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I have an 01 nissan sentra which had been running hot. If i start the car in the morning and turn temp control to hot and fan on and let the car warm up, i will have no heat and the car runs hot. Just starting up and going in the morning i will have heat but when idling at a signal the air turns cool and the car begins to run hot again. I recently had a power flush and thermosat changed about 45 days ago when these problems started. Car ran fine until last week when these issues started again. Mechanic said water pump was fine but maybe it was on its last legs. My main machanic wants to replace my radiator which he will do tomorrow. Any other suggestions before that would be appreciated.

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Any updates? Did you end up letting your mechanic replace the radiator? –  Mark Johnson Dec 14 '12 at 2:59
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2 Answers

I used to have a car that would disengage the AC compressor when the engine RPM was low. This caused the AC to work fine when riding down the road, but the air would almost immediately get warm whenever I was idling in traffic.

This sounds similar to what you are experiencing, so I'm thinking your Sentra may be using the AC compressor for the heater. This will generally happen if you have the HEAT set to Defrost, but not if you have it to Floor. Newer, high end cars seem to use the AC compressor anytime the fan is turned on. (This seems wasteful to me, but I guess you buy a luxury car you expect plush all the time.)

If this is the case (your HEAT is set to Defrost or Vent that is using the AC compressor), then you have a 2 fold issue:

  • One is your AC compressor is acting up. This is no big deal, but it would cause your car to stop pulling heat from the engine when the engine RPMs are low. This could possibly be fixed with a new AC compressor clutch. I'm not sure.

  • The other is the engine heat issue. Since you say you have already flushed the radiator and installed a new thermostat, I would be looking out for either a crack in the engine head or a blown head gasket. This would cause heat from the combustion cycle to go straight to the coolant instead of blowing out the exhaust, and your radiator can not pull that amount of heat out of the engine.

Just things to check.

I don't really think it would be the radiator, unless it has been patched before and has several dead circuits in it.

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Hopefully your mechanic would pressure test the water pump before replacing it to determine if it's really bad.

Having no heat is usually a symptom of a stuck open thermostat, but if that was the case, your temp gauge would not indicate hot...maybe a clogged heater core? Did your heater core get flushed? That wouldn't explain the temp gauge reading hot, though.

Any coolant leaks that are maybe coming from the pump? Is the coolant level at the correct level? Do you have the right mix of water and antifreeze? Was the correct thermostat installed? You might have air trapped in the system. Did anybody bleed the coolant system after the flush to get rid of any trapped air?

I don't understand why your mechanic suspects the radiator, did he explain his reasoning?

If you find the line going into your heater core under the hood, you can feel if it's hot after the engine has warmed up. If it is, but you don't have any heat, maybe the vent flaps under the dash have some kind of intermittent problem? Can you hear them actuating as you change the climate control settings (from heat to vent, floor to dash, vary the temperature)? There might also be a vacuum or electrically activated valve on your heater lines that could be malfunctioning. No coolant flow in the core would explain the lack of heat.

I'd make sure the system was bled and maybe bleed it again. If your temp is still running hot at that point I would think the water pump would start to become a suspect. The heater problem may or may not be the same problem. Check for a valve on the heater hoses.

By now, you've probably got it back from the shop. Any updates?

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