The car will maintain operating temperature if it is moving, but when it is stooped at idle the temperature will increase until it is at the 3/4 mark. When you get moving again it will decrease to reguler operating temperature. I changed the thermostat and replaced the coolant but this did not fix the problem. I think I should change the water pump. I called the dealer and they said it is not the water pump, buit when I ask them what the problem is they only say bring it in.
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I would clean the throttle body, idle air control valve, and the Mass Air Flow Sensor (MAF) if equipped. The throttle body gets dirty and wouldn't allow enough air to pass into the engine. The engine will have trouble maintaining an idle until the car reaches normal operating temperature.
Make sure your cooling fans come on. If overheating at idle it is likely for the fans to have failed. While driving the air rushing through the grill cools the coolant in the radiator which is probably why it's fine while driving. You shouldn't drive the car if overheating as this will cause damage to the engine. Check fan relays, fuses and fans. If you turn the A/C on the fans should come on.
Usually the problem with no heat and overheating is low coolant, thermostat, or a faulty water pump impeller. The radiator may not be full of liquid coolant, or the pump may not be moving the coolant thru the block.
The Idle Air Control (IAC) valve controls the amount of air that bypasses the throttle valve, which controls the engine idle speed. The IAC valve consists of windings, an armature, a return spring, and a rotary slide. The engine control module (ECM) pulses the voltage to the winding, the opposing forces on the armature by the return spring cause it to maintain a fixed position, adjusting the amount of bypass air to maintain the correct idle speed.
Removal & Installation
Turn the ignition switch to the OFF position.
Disconnect the idle air control valve harness electrical connector.
Using a suitable tool, remove idle air control valve.
Installation is in reverse order of removal.
Checking Idle Air Control Valve (IAC) Triggering
Check IAC valve is electrically OK.
Slide up boot on the IAC valve connector.
Fig. IAC Connector
Use scan tool 'Output Diagnostic Test Mode' function 03 to trigger the IAC valve - N71.
Connect VAG 1527B voltage tester, or equivalent, between terminal 1 and engine ground (GND). Voltage tester must flash.
Connect VAG 1527B voltage tester, or equivalent, between terminal 2 and battery positive voltage (B+). Voltage tester must light up.
If specified results are not obtained:
Connect VAG 1598/19 test box, or equivalent, to ECM harness connector.
Check the wiring between harness and connector terminal 1 (in engine compartment) to test box socket D11 . Then check the wiring between harness connector terminal 2 and test box socket D9 using wiring diagram. Repair as necessary.
If the wiring is okay, but triggering still does not occur, replace the ECM.
If the triggering signal and the wiring are okay and the valve does not react, replace the IAC valve.
Checking Mechanical Function
Remove the idle air control (IAC) valve.
Visually check the surface of the rotary slide (arrow) for signs of wear.
Fig. Showing the IAC Valve
Do NOT check for ease of movement by prying on the rotary slide with a screwdriver or other tools that could cause scratching or other damage.
Reconnect the IAC valve, while still removed, to the harness connector.
Use scan tool 'Output Diagnostic Test Mode' function 03 to trigger IAC valve - N71.
Check whether rotary slide moves freely from stop to stop.
If there are signs of scoring, or if the rotary slide does not move freely in both directions, replace the IAC valve.
If the IAC valve does not respond (not triggered) during the Output Diagnostic Test Mode, check triggering.
Remove the idle air control valve harness connector.
Use a suitable Digital Multimeter (DMM) to measure the resistance between the 2 terminals of the idle air control valve.
The correct measurement should be 7-11 ohms. If not, replace the idle air control valve.
At room temperature, the resistance value will be lower. The resistance value will be higher if the Idle Air Control (IAC) valve is measured at under-hood operating temperatures.
Reconnect the idle air control valve harness connector.
The idle air control valve should hum and vibrate slightly when the ignition is turned 'ON'. If not, the ECM or harness may be defective.
Testing with VAG1551:
Remove the idle air control valve harness connector.
Connect the VAG 15278 LED tester to terminals 1 and 2 of the harness connector using the jumpers from VW 1594 adaptor kit.
Turn ignition ON, perform steps from output DTM (Diagnostic Test Mode) to activate the IAC valve (N71) LED tester must blink.
If tester does not blink, check wiring using VAG 1598 pin out box, electrical checks, step 26. Repair as necessary.
If wiring is okay, replace the ECM.
If LED tester blinks but the IAC valve does not operate, replace the IAC valve.
Sounds to me like you are having a cooling system problem, if water is boiling in the tank or radiator then the engine is overheating. If you have not flushed the cooling system, it would be a good idea to do so. Also you need to test the cooling system at idle and see if the engine will maintain normal temperature, without overheating. Overheating can be caused by plugged or restricted radiator, hoses, or the thermostat, also check the operation of the electric cooling fan. Once you solve the overheating you light should stay off. If you do not solve the overheating soon, the engine will suffer major damage, so check it out very soon. Hope this helps, let me know.
The Temp gauge should read in the middle of the gauge at operating temperature. This is normal. The radiator fan should come on after idling (not driving) for 20 minutes. The fan should also come on when you turn on the AC. Are you experiencing overheat problems?
Please reply if you have overheating issues.