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I have a 2002 Ford Focus.

Whenever the car is idling the heat gauge goes up at an alarming rate and goes the farthest into the red that it can if left alone.

Now if I rev the engine a little the temperature goes down (no matter the gear), I took it to a couple of mechanics and they couldn't figure it out.

What could be causing this, and how would I fix it?

EDIT:

As per Tester101's comment I went to my car and checked some things out, here's what I found.

I let the car sit in park for a couple of minutes, so for around 10 minutes the needle stopped at the midway point, after 10 minutes it started going up and the fan didn't turn on. When the needle was at the highest it can go and I was about to shut the car off I heard the fan go on.

I let it go for 5 more minutes but the needle still didn't go back down from the highest.

Anyway I turned off the car for 2 minutes and when I turned it back on, the needle was just starting to go into the red however the fan was on. Even though the fan was on it still kept getting hotter.

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Does the fan come on when the temperature starts to rise? When the car is not moving, the fan should come on to draw air across the radiator to get rid of the heat. If the fan does not come on, you likely have a bad thermostat or a blown fuse/relay. –  Tester101 Aug 19 '11 at 18:46
    
@Tester101: updated the question –  qwertymk Aug 19 '11 at 19:26
    
Have you checked that it's got enough coolant in both the radiator and the expansion tank when the engine is cold? –  Timo Geusch Aug 19 '11 at 19:34
    
@TimoGeusch: Yes there is enough coolant, also made sure there's no air in the system, it goes up to the midway between max and min or whatever they're labeled –  qwertymk Aug 19 '11 at 19:57
    
I assume that's the expansion tank, did you check at the (radiator) filler neck as well? If you have a leak when the engine is cold, the coolant level in the engine can fall far enough so it can't draw in more fluid from the header tank. –  Timo Geusch Aug 19 '11 at 20:12

2 Answers 2

up vote 4 down vote accepted

Usually coolant temperature problems as you described (without leaks) are caused by any of the following or a combination thereof:

  • Partially blocked or missing ducting to the radiator so either not enough air reaches the radiator or it goes past the radiator instead of through it. This should be relatively easy to eyeball.
  • Thermostat not opening fully (or stuck open, so the motor runs cold). The result would be an engine that does run OK for a while but would get (too) hot when the car is either at standstill or under more load than usual. Unfortunately the only way to really figure out if it's the thermostat is putting a new one - I tend to like OEM ones, and I'd go for the OEM opening temperature as well. Don't bother with a thermostat that opens earlier as this might mask the 'real' fault.
  • Partially blocked radiator. This usually manifests itself under heavier loads (think long uphill stretches with the aircon at full blast). Easiest way to find out is if you 'hover' the palm of your hand over the cooling fins with the engine off and see if you notice areas of the radiator being cooler than others. Don't touch the radiator and don't be tempted to do it with the engine running in case the fans spin up. Better way to do is to use a remote infrared thermometer.
  • Issues with the waterpump not flowing enough coolant.
  • One of the fans not operating correctly. A lot of cars with a/c have two fans, one is often supposed to run when the a/c is on, the other one cycles off and on based on coolant temperature. If the a/c fan doesn't work you're losing a lot of cooling capacity as the a/c evaporator blocks part of the radiator.

I'd check the ducting, and the fan(s) first, then consider replacing the thermostat. Thermostats don't last forever and if there are deposits in the cooling system, this can shorten their life considerably.

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I think it might be 1, 2, or 3, how would I check it out? BTW running the AC/Heat on high doesn't cool the engine off, does this mean anything? –  qwertymk Aug 19 '11 at 21:48
    
a/c on high will be detrimental to engine cooling, but if you turn off the a/c and run the heater at full blast you should definitely see the engine cooling down. –  Timo Geusch Aug 19 '11 at 21:58
    
well running the heat on full blast doesn't help either –  qwertymk Aug 19 '11 at 22:02
    
Just to confirm, you do get heat out of the heater ducts if you have the heater on full with the fan at max? –  Timo Geusch Aug 19 '11 at 22:04
    
Not really, it comes out a little hotter than room temp, vice versa for the AC –  qwertymk Aug 19 '11 at 22:07

Another thing to check is the condition of the radiator - if it is missing a lot of fins or full of gunk, it won't cool as efficiently.

If your heater isn't working either then you definitely have a cooling system issue - I would replace the thermostat as a matter of course, and give the whole cooling system a good flush through (including the rad and the heater matrix) to get rid of the rust, much and other debris that builds up in there. Also check the condition of the water pump, as if it isn't pumping the coolant round then it won't cool anything!

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