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Tim West
Tim West, ASE Certified Technician
Category: Car
Satisfied Customers: 4638
Experience:  ASE Certified Service Manager for Bridgstone/ Firestone
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My car is idling the engine temp..gauge..thermostate

Resolved Question:

When my car is idling the engine temp. gauge will go up to 3/4 from H hot but when i excellerate it drops down to almost C cold, is this most likely the thermostate or the fan?
Submitted: 7 years ago via Advance Auto Parts.
Category: Car
Expert:  Tim West replied 7 years ago.
Thanks for visiting Just Answer,
It sounds like the fan isn't working correctly.
When the vehicle is getting warm feel the lower radiator hose. If the lower radiator hose is cold, the thermostat could be stuck closed. Replace the thermostat if needed.
Next verify the fann is working. Hi speed, low speed and with the a/c on.
  1. Cooling fan motors will run at low speed if:
    1. The engine temperature is higher than normal. Cooling fan motors start running at 102°C (216°F) and stops running at 100°C (212°F) .
    2. The A/C is on and the vehicle speed does not provide enough natural airflow. The cooling fan motors will stop running at speeds over 61 km/h (38 mph) if coolant temperature is below 100°C (212°F) and A/C head pressure is below 250 psi . NOTE: The cooling fan motors will not cycle with the A/C clutch. The cooling fan motors will come on when the A/C switch is turned ON.
  1. The cooling fan motors will run at high speed if:
    1. The engine temperature is higher than desirable and cooling fan motors have been operating at a low speed. The cooling fan motors start running at high speed when the coolant temperature exceeds 109°C (228°F) and return to low speed when coolant temperature drops to 106°C (222°F) .
    2. A Wide-Open Throttle (WOT) A/C cutout is used during hard acceleration. A signal is sent to the powertrain control module which then signals the CCRM to cut off power to the A/C clutch field coil. This keeps the engine from being overloaded.

How is the fan acting? Can you let me know?
Customer: replied 7 years ago.
well I'm not rally sure all i know is that if I do not have the fan at it's highest ( when the a/c is on then it barely blows any air when I switch it to the third speed it drops dramatically. is it possible that it needs a new motor?
Expert:  Tim West replied 7 years ago.
Well that fan is a different fan than what keeps the engine cool, so this would be 2 different problems, The fan I am talking about it in front of the radiator.
Check that fan and let me know. The fans should turn on when the a/c is on and as the vehicle gets warm.
We will diagnose the blower motor later on. Let's fix the overheating first .
Customer: replied 7 years ago.
yes the fan does go high and low as needed i took it around the block to get the engine hot and sat and listened and as the gauge rose the fan kicked up but the needle has a tendency to jump and bounce.
Expert:  Tim West replied 7 years ago.
Thanks, XXXXX XXXXX fans are working ok normal overheating diagnostics would apply.
When is the last time you have changed out the coolatnt? If it's been a while it might be time to replace the thermostat and flush out the system.
The thermostat is designed is spring loaded and opens when a certain temperautre is met. IF the thermostat is sticking it can open late or not at all causing an overheating problem.
Old coolant can rust in the cooling system and damage cooling system componets.
Try this next, if the vehicle still overheats we will look at a possible water pump, radiator and then head gasket possibilities. Usually it doesn't get that far.


1. Drain cooling system below level of upper radiator hose.
2. Remove upper radiator hose, then the retaining bolts from thermostat housing.
3. Remove housing and thermostat as an assembly, discarding old gasket. Clean sealing surfaces with gasket scraper. Ensure not to gouge aluminum surfaces as these gouges may form leaks.


1. Install thermostat into housing, ensuring jiggle valve is up in relation to housing.
2. Position gasket onto housing using bolts as holding device, then install housing assembly and retaining bolts.
3. Install upper radiator hose.
4. Fill and bleed cooling system with recommended amount and mixture.
5. Start engine and inspect for leaks.
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