My temp was spiking to red from normal run temp in an instant. This would happen at a traffic light or running on the interstate. I knew my low speed fan was out for quite sometime but interstate driving shouldn't involve low speed fan.

I replaced the thermostat, installed the work around resistor to fix the low speed, and replaced coolant. Both fans now work fine when I test them with the a/c turned on.

Here's the issue. Without turning on the a/c it seems neither fan will energize. I checked fuses and swapped relays, all seem fine.
With the ECU in the readout mode the water will get to 105C and the expansion tank Will boil over every time. (Temp gauger still shows normal).At 105C the high speed fan has not turned on yet.

What are the run temps required to trigger the fans? Any suggestions why the tank boils before the fan. Maybe a bad expansion cap????

 

The fan should kick on at low speed when the engine reaches a temperature of 105 c. (221 F.) When the temperature drops to 101 C.(214 F.) the fan should shut off.

The fan kicks to high speed at 112 C.(234 F.) and remains on until the engine temperature drops by 4 C. (7 F.) at which point it should revert to low speed.
The low fan operates when the A/C is on.

Data above comes from Bentley.

I run a Scangauge and can actually see the point when my thermostat opens, usually around 198 - 200 F. and drops to around 190 F. Maybe you still have a thermostat problem.....hope this info. answers some of your questions a good luck,

 

Are you certain you have bled all the air out of the system? If you can find a slight incline and place the radiator at the top you might be able to get the last bit of air out of the system if that is the problem.

 

The pressure in the system should prevent it from boiling at 100C. Maybe a bad pressure cap on the expansion tank?

Also, is this an S? If so, remember there's a small bleed screw down by the thermostat housing in addition to the one on the upper radiator hose. I forgot about it the first time I changed my coolant and temps started rising (although the high speed fan was definitely screaming). There's a small amount of air that needs to be bled out to prevent the thermostat from being air locked.

 

Update to my post. This afternoon I put a new pressure cap on the expansion tank. Car came up to 105c without boiling over. So far it looks as though it was a cap and not the thermostat. Also low speed fan kicked on right about 105-106 and immediately started bringing the temp back down.

I suspect that I still could have air in the system as I didn't bleed the valve by the thermostat. However, so far so good.

BTW In one of the fan resistor forums someone posted all of the readings you can get when you put the ECU in the test mode. If anyone has the link to that please repost the link. It was very helpful but now I can't find it when I search the forums.