Sorry to thread jack but this is kind of on topic. I've been having overheating issues lately that corrects itself when the heater is on. I've purged the system several time. The fans work fine. Today the coolant overflow tank almost filled up all the way while overheating. Could a bad cap allow air to keep entering the system causing me these problems, or is it the thermostat?

 

you have air in the system and it's not purged properly. If the overflow is filling up and it's not coming out of the filler neck, the cap is doing it's job. If the fans turn on, and the gauge is pinning itself, the thermostat is working

causes of the air? any number of things from a simple bubble, to a headgasket issue

 

Yep. It's not the cap, it's air. I think that the key is to ad d the fluid EXTREMELY slowly, as they mention in the manual. I actually added it that slowly and nailed the whole procedure the first time.

 

i thought the stock cap was .9bar. That Z1 link seems to think its a 1.2, which is already sufficient for FI.

 

stock is .9
mishimoto comes with a 1.1

I removed the mishimoto and replaced with my stock .9 cap because my car never gets over 211* even will full boost runs in the upper 90's ambient temps so my car has no reason to run a pressure cap.