Fuel economy keeps getting worse [SOLVED - reset ECU]

[chairmankaga]

For the past couple of months I've watched my average fuel economy drop more than 2 MPG. Each fill-up I run the numbers through Gas Cubby. I've gone from 17.8 MPG (mostly city driving) to 15.4. It's been gradual, over about 7 fill-ups, each with a lower number than the last. I'm not throwing any codes, the oil was changed only 3 months ago (Mobil 1 synthetic), tires are aired to 35 psi as usual. The air filter is newish and clean, there are no obvious tears or obstructions, all 6 cylinders are getting spark. I even use the same pump at the same station almost every time.

Any ideas? I really don't want to take it to a mechanic with the nebulous instruction to find out why my fuel economy has gone from crummy to terrible.

[EdBwoy]

I'd check the brakes to make sure nothing is dragging.

To eliminate any driveline hiccups, does the car still coast off the throttle like it did before? I know our transmission isn't exactly coast-friendly, but worth a comparison.

[chairmankaga]

It seems to pull away fine from a stop.
I filled up this morning and thankfully it hasn't gotten any worse - still 15.4 avg, which is well below my cumulative average up to February. That's when the sudden decrease began.
I do wonder if it's not something binding though. Maybe a dragging brake pad? You'd think I'd notice that though. Transmission drag?

[steve_c]

Did any driving habits change?
Did you eliminate any highway driving?
Have you changed routes or roadways?
Any traffic pattern changes to your regular routes most traveled (construction, congestion, etc.).
Are you driving with additional passengers, or cargo load (like in the trunk)?
Is there a seasonal ethanol additive changeover in the fuel at the station you are using?
Did you change footwear that might be giving you different gas pedal/throttle feedback?
Does anyone else use the vehicle?

[chairmankaga]

My drive is consistent. Same route, same times of day. Traffic is always bad (Austin). Time in the car is about the same, although some days of the week are worse than others, but that's consistent week to week. My wife will drive it occasionally, but she's far more gentle on the throttle than I am. Not sure about ethanol content, but I'm going to try a different station and brand next fill-up. And I have maybe three pairs of shoes for work, depending on how fancy I need to be that day. No bigass boots or anything though, just plain old shoes.

I'm going to pull the plugs again, and try to get to the downstream O2 sensors. I replaced the EGR valve a while back because the old one was clogged, so MAYBE the burning oil sooted up the sensors?

[Mikesvagen]

Winter blend fuel?

[chairmankaga]

I'm honestly not sure we even get winter blend in Texas.

[chairmankaga]

Well, I did the tap dance to reset the ECO and unplugged the battery for a while (I was cleaning the engine bay anyway), so we'll see if that helps. I only pulled two plugs (and what a PITA that job is) but they looked perfectly fine, with correct gap. No other obvious trauma, although I think I could use a new air intake snorkus, as mine seems to be starting to crack. Air leaks post MAF, no bueno.

[chairmankaga]

Update: Since resetting the ECU I've burned two full tanks of gas. According to Gas Cubby I've gained almost 3 mpg. Prior to the reset my MPG average was 15.4, and both of the past two have been 17.9. Same daily drive. No significant alterations to anything. So I guess that for whatever reason, that was the trick!

[chairmankaga]

Settling in at 17.8 MPG. Resetting the ECU seems to have been the solution.

[EdBwoy]

Thanks for circling back Mr. Chairman.

Although it's odd why it would suddenly need a reset. If Ken Block had borrowed it for the past 5,000 miles, it might learn those preferences and stay pretty aggressive, but you say your driving is fairly relaxed.

I feel like some things like your throttle body could use a reminder every once in a while as dirt and grime builds up affecting the true set-points, but you didn't touch that either. Oh well, all is well that ends well. I'll mark the topic as solved.

[chairmankaga]

Well, I bought the car about a year ago from its second owner with 89k miles on the clock. Looking at my Gas Cubby chart the fuel economy has dropped steadily over the months, but really bottomed out in March or so, tanking from the mid-16s to the mid 15s. I do know my daily drive is VASTLY different from the POs so maybe that disparity just confused the fuel maps enough to reduce efficiency. Regardless, it worked! Still clocking in close to 18 MPG each tank, which is better than my very first tank after I bought the car.