Why is coolant bleeding so difficult at times. ('03 with 3.0 six cylinder)

[jpilk99]

I bought my 2003 X5 3.0 about 3 months ago. The seller told me about a slow leaking expansion tank, and after a few weeks, I bought a "BEHR" replacement. After swapping it, I went and looked up the bleeding process for this engine/vehicle and found it very similar to all the 325/328 and 740iL bleeding processes; i.e. http://forums.bimmerforums.com/forum...ystem-Bleeding

Interestingly, my mechanic - who I trust greatly - has varied minorly in the bleeding process outlined above whereby he says to, essentially:

Fill the expansion tank to the appropriate level, with the bleed screw open
Have someone start the engine, heat on high/fan on full, and hold the engine at about 2,500-3,000 RPMs.
Coolant flows through the little "piss hole", close the bleed screw, and continue to monitor the flow of coolant through the little hole back into the expansion tank.
Once the flow is free of any air bubbles, close the expansion tank and begin operating.
Monitor accordingly

That is a minor paraphrasal of my mechanic's recommended process. The big difference being that he says to have the engine running while most other DIY and video's just say to turn key to position 1 (without starting), heat on highest temp and fan on low.

So, after putting a different BEHR expansion tank in my vehicle (of a much newer X5 with same engine at the junk yard), I followed the above linked procedure. What was ...crazy to me was that coolant never slowly bubbled up & out of the open bleed screw hole ...I hear the internal workings of the coolant/heating system kind of wind up and then GUSH, like a geyser, it shot out the bleed hole about a foot high, and then retreated back into the hole, as I continued to pour in coolant. Then, while adding more coolant, after about 5 seconds, I heard it coming again and BLURRRRP up went the geyser. I did this about 5 or 6 times and realized it wasn't going to stop, so as it continued to flow upward, it didn't look like there were any bubbles, so I quickly re-inserted the bleed screw and tightened it down. Then I filled the expansion tank a little more until the red "fill level" stick was protruding an inch or so out of the tank and then screwed the cap back on. Rinsed it all off and took it for a drive.

Coolant temp climbed up normally to dead-on middle and stayed there. I drove about 10 minutes, accelearating a few times quite quickly and putting a good load on the engine. Got home, (No "Check Coolant" lights or anything), but when I opened the hood, I could see a minor leak from what looked like the cap. And I could hear the slight hiss of air escaping.

I'm thinking of trying a different cap, but, ...this is the game I've been playing for a few months. I could certainly run out and buy a new expansion tank, radiator, water pump, new hoses all around and then bring it to a dealer to have them "bleed" it, but, ....(obviously being a little baby here) that seems excessive.

Anyone ever had the "geyser" issue I mentioned? Can caps be this problematic? I see no cracks in the cap, it has both rubber gasket rings...

Just don't know why it's so tough to bleed these machines. Thanks in advance,

Jay