist. Never had a drop on the ground that I’ve seen… It was the lower coolant level (and lack of a cracked coolant tank) that queued me into the test. Temp gauge has never moved over 1/3rd of the way up its scale (normal operating temp) in the entire 13 year life of the vehicle.

I’ve always pressured them up and let them sit for 10 minutes at a minimum and expect to see zero pressure loss after that time.
The problem is that sometimes you never really know if a pressure loss is caused by the car or the cooling system tester as the testers (just like a vacuum testers) can be a bit contrary.

Agreed that the cap sounds a bit shaky.