A defective thermostat.
It could be the thermostat on the engine. If the engine does not warm up adequately the heater will never blow hot air.
catalytic converter
The most likely cause of a long warm up time would be that the thermostat has jammed in the open position
Maybe a little bit since it is a bit more coolant to warm up, but check your thermostat it may not be closing as it should and would be a definite cause for slow warm up and might also cause it to cool off a lot when you are traveling fast in cold weather.
in most cases your engine thermostat is stuck oped not allowing your engine to warm up fully. a new thermostat and gasket is usually under $10.00
there is a little CPU chip which is smaller than your hand, the CPU chip is a computer it revs the car to warm up the engine, if you live in a cold place that is most likely cause
not typically , extreme lower temps won't allow the engine to move quickly = it is better to warm the engine some 2-3 minutes minimum. the cracks could be from frozen water (not enough antifreeze) in the coolant passages .
Are you sure warm has anything to do with it? I thought my mx-3 getting warm then bogging down and it ended up being that the catalytic converter had fallen apart and was restricting the exhaust.
No
The most common cause of engine lockup is a lack of oil. Also, an engine overheating from lack of coolant can cause an engine to lock up.
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No removing the thermostat would cause your engine not to warm up as quickly as it should. Your water pump may not be doing its job correctly.