So the other day, I noticed that when I first back out of my garage, I could hear the water flowing through the heater core....(Note: I had the heater core replaced ($1200 job) just in 2019. It's been warm so have used heater or defrost. Well.... this morning I start up the truck, let it run for about 15 minutes, no defrost or heater, and truck is running about 200 degree (it usually takes for ever just to get to 140 idling). I checked the fluid levels (all good) last time I drove it (last week as I don't drive it every day).. Any ideas would be awesome!

 

These trucks are notorious for being difficult to get all the air out of the heater core.... Might try burping the system, and see if anything improves. Getting the front end up as high as possible also helps.... Generally, the more 'pucker-factor' the more effective. I found a nice steep hill to park on.... (yeah, that was...... interesting.....)

 

Make sure your blend door is truly moving to heat. '99 should still be cable operated?

If yes, then yeah, I'd try to burp the system on as much incline as possible. Get a funnel for just this, they're cheap.

If THAT fails I'd look at core obstruction. Doesn't matter if it was just replaced. If you have crud circulating in your system, it could clog it. Remove heater hoses and blow air BOTH directions repeatedly. Try to disconnect closer to the motor -- I prefer to not tug on the core any more than necessary, PLUS the cowl makes access a PITA

But first be certain the blend door is calling for heat or you're punking yourself. And I know what your gauge says -- and it's probably right -- but it never hurts to verify ECT in live data. If a t-stat is stuck open you're again just punking yourself