nissan heater question.
 

first post here, but not my first time working on stuff.
i am diesel mechanic, so i am a little lost on these small cars.


the kids across the street have 2 nissans.
03 sentra 4 cylinder and a 98 maxima V6.
both of these cars only have heat when driving at speed.
i flushed the coolant systems on both, and the crud that came out resembled red mud. i think it was the original coolant in both.
neither overheats, but blow cold at idle even when hot. you come off the highway and the heat goes to cold air after sitting idling 5 minutes.
both have new t-stats installed with the coolant flush.

i am thinking the impellers on the water pumps have rotted down enough to not push enough flow to supply the heater at idle, but will flow enough at speed because the pump is spinning faster.

ideas??

Possibilities:
Sounds like you narrowed the problem down to one possibility