No heat from heater during idle

1997 Chevolet S-10 Blazer. I have changed water pump (leaking), changed radiator hoses, replaced Thermostat(checked temp, opens around 175 F) replaced radiator cap, flushed system, bled air from system and replaced coolant. The vehicle heats up quickly (if) running down the road.

If the hoses going to the heater core are hot, and I assume they are considering all you’ve done, the problem is most likely with the HVAC system under the dashboard. The air temperature is controlled by a door, or flap, which moves (or should) according to where you set the temperature control. When you ask for warm air, the door directs air through the heater core. When you want cool air the air flows through the AC evaporator.

The device that moves the temperature door is often vacuum controlled. Engines at idle produce high vacuum. Engines at part or full throttle produce less vacuum, so if there is a leak in the system somewhere, the door can move on its own, depending on engine speed, causing exactly the symptoms you describe.

I’d start looking for a vacuum leak in the hoses or the vacuum devices under the dash. This is a very common problem. You may even be able to see or hear the temp door moving if you crawl down in the passenger footwell and do some experimenting with the throttle. NOT while you are driving, please. By the way, a service manual, such as Haynes, can be very helpful in diagnosing something like this.

I agree with MC, it sounds like a weak vacuum.

One point of clarification, please. When you say “The vehicle heats up quickly (if) running down the road” do you mean the cabin of the vehicle or the engine temp as read by the gauge? If the latter, one thing you didn’t mention was the thermistor controlled fan clutch-- if the fan’s going full-speed, it can stop your motor from warming up until you start driving around.

The vacuum theory’s a possiblilty, although in my experiance, you get the opposite effect from a vacuum leak-- heat at idle, but not when you open the throttle. I haven’t had the issue with later-model GM products though, so perhaps they’re designed that way.

The cabin warms up quickly, when driving. Engine temp is about the same.
The fan turns freely, when engine is off, and I can tell a diverence in the fan speed periodically as the motor is running.

Thanks, I’ll start looking for a vacuum leak.

Which “fan” are you talking about? Are you talking about the engine fan, or the heater blower fan?

I had a the same issue on my 2002 Nissan Altima. The only thing that worked for me was to install a dedicated water pump for the heater. A DIY kit can be found at amazon by searching for vehicle heat pump.