Heat stops coming out when driving downhill?

[maxpowers]

yep, that is about it. its very strange and immediate. hot air on level ground and up hills and as soon as I decend it gets cold. fan stays on but the heat just stops.
if I drive up and down then the heat goes on and off.
any ideas?

[mararmeisto]

Heater core is blocked, almost completely shut.

[strada-caster]

Heater core is blocked, almost completely shut.

possibly in combination with a poor water pump?

[mararmeisto]

Heater core is blocked, almost completely shut.

possibly in combination with a poor water pump?

Possibly. One has to remember that the heater core inside the cab is at the top of the circuit, and the piping is smaller than the rest of the circuit. Also, there is a flapper valve which allows/disallows the hot water to circulate through the heater core - there might be a problem with that as well.

[thedjjack]

When was the last time the cooling system was serviced?

We talking long down hills?

1. Is it full of coolant, clean coolant?

2. What is the temperature gauge doing? It could be a stuck thermostat that is not closing when going down hill (over cooling the engine which is not good for the motor either).

3. When was the timing belts ect. done? If you are going to service the cooling system it is a good time to change belts etc...

I think thermostat stuck open (fail safe ones are designed to fail open) and I would flush the system good (especially the heater core).

[maxpowers]

short hills long hills, it is a very immediate change.
all work has been done at CVI in the past, new belts about 8 months ago,
cooling system was flushed, a year and a half ago.

the temp gauge is totally normal, heats up and stays at its regular spot. it doesn't change with the hills. its just the air coming out of the vents,

thanks for the educated guesses that is what I was kinda leaning towards.