Poor gas mileage

I have a 99 SE that runs fine but in the Winter the gas mileage is terrible. There is no performance problem or difference between performance now and during the summer. Engine does struggle to get to operating temp. I replaced the coolant temp sensor. Theoretically, mileage should be better in cold weather (New England) winter (cold air is densor so greater volume fits in cylinder.) but it is very much worse in Winter. Thanks for any thoughts.

A hotter engine is more efficient than a colder engine. Maybe your engine isn't getting up to operating temperature soon enough or not at all. I"d put in a 195 degree thermostat. I"ve found out with my Escort that if I make short trips in cold weather I only get about 25 mpg. What kind of mileage are you getting?

I am getting only about 16 mpg around town. The existing thermostate is working, no idea what the temp rating is on it though.

16 around town is way too low for an Escort. Is the air cleaner element clogged? Do youi keep 32lbs. in the tires? It could be that your timing belt has slipped a notch. That would retard the valve timing and ruin your mileage. Are you getting a CHECK ENGINE light? You should be using 5w-30w oil. Anything higher than that will hurt your mileage. You should be getting 30+ mpg. As bad as the mileage is, I'd take it to the Ford dealer and have him scope it out. You're getting pick up truck mileage out of an Escort. There's something really wrong with your vehicle.

Regarding the theory of better mileage in winter - it's actually the exact opposite. For one, like Davescort said, the warmer the engine (to a point) they more efficient it performs. Around 190F is where it operates at peak efficiency. If it struggles to get up to temperature, it's not the sensor that needs replacing, it's the thermostat. That regulates the flow of hot coolant to the radiator.

And other factors hurting winter mileage include denser air makes for greater parasitic drag (mostly affected on the highway), drop in tire pressure (check every week or two!), and finally the addition of up to 10% ethanol in your gasoline. Ethanol has a lower thermal energy than gasoline, therefore you get reduced mileage on the "winter mix."

Davescort97: Air cleaner is fine, engine runs well, lots of pep (as evidenced by my daughter being clocked at 90 mph in it - that may have hurt mileage but she hasn;t driven it in a while :) so thats not the problem), no check engine light, I check tire pressure regularly. This is the second winter where this has happened, summer mileage is fine which leads me to believe it must be thremostat beccause its not warming up easily. I changed the temp sensor because I know the thremostate is working (just not properly apparently).

Cold air has benefits in terms of HP (hence all the cold air intake mods), I guess mileage does not also benefit.

I will change the thermostat to a 195 degree if I can get a warm day without rain (no garage) and see what happens

I hope you get a nice day that's not too cold to replace the thermostat. My Aunt lives in Franklin NH. She says it really gets cold there this time of the year.

There's an easy way to warm the engine up. Get a piece of cardboard and block off about 2/3rds of the radiator. See what happens then. The engine will run hotter. If your fuel mileage remains the same then don't bother with the thermostat. Last time I checked I was getting 37MPG on the highway. 16MPG is pretty bad :-(

The cold temperatures effect many things about the car. Cold starts use more gas because it takes the car longer to get up to normal operating temperature, cold air is more dense which means the 02 sensor reads more oxygen passing into the engine therefore it dumps more gas, all the other fluids such as transmission are cold and thicker, grease in the wheel bearings is stiffer, winter blend gas which I think they start selling in most areas around the middle of October and sell until about March isn't as good of gas. 16 does sound a little low even given all the variables. A bad 02 sensor can also have an extreme negative effect on fuel mileage. I've had them go bad on my cars before and see as much as 25-35% decrease in mileage. By theory if the 02 sensor is bad you should have a check engine light, but if the 02 sensor is just lazy and not updating the information to the ECU like it should it might not set a CEL. Oil weight is not going to make that much difference in fuel mileage, I use 10w40 year around and even add a bottle of STP oil treatment at every change to my '88 Escort Pony with 503K miles on it and got over 39 MPG mixed city/highway on the last tank. If the car is not getting up to normal temperature the temperature sensor that feeds information to the ECU might be bad causing the car to run in open loop all the time. Your Escort has two temperature sensors, one that feeds the information the the temperature gauge, the other one feeds the information to the ECU. Do you start the car several minutes before starting to drive to let it warm up like lots of people do in the winter? If so you have to stop and realize during that time you are getting 0 MPG which would drop averall mileage considerably. Hope some of this information will be helpful!!

Thanks for the suggestion AZTumbleweed, I wonder if they make a radiator cover for a Scort like they do for the big rigs.

FordMan: I did change the coolant temp sensor - I had no idea there was another sensor. The dense air fooling the 02 sensor is an interesting thought. I have some diagnostic software (AutoEnginuity) but I am not that proficient to use it other than to read codes, plus I dont think it reads all of the Ford sesnro without some additional software. I appreciate the suggestions and will give them a try.

I think the other temp sensor is for the gage.

Does your engine ever reach normal temp? How long does it take to get there?
How do you know that your thermostat is working properly (stuck open), or that coolant is not getting around it?
When the car is warmed up does the upper radiator hose feel hot?

In older models there was a bypass valve in the thermostat housing. If that valve is allowing coolant around the thermostat then that would explain your trouble warming up. I don't know if the 99 has the same type of valve though.

Hehehehe... ZZ may have a point. Maybe the 90mph daughter is borrowing the car without your knowledge. ;)

I too notice that frigid temp mileage (20 Fahrenheit) is much, much worse than in warmer, 40+ Fahrenheit temps.

I have an older model scort ('94/95) 1.9L SEFI ~240k and have intentionally not fixed the thermostat bypass valve, allowing it to run cold. (reason being to delay or prevent a second head gasket failure)

But even with the well below normal engine operating temp, extended idling periods, and poor, slippery, hilly-terrain, 10-30mph road conditions, I don't think the mileage is that bad.