I have a 1993 940 Turbo wagon. One of my coolant hoses needed replacement. First I pressurized the coolant system at the overflow tank to 22 psi. That is the correct pressure for this vehicle. It held at that pressure just fine. Then I drained the coolant and replaced the hose. With the coolant system empty, I pressurized it again to 22 psi to make sure there were no leaks before refilling with coolant. With just air in the system, the pressure dropped to half within 3 minutes. I checked for leaks at connections and found none. I filled the coolant system with water and re-pressurized to 22 psi. With liquid in the system, the pressure held fine.

I understand there is a jiggle pin (wobble pin) in the thermostat to release air, but that only releases air from the engine to the upper coolant system. Mine is installed correctly. It would not leak air outside the coolant system anyway. How is the air escaping when the coolant system is pressurized with no water or coolant in it?

 

Must be a leak somewhere, your pressure test shows that... Air is easier to escape than liquid. Probably in the early stages of developing a head gasket leak unless you can pinpoint an external spot. These engines do need a HG change now and then, and this can be indicated by various symptoms. Many b230 start to leak slight amount of oil externally, etc. Doesn't mean you have to do a HG right away, it may go for years like this, just make sure it doesn't overheat.

For engines that need a HG but the owners are unable to do it themselves, there is an additive called Blue Devil which works great for sealing coolant HG leaks. It's pricey as additives go but works great if you follow the instructions. I don't like the "pour-n-go" version, I use the full strength and remove the thermostat, etc. Beats the $1,500 bill most shops want for a HG.

 

If you read the instructions on the coolant system pressure tester it tell you to have the complete system filled with fluid. The reason is if there is any air in the system it will compress and give you a lower reading during the test which will be misunderstood for a leak. If the system maintains pressure when it is full you do not have a leak.

 

Pressurize the system when you can let it sit a couple hours and then start the vehicle and see if you have any unusual puff of smoke out the tailpipe on startup. That would indicate a head gasket leak.
Might try pulling the spark plugs and seeing if one or two look noticeably cleaner than the others ??