e30 overheating

hello GRM, I have a 90' 325is. I have had my bimmer for a couple months now, when I first bought it it had a weird problem, it would run hot (creep up to the 3/4) but only when i was idling or running at very low RPMs. i thought it was the fan clutch but after about a week and then it just completely stopped. so i left it alone. but recently i decided to check on the coolant when the engine was completely cool. as i unscrewed the overflow tank cap it slowly hissed for about a minute. now the same problem came back as before... why was there pressure in the cooling system when it was completely cool? did this pressure somehow fix the problem i was having with the idle? ALSO the coolant fan switch wires are messed up and possibly not making any connection. but it was like this the entire time ive had the car.

thanks for your help guys.

might always be some residual pressure.. and the way a BMW cooling system is set up.. it keeps the coolant out of the reseviour. you most likely have an air pocket now. You will need to open up the bleed screw at the top of the radiator and top it off till coolant comes out.

It will leak some for a while afterwards.. and then calm down

Yes, it sounds like air in the system - with the engine off open the screw and add coolant until it runs out the bleeder. Tighten it up, start the car and get it up to operating temperature, then while running crack the bleeder screw again until the coolant coming out is free of air bubbles.

Thanks guys I'll try bleeding it tomorrow and let you know how it goes