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Bad gas mileage in the cold

My 91 Subaru Loyale is killing me!

When it gets cold, and it is cold in Colorado 9 months a year, my gas mileage drops from 30mpg to 22mpg. It is costing me hundreds of dollars a year!

It is NOT my a/c - yes, I get 22mpg when I run the a/c - but the a/c is disconnected to take it out of the equation. I also lose no power in the cold.

My o2 (Oxygen) sensor is disconnected. It is bad (Engine light is on) but it is fused into the exhaust and I can’t extract it.

One mechanic said without an o2 sensor my computer is not able to determine how much gas to squirt in the injectors. So why would it squirt 25% more in the cold?

This same mechanic suggested cutting a new hole in the exhaust and putting a new o2 sensor in.

Is this the right solution, and what should it cost (he says $150 labor!)?

The computer cannot meter fuel correctly without the O2 sensor operating as designed. Maybe a good mechanic can extract your bad O2 and replace it without any welding but welding in a new O2 base is somewhat common on today’s automobiles.

The O2 sensor is very important. It tells the ECM how rich or lean the engine is running, so it can fine-tune the fuel mix. Without it, the ECM runs in ‘open-loop’ mode, in which it simply uses the metered air and air temp to guess at the required fuel, and it tends to guess on the rich side to avoid expensive engine damage due to leaning out. You will not see 30 mpg again until the O2 sensor is fixed.

It is natural to get worse mileage during cold weather. Warming up the engine takes longer, and there is lots more friction in the drive train until everything is warmed up.

However, I agree with the others that on a modern car all the sensors have to be working properly to get the best mileage. So you should get it fixed properly.

A colder engine will also use more fuel just because it’s not running as warm.

Yes, replace the oxygen sensor. Either have your mechanic remove it, repair the threads and install a new one, or have him install a new 02 sensor bung near the old one. Either way, anything you do to try to improve mileage will be useless until the oxygen sensor is functional.

Has the thermostat ever been changed?
They’re cheap and I’d replace it every 5 years with the coolant.

@oneye in addition to the other suggestions, I 'd suggest checking out the engine coolant temperature sensor. If it’s faulty, and the engine control module thinks the engine is cold, more fuel will be used.

Even with the oxygen sensor fixed, you are going to get worse gas mileage in the winter than the summer. My old Buick would get 30 mpg in the summer, even with the a/c running, but would get 24 in the winter. There are many, many scientific reasons that conspire to make this happen, like additional parts friction, thicker oil, denser air the car has to push through, etc, and some behavioral reasons as well, like letting the car idle while you scrape frost off the windows, or waiting for the heater to do that so you don’t have to.

What oil weight are you using?