Edmunds Answers

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  • avatar MrShift@Edmunds 10/04/10 11:26 am PST

    Okay, then, to convince yourself why not hot wire the fan directly so that it runs all the time, and then see if the car overheats? So by-pass the sensor in other words.

Answers

  • MrShift@Edmunds 10/03/10 8:13 pm PST

    If it overheats very quickly, and if it overheats while you are moving through the air, then it's not your fan circuit.


    This really leaves two possbilities. One would be a stuck thermostat, which can cause a pretty fast overheat, and the other would be a blown head gasket.

    There should be no mystery about this. Both a stuck thermostat and a bad head gasket are easily testable, so there needn't be any guesswork.

    If you can get the thermostat out yourself, you can test it by placing it in a pan of hot water and watch it open. If it doesn't open, well there you go.

    For the head gasket, the best test is to pressurize the cooling system with a pressure pump, leave it pressurized and then extract the spark plugs and look for coolant on the tips of the plugs, or use a bore-scope to see coolant in the cylinders.

    Sometimes the head gasket will not fail in this manner, but instead either a bad gasket or a cracked head will force combustion gases into the coolant.

    You can buy a chemical that tests for combustion gases in your coolant.

    your mechanic should have been able to demonstrate to you scientifically how he knows the head gasket is bad.


  • jag2856 10/03/10 10:09 pm PST

    Mr Shiftright, Thanks for the reply. Here is more information about the problem. The engine also overheats wile idleing in park (no air movement across the radiator). When I start the engine cold, the temp gauge goes to about 1/3 mark in about 10 minutes and holds there for a while. Then in another 10 or 15 minutes, the temp starts climbing and goes to the red zone. The cooling fans never come on while the engine is running, but when the temp gage overheats, I can bring it down by turning the heater on full blast. This is why I thought the fans are the problem. If the heater fan can bring the temp down, both thecooling fans should be able to do a lot better. When I turn the key off, the one cooling fan comes on for a while. Why does neither fan come on with the engine hot & running, but one fan comes on when I turn the engine off?

  • MrShift@Edmunds 10/03/10 10:13 pm PST

    Okay, that is clearer. But anyway, let's back up. Upon what basis did your mechanic tell you the head gasket was blown? I mean, what evidence...or did he just raise the hood, say headgasket, and leave?



  • jag2856 10/03/10 11:37 pm PST

    I didn't think about asking that at the time, because I definitely don't want to sink $1,500 to $2,000 into a car that old. If I fixed the head gasket and it didn't cure the overheating, I didn't want to start spending more money on water pumps, etc. If it is the head gasket, I'll just get rid of the car.

    I think that the fans should come on when the engine is running hot. I'm trying to see if there is a possibility of a simpler and less expensive solution.

    If I can't get the car running for a couple hundred bucks or so, I'd rather just spend $2,000 on a down payment for a new car.

  • zaken1 10/04/10 12:29 am PST

    There is a radiator fan control thermal switch located on either the water outlet or the thermostat housing. There are different models of fan switches used on this vehicle; depending on whether the car has a temperature gauge or a light; and whether the switch is mounted on the water outlet or the thermostat housing. This part triggers the fan relay, and would keep the fan from running if it failed. These thermal switches are notoriously unreliable, and I would replace it before condemning the engine. As you observed; the fan should work when the motor heats up.

  • jag2856 10/04/10 1:21 am PST

    The puzzling thing is that the fan runs, when I turn the engine off. If the fan thermal switch is bad, wouldn't that prevent the fan from runing, when you turn the engine off?

    Because the cooling fan does run (when the engine is switched off), it seems that the fan motor, wiring, relays, and switches are all good. The problem is that the fan won't run while the engine is on, it fires up only when i switch off the engine.

    I did unplug and replug the fans when I changed the radiator, but i don't think I could have reversed any wiring, because the plugs only fit together one way.

    I saw a wiring diagram, showing all the switches and relays and they all lead to a "Fan Control Module" which I assume gives the signal to the relay to run the fan. The Chilton's manual doesn't say much about this component. Is it replaceable?

  • zaken1 10/04/10 11:20 am PST

    Unfortunately, I have no information about the fan control module. That sounds like a dealer only item. Call a dealership parts department to get prices and availability. I will say that the cool down function on engine shutdown is done by the module (which apparently works, and is independent of the thermal fan switch) For that reason, I would not suspect the fan control module. This simply sounds like a defective fan switch.

  • MrShift@Edmunds 10/04/10 11:26 am PST

    Okay, then, to convince yourself why not hot wire the fan directly so that it runs all the time, and then see if the car overheats? So by-pass the sensor in other words.

  • jag2856 10/04/10 11:25 pm PST

    OK, I unplugged the thermal switch near the thermostat and jumpered the wires. Both fans came on, so the fan problem was a bad fan switch. I tried driving the car to see if it would still overheat. It lasted a little longer with the fans running, but within about 10 minutes the temperature started climbing. Turning the heater on helped control the temperature, but it's still running hot. I guess now I'm convinced that a bad head gasket is the likely cause of the overheating. I'll try another mechanic, to see if I can get it changed out for less than the $1,500 quoted before.

    Thanks for all your suggestions!

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