Pressure building in cooling system....
 

Ok after about 12 hours of sitting I uncapped the cap on the filler neck and got a bit wet with coolant. What is going on here? I have some suspisions but not sure about it. I replaced my water pump housing with an 88 model my car is an 87 they had different caps the 88 had the overflow tank outlet on the radiator and the 87 had it on the filler neck. I removed the 88 cap from the water pump housing and used mine. Would that be causing my problem? I am thinking of switching it since I am changine to an 88 radiator with the overflow on it so would this likely fix my problem?


Santiago

Thanks for the help guys. :)

The coolant expands and pressurises the system when it heats up. To maintain 13psi some coolant is bled into the overflow bottle during running. When the system cools it's sucked back in. This is all controlled by the radiator cap. If you're having pressure problems, the first thing you should do is go and buy a new one from Mazda.

The S5's have the filler on the radiator, S4's have it on the thermostat housing. The The cap on the S4 filler neck is a plain cap, not a pressure relief cap. The radiator cap on both models is a pressure relief cap.

You must have a pressure relief cap on the radiator or you'll overpressurise the system and blow something up.