A couple of things . . .
- Pull the heater supply and return hoses off the engine and see if you can blow through them, just to make sure there isn't something blocking them and/or the core.
- Try the reverse and blow INTO the engine outlet port; maybe a piece of rust or other deposit in the water jacket shifted and is partially blocking the outlet.
- Check your tranny cooler on the raw water intake side to make sure there is no gunk like weeds partially blocking it. My heater was noticeably cooler last week and I discovered lots of weeds in there. Strangely, the engine temp still read normal.
I did the heater install myself last year and it works fine. I used the Y-adapter from SkiDim that was mentioned above and I get good heat at idle and great heat when under way.
2005 X2, Viper Red, MCX, OJ TZ-X5 prop, PPass, rear 750 sacs, KGB, IBS, Bennett Wake Plate, custom wake shaper
(previous) 2001 X5, 1991 TriStar 190