I'm new here, and before anyone says search around there are tons of threads on heater problems I just want to say you're right and I've been reading thread after thread for over an hour. I've read about similar problems but I've not come across one that makes me jump out of my seat and say THAT'S IT! Although I am leaning towards a blend door problem, but not sure what exactly.

The important stuff. I have a 97 1500 4x4 5.9. Coolant is up to level. Engine gets to operating temp, has a 190 thermostat. Both inlet and outlet hoses on the heater core are hot.

Heres the deal. I can let it warm up in the morning gets fairly warm. I start driving, it's ok. I get on the expressway it it gets cold.

What I've noticed is, sitting at a light, I drop it in neutral, rev the motor to above 2000 rpm and heat starts pooring out. If I let off the gas, let it sit at idle it cools back off after a few seconds. When I am on the expressway going around 60/65 in overdrive the rpms sit around 16/1800 and the heater blows cool air. If I turn OD off it gets warmer but not like when I'm in neutral.

Any thoughts?

 

Maybe your water pump isnt pumping enough coolant at idle or cruising speed to get through the heater core.

 

does your temperature gauge stay on normal 190'ish, or does it drop off.

i'd say your heater core is stopped up and it only flows coolant under the higher water pump pressures of high rpm.

carefully disconnect (or cut) the heater hoses and backflush the heater core both ways with a garden hose. be careful on those hoses. you don't want to crack a pipe or break the heater core cause its buried behind the dash. if any doubt, cut the hoses in a convenient place and then splice them back together when your done.

 

ck your fan clutch.

 

another guy on here stated same thing....hot at both ends of heater core....pretty sure he fixed it with a flush although not positive....

 

^ agreed.. try to back flush your heater cord