I've been reading a lot about O2 sensors and how they work in conjunction with the computer to tune the fuel curve. I also have read that until the O2 sensors "warm up" the engine runs in "open loop" and the fuel is metered by the MAF sensor. Since it has gotten a little colder here in TN, I have notice my mileage has really went to crap. I was wondering, how long does it take the O2 sensors to warm up? I only drive like 8 miles or 10-15 mins to work most days. I don't really even notice the temp gauge starting to rise until I get on the freeway. I was considering that maybe the MAF was dirty and I didn't really notice in the summer because the O2 sensors would maybe warm up quicker.

Anyway, thoughts?

Bye the way, 2001, scab, 4.6L 40K miles, bone stock.

Thanks!

 

These are heated sensors, i.e., they don't just rely on the exhaust to warm the up, they have electric resistance heaters internally. They should only take a minute or two to heat up and be ready for closed loop.

One of the requirements for closed loop is that the engine is warmed up. A bad ECT (Engine Coolant Temperature sensor) is one reason for poor mileage. In closed loop, the fuel calculations are made by (technically they do this by adjustments call fuel trims to that calculated by the MAF) the O2 sensor but the O2 sensor can only maintain a 14.7:1 air fuel ratio. Until the engine is warm and at wide open throttle, the A/F ratio may be as rich as 10:1. If the ECT fails open (-40 degrees F), the engine will stay in open loop and run rich.

All of that is true, but if it is the problem you're having it would be coincidental that the ECT failed at the same time it turned cold.

 

Thanks for the reply. That clears up a couple things. I suppose it could be the ECT causing the problem. I had one in my 88 F-150 about get me killed a couple times because it was killing the ignition system at very unopportune times I might add. The other thing that makes me wonder about a rich condition is there is a little black ring around the tail pipe. Back in the days of carburation I would have said it was time to lean it out a little. With these computer trucks I really don't have a clue. I know the computer is suppose to do all that for you but if it is not getting the proper feedback it cannot do it properly. I have lost a couple MPG steadily over the last 3 or 4 tanks. It ran between 15.5 and 17 all the time before now it is running between 14-15 all the time and the only change I know of is outside temp since this really started.

Any other insights or places I should start looking for those couple mpg?

 

The ECT is just one reason you could have poor gas mileage. It probably would happen suddenly, not over several tanks, and would probably be worse than a couple mpg.