Thanks for the reply!

No Ihaven't pulled the pipes out yet. I was uncertain as to whether they could come out at this stage or not.

Next I need to see if the o-ring is bad or if it is truly a bad heater core.

I will keep everyone posted.

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)