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EricFromCT, Ford Technician
Category: Ford
Satisfied Customers: 3803
Experience:  20 years dealership exp., ASE A1-A9 L1 L2, Ford Lincoln Mercury Senior Master, Diesel Certified
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I have a 98 f150 with a 4.6. It is overheating by the gage,

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I have a '98 f150 with a 4.6. It is overheating by the gage, as soon as I turn the engine off, it only takes 3 to 5 minutes before the gage shows back to normal.I have replaced the thermostat, the coolant sending unit, and both upper and lower radiator hoses. It might happen after driving 20 miles or 50 miles I just don't understand how it cools down so quickly after shutting off the engine.

Hi. I agree that the engine seems to cool down rather quickly. I understand you replaced the coolant temp sending unit. Just to clarify, the engine has a coolant temp sending unit (for gauge) and coolant temp sensor(for pcm), and cylinder head temp sensor(for vehicle failsafe mode).

I first want to verify that you replaced the sender and not the sensor. The SENDER is located on the front of the engine and has wire colors red/white and yellow/red.

The SENSOR is also located on the front of the engine and has wire colors gray/red and light green/red.

Your concern does sound like a faulty sending unit. I have seen problems with the CHT but usually, the vehicle goes into a failsafe mode with limited power if that sensor is the cause.

Please verify correct sensor replacement, then we can go from there.


Thanks, Eric

Customer: replied 7 years ago.


thanks for your response. I changed the other sensor you mentioned and took a test drive. I drove for 30 miles with no problems, temperature ran normal. Then the gage shot straight to hot again. I stopped and turned the engine off and the gage went back to normal. It ran normal for about 2 miles and shot straight back to hot.When I got home the service engine light was on.I read the fault and I got a code P1299 "Manufacture Specific see service manual" I looked in my service manual but didn't see anything that applied to this.

Ok. Well its great that we got a code now. The code indicates that the CHT sensor sensed an overheat condition and went into failsafe mode(see below). As long as the coolant level is ok, I would recommend replacing the CHT sensor. It is located in the left hand cylinder head in the front. Has a 2 wire connector, gray/rd and yellow/light green. Below is a picture of its location.

Hope this solves your issue.


Thanks, Eric



Cylinder Head Temperature (CHT) Sensor C179 (Coordinates F8)


    • Engine cooling system concerns.
    • Base engine concerns.
    • Low engine coolant level or loss of coolant.
  • Diagnostic Trouble Code (DTC) P1299 indicates an engine overheat condition was detected by the cylinder head temperature (CHT) sensor. An FMEM Strategy called Failsafe Cooling was activated to cool the engine. (Refer to Section 1A, Electronic Engine Control (EC) System , for more information on Failsafe Cooling Strategy and the cylinder head temperature sensor.)

    Possible causes:

  • Check engine coolant level.

Is the engine coolant level fill correct?

Yes No
GO to the Powertrain/Engine Group, Engine Cooling System Section in the Workshop Manual for symptom, Engine Overheats. GO to the Powertrain/Engine Group, Engine Cooling System Section in the Workshop Manual for symptom, Loss of Engine Coolant.

Customer: replied 7 years ago.


Thanks for your help, I let my truck idle for 2 hours in the yard last night and drove it for 50 miles this morning. Everything seems normal. Good job!

Ok great! If your satisfied with the result please click the accept button. Feel free to request me with any more questions that you may have.


Thanks, Eric

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