I have a 1999 gmc heating fine driving down the road or with idle up but let off at a light and heater drops off fast temp. gauge reads normal but no heat , tried bleeding several times still not much better any help ?
Might be the heater core's starting to plug, also could be the water pump impellor is beginning to wear away from corrosion.
Try back flushing the heater core good. or is coolant level low??
you say tried bleeding. was the system opened for service/replace componet? that has 2 tstats could b one stuck open happened on mine. does it take a while to warm up?
Pull it out, throw it in the garbage, and put a 454 in it.
Bet your heater core is fried..... had a f-800 ford do the same thing. Couple weeks later the core cracked a leak.
pull the hoses off the heater and hook together. the hoses should get real hot, this will eliminate a pluged heater core
6.5s have real problems with coolant pumps. There is a replacement that works better and lasts longer. Check with your dealer. You may also have a problem with temp regulator, or with heater core. 6.5 is not a bad engine, properly maintained and set up. (I have two) Both of mine have improved turbos, performance chips, many mods inside the engine itself, and will run on nearly anything you can put in the fuel tank. I run B99 often, but also #2, or #1 if it gets real cold. No additives. Both units also exhaust via 4" tubing and freeflow muffler. Newer one has a cat off an 08 Duramax, just for visual effect at emissions test station. Last but not least, get online with the 6.5 diesel forum. http://www.sixfiveturbodiesel.com/ There you will find many answers to 6.5 problems, and the vehicles they are installed in. Good Luck!