Heating system cools when sitting idle

Have the system serviced as it sounds like it is low on freon. You have enough to make the system work at idle but when run at higher rpm the low side pressure may drop and the pressure switch will shut off the compressor.

If the engine temp rise while sitting at lights then you have an air flow through radiator problem. Electric fans not working or viscous fan hub is faulty. high heat except at speed may indicate an air lock in the cooling system or coolant level in the radiator. Check at radiator cap not the reservoir.

Have cooling system flushed properly which should improve circulation through heater core at idle, or low rpms. I usually flush heater core with C.L.R. (calcium, lime, rust remover) from hardware store before re-filling cooling system by removing hoses to core at firewall, and hook up to a small drill driven pump (also available at hardware store for approx$10). I use that to circulate it through core, and reverse hoses and pump it through in both directions, then let it sit in core overnight if at all possible, just for better cleaning. Drain it next day after circulating a little more, and re-fill cooling system. Be sure to leave rad cap off and heater turned on when re-filling coolant, and allow rad fan to come on and off 2 or 3 times before topping up and putting rd cap back on to be sure all air out of system. Should see improved heat at idle.

Change engine cooling system thermostat.

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Sounds like you have a problem with the cooling system. First thing you need to check first if the auxillary fans are working from the time you are the engine is overheating. If the auxillary fans is not working then the problem would be the aux fan itself or relays. if Fan is working then you might have a poblem with the water pump.

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mamoore4u . check for a faulty vacum line at your heater by pass heater hose,

your thermostat might be stuck open so while idle it can warm up but as soon as the motor increases speed it cools down quick

is the fan working? if the fan isnt working it will overheat when idling, as it wont be sucking cold air through the radiator. a quick way to diagnose that your cooling system is fine is when stuck in traffic, turn your heaters onto hot, if the engine temp gauge shows that the engine is cooling down, the cooling system will be fine 9/10 times.

hope this helps.

This whole problem is most likey due to a sludged up intake and idle speed control valve, do the following procedure and it should take care of the problems, also run a couple of cans of LUCUS brand fuel injector cleaner through, I know u will see it is more expensive than other brands, but the thing is it works!
Here is the most common cause of surges and stalls at stops and low erratic idle speed and rough idle, it is the idle speed control air-bypass valve and throttle valve (IAC for short), they get full of gunk over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake