I recently got a 2000 Grand Marquis with around 70,000 miles owned by an older guy who couldn't drive anymore. It would take a while for the engine to get to operating temperature so I assumed the thermostat was the problem. When I checked the coolant, it was extremely dirty and had rust particles, so I flushed out the system (without any cleaner, just ran a hose through block and radiator and then a couple cycles with distilled water until it came out clear). I also replaced the thermostat with an OEM motorcraft. Now when I first do a cold start, it takes about 10 minutes for the engine to get to operating temp, so I'm wondering if it's a faulty thermostat or if there could be another problem? Another issue is that now whenever I come to a stop and then accelerate I can hear water sloshing around in the dash. I'm guessing it's just air trapped in the heater core and just needs to be bled a little more?
Definitely could be air still in the system or another bad thermostat. Is it 10 minutes of driving where it's not heating up or 10 minutes of sitting still? Because to be honest, especially if you have any sort of HVAC on, 10 minutes to warm up standing still is not too bad. If on the other hand, it's while driving, there could definitely be an issue.
No just idling. It's behaving just like before, where it stays cold while idling, and then I'll start driving and after about 5 minutes it gets to normal temp.