Temperature fluctuation - Ford Mustang Forum
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post #1 of 4 (permalink) Old 01-20-2020 Thread Starter
khatx's Avatar
Joined: Apr 2019
Posts: 1
Temperature fluctuation

2010 4.0 AT Coupe, thermostat housing sprung a leak at approx 100,000 so I ordered a metal replacement which included a thermostat and temp sensor. Installed, got the air out of the system, car ran fine for a couple of weeks. Heater was blowing very well, cold to hot. Now it'll run a few minutes, temperature warning light comes on, temp shoots up (not gradually but instantly) for C to the mid point on the gauge. The temp gauge will then drop back to C very quickly, then temp waring light comes back on, and the process repeats. At this point, the heater is blowing luke warm. I've parked it until I can figure this out. My first guess is they put a cheap thermostat in the housing to keep the price down.

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post #2 of 4 (permalink) Old 01-20-2020
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Definitely the thermostat...cheap, well, maybe not...not unusual for a thermostat to get stuck but they are suppose to fail in the open position....you might have some 'gunk' that flowed into the thermostat causing it to stick closed.

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post #3 of 4 (permalink) Old 01-20-2020
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or maybe an air bubble?

if the thermostat was stuck, I don't understand why it would seem to be opening and closing

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post #4 of 4 (permalink) Old 01-20-2020
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Wild swings in temp is an air pocket or a small possibility of a bad sender. Check your fluid levels.
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