I have a 03 Nissan sentra, 1.8 liter. Ive been trying to fix the heat, at stops lights it cools down ? Ive tried thermostat, bleeding system and even a heater core. any sujestions ? would the water pump do this. It gets realy hot then at idle sets and cools right down ? is there any special bleeding techniques to use.
The problem is most likely in the HVAC system, not the engine. If the blend door, which controls air temperature, is moving when it shouldn’t, you will have this problem, especially if the blend door is vacuum controlled.
At idle there is high vacuum, as you move away from idle there is less vacuum. I think you need to start looking behind the dashboard instead of under the hood.
There are shops that specialize in automotive HVAC. Find one.
The water pump may have impellers that are damaged which would affect circulation. You could try this: Park facing uphill (45 degrees) when your engine is cool/warm. Remove the radiator cap with engine running and keep adding coolant a little at a time until all the air is out of the system. Quickly replace the cap and see if the heat improves. You may need to see a good mechanic if heat does not improve.