I have not been pleased with my gas mileage (about 31 mpg for a manual '95 DX - rated for 40 and my girlfriend's brother said that he has queezed 44 out of his when doing everything he can). I also must have a bad coolant sensor, as the gauge sometimes (rarely anymore) gets up to the correct level, but sometimes does not register anything at all (and other times, strangely goes up about a fifth of what it should). Anyway, I saw in another post the following from RonJ: "A cold running engine can cause the engine to run rich. Check for a stuck-open thermostat or a faulty coolant temperature sensor." 1) The sensor does not actually dictate how hot or cold the engine gets does it? I thought that it was a passive instrument. Is the problem possibly that the thermostat is opening too quickly? I've only ever experienced a thermostat opening too late and letting the engine get too hot (in other vehicles). 2) How can I know if my problem is the sensor or the thermostat? Thanks again,

Andy

 

the sensor tells the pcm what to do regaurding the temp cold engines do require more fuel but im not sure wether it being messed up would make it run on open loop constantly but 8mpg difference isnt alot whats wrong withe the sensor why dont you get a new one/

 

Replace the thermostat. It seems to be stuck open. You may even see that it is not fully closed when you remove it.

 

I agree that you should probably just replace the thermostat, mostly because its an easy potential fix...
But would a bad thermostat cause a car to run cold? The point of a thermostat on a car is to help the block warm up faster, and once up to operating temperature, allow the coolant to flow thorough the entire engine. Many people, I am not saying this is a good idea, run their cars without the thermostat in place. Without it, the vehicle will definitely take longer to warm up (and get worse mileage while warming up) but once the vehicle is up to operating temperature, it runs the same as it would with a thermostat.
Correct me if I'm wrong...
I'm leaning more to the temp sensor and its making your car run rich... is the exhaust dark?

 

asechrist: The coolant temperature sending unit provides the temperature signal for the dash gauge, whereas a separate sensor (the coolant temperature sensor) sends coolant temperature measurements to the ECU, which in turn uses this information to determine proper air/fuel ratios. Therefore, if the coolant temperature sensor malfunctions, then the engine may run rich. As a bad coolant temperature sensor typically (not always) throws a CEL code, I would suggest that you may have a bad thermostat, which also fits with the symptoms you describe.

rcherr642z: (1) In very cold weather when driving on the freeway even with a hot engine, the thermostat may need to close to maintain proper engine temperature. If the thermostat remains open, engine temperature may decrease below optimal, causing the engine to run rich. (2) The longer an engine takes to warm up (stuck open thermostat), the longer it will run rich. Obviously, this problem is worsened in the colder weather of winter. Therefore, a stuck open thermostat can substantially reduce gas mileage in the winter. (3) Running an engine without a thermostat will initially make the engine run cold and therefore rich (as mentioned earlier). Interestingly and somewhat surprisingly, once an engine without a thermostat is warm, it becomes prone to overheating -- yes overheating. This is due to the fact that the thermostat also reduces the flow of coolant through the cooling system. Fast moving coolant just doesn't cool the engine as efficiently as does slowly moving coolant. Therefore, never run an engine without a thermostat -- never.

 

Is it reasonable for the gauge to not move up at all after about 20 minutes of highway driving with a bad thermostat?

 

Do you live where it is very cold in the winter? If so, a stuck-open thermostat could significantly delay the engine from getting hot.

 

Yes if your thermostat is bad the gauge may not move up at all on the road, except when sitting still and there is no cooling to the radiator until the temperature comes up enough to turn the fan on. You will also get very little heat from the heater. If the heater seems to be working normally, meaning the engine is actually hot, check the gauge system.

The first thing to do would be to disconnect the wire from the sender (this is the sensor with ONE wire below the distributor), and ground it. The gauge should then go all the way up to hot when you turn the key on. If it does, it's probably the sender, or you had a bad connection at the sender. Sender resistance should be about 100 ohms give or take when warmed up to at normal temperature, and higher with the engine cold. If the gauge doesn't respond to grounding the wire, check the wiring and cluster for problems.

 

My '92 had a faulty thermostat when I bought it. ~150mi drive home on the highway and the temp gauge never even rose 1/4 of the way. Heater well, didn't.