2008 Grand Marquis temperature gauge
 

Temperature gauge warning light and needle spike at travel speeds over 70 mph on interstate. Repair scan tools from two different mechanics show no problem codes. I have already installed new thermostat & serviced fluid. Step 2: replaced fan control module. Problem still occurs. I cannot locate a coolant temperature sensor/sender/switch. Diagrams from internet do not match 4.6 L engine I see under the hood.

YO, 1572 !!
My FOMOCO 4.6-liter engine has two (2) temperature senders in the Intake Manifold. 1 is for the ECM. and the other 1 is for the Temperature Guage.
Probably get good ones from NAPA Auto Parts. Avoid cheap parts for best results.
1995 Grand Marquis Restoration - Mercury Forum - Mercury Enthusiasts Forums (1)

Myself, I would connect a scan tool to display live data to actually see the engine temperature at the cruising speed and compare that with your dash gauge just to make sure the engine isn't running hot. A code may not be set for your problem. Your symptom does not sound like a sender unit problem since your issue only acts up at high speeds but more like a water flow issue.Senders usually act up all the time. I would want to make sure by using the scan tool.