E30 Zone

Right,

Ive installed an M52b28 into my E30, just been finishing the last few things off my check list before i start using it as a daily and now ive run into a bleeding problem. I know BMW's can be an utter twat to bleed but im unsure if this may be pointing at something else wrong.

Car was heating up and self cooling using an electric pusher fan wired to a rad switch, i took it to be MOT'd and it decides it wants to spew its coolant out of the rad cap on the ramp, figured it could be an air lock as the car hadnt been used since the engine had gone in and it felt like it bled at little to easy. Today ive had a good go at bleeding it, got the heaters blowing nice and hot, it had a 50/50 mix of the green stuff which now has been diluted down quite a bit as i just wanted it bled an running with a view of sorting a proper cooling mix at a later date.
Anyway after a good hour or so of bleeding with the front end jacked up it appeared to be air free, no coolant explosions and it was regulating its own temp using the electric fan. Took it for a drive round the block and hey presto, coolant explosion again! both rad hoses solid as a rock and no fans kicking in..

Any ideas?

Just so people know it had the following before installation into the car

New Water pump
New stat
New ally stat housing

Also had a compression test a few weeks ago for the sake of it as i had the plugs out anyway, all cylinders uniform making around 130 PSI.

Definitely no leaks? If the system can't maintain pressure the coolant can boil, causing the symptoms you describe.

Have you tried a different cap? Does the gauge go above half way?
A 2.8 should be near 200psi on compression test, sounds like you had the throttle shut which gives false readings

I've got three different caps all of which have the same problem, I had considered a leak but both the stat housing and water pump look dry to me. The temp gauge appears to read correctly, the fans kick in just before the segment after twelve o'clock.

I can hear a slight hissing coming from the lower front of the engine but no leaks, I may have to whip the rad out and have a better look around the water pump.

Solved it, pressure tested the cooling system yesterday morning at at around 15psi the water pump starts to leak from the edges. Reinstall, RTV, problem solved! Thanks for the input folks.