Probably the heater tube is rusted through, or the oring on it ...somewhat common on northern cars....intake and lim have to come of (its under the lim)
My MPG hasn't gotten back up to the 26 mark, just 24 with the DP Champions.
I fixed the coolant leak, was a leaking o-ring on the tube, just like you said adu. Also the lim gaskets were in bad shape around the coolant passages.