No heat at idle. any ideas??

Alright I've been searching for awhile now and I'm not finding what I'm looking for so here it is:
92 xj 4.0ho 5spd tons of miles ect.
so long as I'm driving and my rpm's are up above say...1500 everything is peachy. my heat blows hot air my coolant temperature stays right at 210. no problems. then I come to a red light or anywhere else that I'm just sitting at idle. My heat turns frigid cold within a minute or so and my temperature starts climbing fast to the "red zone" then as soon as I start moving again the temperature goes back to normal and the heat inside is kickin again.
I assumed it was the thermostat even though I replaced it 6 months ago along with both temp sensors. so I put another one in yesterday and no change.
Oh...one more thing..a week or so before I started having heat problems I ran some amzoil engine (oil) flush through..(which worked great) I dont see how the two could be connected but that was the most recent thing I'd done..
so, any ideas?

it sounds like your water pump is unable to move sufficient coolant at low RPM. You might have a pump with an eroded impeller. If it's not fairly new, I'd probably change it on general principal.

Shorty said:

it sounds like your water pump is unable to move sufficient coolant at low RPM. You might have a pump with an eroded impeller. If it's not fairly new, I'd probably change it on general principal.

Click to expand...

I'm with Shorty on this.

I would also recommend you replace the fan clutch if it hasn't been done already.

Fan clutch is new, and the water pump is a couple years old..I thought about that too but would that affect the heat in the car?

absolutely-- the heater core is farthest away from the pump and has the smallest lines running to it. If the pump moves less coolant at a lower pressure, the heater core will receive the least coolant.

ColoradoDave said:

Fan clutch is new, and the water pump is a couple years old..I thought about that too but would that affect the heat in the car?

Click to expand...

You might suspect a vacuum issue to the airbox EXCEPT for the engine overheating at idle. Engine overheating at idle and Ok driving you would suspect the fan clutch EXCEPT for the lack of heat in the cabin.

All of that brings us back around to the WP--likely a loose impeller or its suffered some erosion.

NEVER buy a reman WP from anybody. Period.

ive had the same problem a couple times. it turned out just to be air trapped in my coolant system and heater core.

jakbob said:

ive had the same problem a couple times. it turned out just to be air trapped in my coolant system and heater core.

Click to expand...

i second this ...if its overheating at idle it is air pocket in cooling system...
I suggest u get a cooling system burping funnel connect to radiator and fill about half full...run engine at about 2000 rpm with heat on for a few mins then idle then repeat..make sure thermostat opens then remove funnel and replace cap.

this should work whether you have the closed or open cooling system...if closed put funnel on overflow bottle..

Other possibilities are Low vacuum from a leaking brake booster at stop lights (foot on the brake), and or or other vacuum leak affecting the damper doors (vacuum actuator), and or the vacuum actuated heater valve, and a loose serpentine belt slipping at idle, or partially clogged radiator, marginal fan clutch....but my bet is a clogging radiator. Have you check the electric fan and when it turns on and off, what temps?