runs hot

It's a 1990 Mercury Grand Marquis 5.0 EFI.
Changed the radiator, thermostat, water pump, fan clutch and all hoses.
Normal highway driving the car runs cool but as soon as the traffic
becomes bumper to bumper and/or stop and go, the temp shoots right up. Not overheating but the needle shoots to just below the line before the "H" and if I don't get back to a long run drive on the highway the needle stays up there.
Also, it does not lose one drop of coolant no matter how hot the car gets. Is there something else that I missed changing? The all around running of the car is very good except for this.
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Sounds like the temperature sensor is defective.
-- javatek?@usa.com
remove the ? to reply.

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Measure the resistance of the temp gauge sender hot. Should be about 9.7 ohms hot.

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Did you have the coolant changed just before this happened?
Buy a cheap tester and find out what the mix is.
Thomas Moats opined

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says...

New doesn't necessarily mean good. Learned that lesson many times. Start the car and get it up to operating temperature. Stop engine. Now spin the fan. How many times does it go around. Another test is to put a piece of masking tape on the fan, tying it to a water hose or whatever is handy. Use 3/4" wide plain Jane masking tape, near the tip of a blade. Now start it. The tape should break. If it pulls off, repeat and use a longer piece maybe wrapped around the fan and hose on each end. You only want one piece holding the blade still. If the motor runs without breaking the tape, the clutch is bad.
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Try the obvious. Replace the radiator cap. You'd be surprised.
wrongdoi wrote:

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This may be quite normal, when the car is not moving and idling there is very little air passing over the rad. When sitting in a traffic jam try putting car in neutral and rev the engine just a little, this will pass more air over the rad and cool the engine.

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another thing to do if ya stuck in traficck and realy need to cool your car off roll down all your windows and turn the defrost on to heat an turn it full blast.. may be un confortable in hte car but it wil help to cool it down.
Tom Kirkpatrick wrote:

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Running the defroster on cars equipped with air conditioners is going to make the air conditioner run by default, producing more heat in the engine compartment.
Bill

up.
the
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The weather has cooled off here quite a bit but the temp. gauge still runs upward during stop & go traffic just fewer times.
Ironically I had put a new radiator cap on a while ago but I did save the old one. I replaced the new with the old since this problem has not happened in the far past and am waiting, and will be glad, to be surprised.
Thanks for the reply.

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