2001 Ford Focus Poor Fuel Economy

I have a 2001 Ford Focus ZTS 5-speed. Thats the 4 valve. I drive around town in Portland, Maine and get 18 MPG pretty consistently. The EPA rated it at 22 city. I replaced HO2S1 and STFT is about 6 and LTFT is acceptable. I’ve been through the ford service info for the problem to the best of my ability. New fuel filter, new plugs/wires like 4 years ago. No MIL. My exhaust is dented a bit after the cat, haven’t noticed any loss of power. Been having this problem for about 2 years; I think I drive pretty gently, I did notice the economy drop at one point. Did have a problem a few years ago where sometimes it wouldn’t idle and you had to give it gas. Seemed to have gone away on its own. Did set a MAF code when that was going on, although the MAF is clean and the readings seem accurate. No other running problems, no dragging brake or tire pressure issue.

Sometimes now it seems like when I start it, I release the key, after it seems like it starts, and it stumbles and actually starts a moment later. Everyone pretty much tells me there’s no problem, and they’re knowledgeable, but 18 is still pretty bad. Not much point to having a focus if that’s what you’re getting. Feels pretty bad to have to fill up every 200 miles. I bet Ray would know…

How long have you had this car? Have you been checking the mileage since new, so this is a sudden problem, and if so when did it start? Was it sudden or gradual? Or, is it possible that it was always bad and you recently started checking? Whenever you have an average, some units are higher and some are lower, maybe you just got unlucky and got one at the low end of the range.

Odometer reading?

Those EPA estimates are tricky & I don’t even know anything about how they are done. I can say, though, that back in '01 an underlying assumption may have been that the car would be running on 100% gasoline. But now all you can buy is E10 (10% ethanol). You get worse mileage with ethanol. I’m guessing 18 = about 22.

That said, 4yr old plugs are probably ready to go. And the wires won’t be that expensive so why not? Same for the fuel filter. How’s the air filter?

I’d check the coolant temp sensor.

The condition of the tires (independently of air pressure) and the alignment can make a noticeable difference. You’re 4 years on the same fuel filter. When was your last alignment? How are the tires?

What does “acceptable” mean to you on the LTFT? I assume you have some kind of scantool.

(BTW: you have 4 cylinders, not 4 valves. I don’t know which engine is in the ZTS, but it will have either 8 or 16 valves).

I have 98k miles. I did the fuel filter about a year ago, new air filter. Alignment and tires are good. Acceptable on LTFT is under 10, per ford, it was pretty near zero. Sorry, I meant 4 valves per cylinder. My dad bought the car new. I have been getting consistent mpg readings, and I’m rigorous about it. It seemed pretty sudden. I admit that I don’t know what I was getting before, but I’m guessing 25 city. I remember getting that for one tank before.

I knew about ethanol, but I don’t think it can explain for an 18% drop. Imagine if they took away 10% of the gas and didn’t even add anything else. That would only mean a 10% drop in fuel efficiency.

I would replace the thermostat.
It’s cheap and old enough (if original) to be sticking.

Ethanol has 30% less energy per gallon, so all other things being equal, E10 will have 3% lower gas mileage than pure gasoline. I know lots of people claim a bigger loss, and I would love to test it if I could find a gas station to sell me gas, but here in NY I can only buy E10.

If you have a scan tool, will it read out the engine temp while you have it running, or does it only read stored codes? A bad thermostat is definitely a top cause of poor mileage.

If you are handy, pull the plugs and look for usual wear that might signal a problem (burning oil, coolant, poor ignition, etc).

Thermostat was replaced about 2 years ago. I’ve looked at all the obvious stuff, looked at it with the genisys a lot.

Just because the spark plugs were replaced 4 years ago or the thermostat was replaced 2 years ago doesn’t mean they can’t be bad again. If you have a scan tool, does your car run at the correct operating temperature after you have been driving for a few minutes?

Could be a bad ECT sensor sending the wrong information to the ECU keeping the engine from going into closed loop. If your scan tool doesn’t read out engine temperature you need to find one that does and see what temperature it being reported to the ECU.

Have you checked for dragging brakes? Checked tire pressure? Have you cleaned the MAF sensor?