Check engine test coming back p1299. Thermostat replaced last year. Was overheating discovered there was NO coolant in reservoir. Used an entire gallon. Has not overheated since, but concerned as to where the coolant went. No milky substance on cap, and seemingly no smoke coming out of exhaust or apparently leaks. I did notice I was missing a high pressure cap, but was told that wouldn't cause all of it to evaporate. AC is working fine since coolant added.
How long has your 2004 Mercury Grand Marquis had this problem?
less than a week, maybe 3 or so weeks of the ac/heat taking progressively longer to kick on. Solved by coolant
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Hi, this is Bo, an auto expert on RepairPal's Car Genius team!
The mystery of the missing coolant!
As you may already know, code P1299 means your engine went into protection mode due to overheating. If the coolant was empty but no leaks or smoke were obvious, the missing high-pressure cap could be the culprit, letting coolant escape over time. Adding coolant and not overheating is a good sign, but keep an eye on the level and watch for leaks. If the coolant level drops again, you might need a pressure test to find a hidden leak. Let’s get that sorted to keep you cruising smoothly!
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