When I drive around the city and what-not, the engine heats up to normal operating temperature, and the fan turns on correctly to cool the radiator when I'm not moving.

However... at freeway speeds the engine temp will read 0, and during my hour-and-a-half drive to school, the "Service Engine Soon" light will turn on and off every 15 minutes or so. I checked the thermostat, and it wasn't stuck open or broken... could it be my water pump?

No, your thermostat is bad. The spring gets weak and at high speed the coolant flow floats it open so it can't control the temp. Replace it with a good brand,new one and you should be ok. I've had this happen on many cars. Sure kills your gas mileage too.
Humm interesting it alway happens on the GM's, the Chryslers never do it...

Think about your statement. If the pump was bad you would not be cooling enough and would be overheating. You have a good pump!

[This message has been edited by Dodgerunner (edited 04-09-2007).]

Also, I suspect your temp sensor/gauge isn't all that acurate any more. It's not really possible for the engine to get that cold while running and driving in north america.

quote Originally posted by Jax184: Also, I suspect your temp sensor/gauge isn't all that acurate any more. It's not really possible for the engine to get that cold while running and driving in north america.

True!......True!

yes, the connector for the temp gauge is VERY brittle. its the sending unit right near the coil & dist. bolted in a hole in the head. (on a V6 - dunno on the 4)

quote Originally posted by Enix: However... at freeway speeds the engine temp will read 0, and during my hour-and-a-half drive to school,

By "0" do you mean all the way to the left? It's not zero degrees F of course. My 2.8 V8 will indicate very close to the left most hash mark when I'm driving. That's completely normal, (desired even). But if your check engine light is coming on, it probably indicates a faulty thermostat

On a properly functioning temp gauge, doesn't the needle sit near the middle at operating temp? If so, then you Don't want it to be nearly pegged cold. Engines have thermostats for a reason. Running them too cold will cut into gas mileage, without giving you anything in return.

thanks guys, i'll swap out the thermostat and take a look at the temp gauge and it's connections/sensors just in case. and out of curiosity, the last Fiero GT i owned had a similar problem... but every time i would replace the thermostat, it would destroy and mangle it inside its' little housing. literally after a 20 minute drive on a new thermostat the problem would continue, so i'd bust open the therm cap, and the therm arm/handle was bent to nothing and the whole thing was floating around in there. what could that be??

quote Originally posted by Jax184: On a properly functioning temp gauge, doesn't the needle sit near the middle at operating temp? If so, then you Don't want it to be nearly pegged cold. Engines have thermostats for a reason. Running them too cold will cut into gas mileage, without giving you anything in return.

I've never driven a new Fiero, so I don't know for sure, but my low miles GT has always run very close the the first mark. Being here in Texas where it's summer 8 months out of the year, realize that means the a/c is running and the fan is on. With the fan on the engine temp is going to be on the low side. With the fan switch controlling, it's going to run hotter. I am happy that I don't have to deal with overheating.

My 88 I checked when I did the pegging fix and made sure the gauge was cal'd correctly. It was and reads right at 200* just to the left of center. My 85gt I have not fixed yet, it currently reads about the 1/4th mark at 200* plan on seeing if I can adjust it.

quote Originally posted by jscott1: With the fan on the engine temp is going to be on the low side.

Again, the thermostat sets the lower engine temp. You can blow on the radiator with -20 degree air and the engine shouldn't get any colder than ~195 degrees. I suspect that years of pegging when the car is turned on has thrown your temp gauge needle off. Not really a bad thing, as long as you know where "normal" is, but the next step is for the needle to fall off entirely.

I went through 3 thermostats until I caught on and bought the expensive variety with better metal in the chassis.

Since I did that, no more bent thermostats.

Arn

quote Originally posted by Jax184: Again, the thermostat sets the lower engine temp. You can blow on the radiator with -20 degree air and the engine shouldn't get any colder than ~195 degrees. I suspect that years of pegging when the car is turned on has thrown your temp gauge needle off. Not really a bad thing, as long as you know where "normal" is, but the next step is for the needle to fall off entirely.

I don't disagree with your point, but you are missing my point. The Thermostat will open at at 195 degrees, but if the cooling system isn't providing enough cooling, like if the fan is off and the car isn't moving, the car will run hotter than 195. Considerably hotter, until the fan switch turns on the fan like at 235F.

If you turn off the A/C then the lower set limit of the engine is set by the fan switch and not the thermostat so it will run hotter.

By the way, I'm not running stock gauges, I don't have the pegging needle issue, (didn't hook them up that way) and they are completely accurrate.

[This message has been edited by jscott1 (edited 04-10-2007).]

quote Originally posted by Jax184: Again, the thermostat sets the lower engine temp. You can blow on the radiator with -20 degree air and the engine shouldn't get any colder than ~195 degrees. I suspect that years of pegging when the car is turned on has thrown your temp gauge needle off. Not really a bad thing, as long as you know where "normal" is, but the next step is for the needle to fall off entirely.

Your question made me consider two things... 1) it assumes I have a 195 degree thermostat, and 2) that I have stock gauges.

I honestly never checked the thermostat before and I just found out I have a 160 degree thermostat. That's one reason for it to run cool, and second, 160 is the lower limit on my gauges...

Heres my hair brained idea,if you havent added atifreeze in a while and you live in an are wear the weather is cold,then the water in the radiator may freeze over night takeing alot longer for the the car to warm up.Allso your car may run cooler at freeway speeeds because air is constantly going through the radiator,regardless if the fan is on on or not.Just my hair brained 2 cents (brain fart,ha,ha).

quote Originally posted by jscott1: Your question made me consider two things... 1) it assumes I have a 195 degree thermostat, and 2) that I have stock gauges. I honestly never checked the thermostat before and I just found out I have a 160 degree thermostat. That's one reason for it to run cool, and second, 160 is the lower limit on my gauges...

It's probably safe to assume though that Enix is using a stock setup, so the temp should read somewhere from 1/4 to 1/2 of his gauge, not all the way to the left. My Formula will range from just left of the 1/4 mark, to just about the 1/2 mark. With the fan on it stays at about 1/4. My old 4cyl's could maintain the 1/4 mark no problem though, didn't seem to generate as much heat.

Enix:
With the engine cold, like when you first go to it in the morning, what temperature does your gauge read when you turn the key to "Run"? Typically it will peg all the way to the right, then it should sit at 100, the leftmost reading. If it sits somewhere else the needle may be off.

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[This message has been edited by Xanth (edited 04-24-2007).]