strange CE light on long trips.. heat issue?

[smak]

i've recently gone on a few long road trips with my gen2 mtx sho.. after about 5 hours of straight driving at moderate speeds (holding maybe 2.5-3k rpm steady) the CE light turns on.

the temp guage holds steady below/mid normal throughout, but does sometimes float around in this range (doesent steadily rise or fall often, usually skips up or down then stays there for a while, as the tstat opens i assume)

when i get off the hughway and give the car a rest she is okay for a while longer.. checking the oil during these breaks it seems unusually hot, and the stereo head unit i've got installed gets really hot also.. rad is full and overflow bottle has some fluid in it also. just gave an oil change using 10w30. the outside temps during these trips have been well below freezing also fyi.

do you think the dummy guage is toast and giving me an inaccurate normal reading? should i just go in and replace the t-stat even though it seems to be working according to the way the dummy guage fluxuates?

thought i'd check in with y'all before i go and run codes. anyone seen this problem arise in the past?

 

[sdpatt]

Don't take any action until you read the codes and find out what the problem really is. My guess is an O2 sensor not switching, but please don't take my WAG. Read the codes.

 

[Bizzy]

Ditto what Scott said on both.

 

[rangerj]

The fluctuating temperature guage could be due to a couple of things. First check the connection at the temperature SENDING unit, not the temperature sensor. It is a single wire. A poor connection and/or momentary grounding will cause the gauge to go up toward hot.

Check the radiator cap. If it is worn, either the seal or the spring tension, it will not hold the required pressure. The pressure raises the boiling point of the fluid. If the fluid boils the hot gas will spike the temperature until it is "burped" into the overflow tank. As SDPatt has suggested an O-2 sensor may not be "switching" (from rich to lean).

If an O-2 sensor is stuck in the rich position the computer will adjust the fuel to lean out the mixture. Running lean will cause a rise in operating temperature. O-2 sensores frequently fail slowly and they slow down the "switching". So yours could be sticking on rich for a while then switching to lean for a while etc. If the sensor is switching slowly, or has stopped switching all together, it should be replaced.

Lastly, check the hoses and connections, including the hose from the radiator cap to the overflow bottle. A loose hose will allow air to be sucked into the system when the car cools down as the fluid contracts and causes a vacuum in the cooling system. Again air in the systen will cause hot spots. When the hot gas gets to the temperature sending unit the temp gauge spikes a temperature.

Just some thoughts!

As SDPatt said "don't take my WAG", check the codes and check the components of your cooling system. rangerj