Slow warm-up, lack of heat- 2004 Mazda MPV

My 2004 Mazda MPV with just over 200,000 miles, started experiencing slow warm-up (during warm weather) approximately 5,000 miles ago. Shortly thereafter, I noticed slowly increasing coolant loss. The coolant loss was diagnosed as a cracked radiator, and, while a new radiator was being installed and the system flushed, I had the original thermostat replaced at the same time. The repairs cured the coolant loss problem, and for a short while the car seemed to warm up normally. However, once the weather turned cold, I started experiencing very slow engine warm-up and also have a minimal amount of heat, even after the temp gauge indicates that the engine is up to operating temperature. The rear heater is experiencing the same minimal amount of heat even after the engine is up to temperature.
Any ideas as to what the problem(s?) might be?

Was the thermostat replaced with another OEM (from Mazda) unit?
Many aftermarket thermostats are ill-suited junk.

Not sure- I’ll check with the shop that did the work tomorrow morning.
And, aftermarket parts can be problematic…

Also, there could be air in the heater core. The system may need to be bled.

I’m beginning to suspect the system was not bled properly when the radiator and T-stat were replaced. Also, I just discovered that the coolant shut-off valve to the heater cores was partially closed. I’ll know better tomorrow morning if fully-opening it had an impact

My guess is that the system was bled without the heat having been turned on, leaving an air block in the heater core.

However, you might want to check that the cooling fans aren’t running constantly. You should have one that turns on when the AC is on and one that only turns on when the engine reaches operating temperature. If with the AC off and the engine cold you have a fan running, you need a new temp sensor.

There was air in the system. I’m not sure if it all is gone, but both issues are definitely better today. I’ll bleed the system again properly over the weekend just to be sure. Apparently there was a design issue with the 03 & 04 models that had the rear HVAC unit, so they added a control valve at the firewall to assist in bleeding the rear unit. Mazda issued a service bulletin- US Bulletin No. 07-004/04. Applies to 2003-2004 MPV Dated 08/2004

Thanks for the help.

Thanks for posting the resolution. TSM had it spot on.