Details. Same driver, same routes, same load (or lack of it), same gas, 50K miles in 3 years. Kept detail MPG and other data on 95% of all gas fillups. So I can compare data during the same season. I understand summer gas and winter gas.

Tire pressure checked, oil changed according to MM. ATF changed 2X. Air filter cleaned. In other words, all service done and to date.

The only thing is I could use new tires soon, and by feel, alignment is good, like new. Track straight.

The last few tanks have been about 3-4 MPG lower than normal. I have enough data to compare apples to apples. And calculations are/have been done right, and consistant.

What else can I do, or check to see if there is cause? An engine at 50K miles is broken in and at it's prime, so it's not like it's old or factory new.

TIA

 

change O2 sensor?

If it is reading on the lean side, closed loop will dump more fuel to get back to stoich. Might not be bad enough for check engine light to come on.
Knowing your past short/long term fuel trims would be the only way to know if they have changed. If you change O2, reset the ECU for it to re-calculate fuel trims.

 

Ok, I hope not. I'll do a ECU reset first.

BTW, could it be MAF?

 

Fuel with high ethanol concentration will skew fuel mileage if you are comparing it to regular unleaded fuel.

Driving straight does not mean the alignment is good, it just means that the current alignment settings allows the vehicle to track a straight line. How about your tire wear, is it even? Rough?

O2 sensor problems will be caught by the PCM, what needs to be checked first if you believe it is engine related are the fuel trim readings. If your fuel trims are deviating +10% or more, this will account for the bulk of your problem, the only question left is what is causing it. Long term fuel trims generally over 20% will set a too lean condition, as the computer is adding the most fuel it can and remain in control. It will add fuel to keep the O2 sensor cycling in the proper ranges.

Doing an ECU reset will do nothing, it will force it to relearn what it already knew, which it does quite frequently on its own.

I personally would think of something environmental, such as in town or highway driving style, uphill/downhill, cruise control or no cruise, A/C on or off. FWD or AWD, poor tire choice, dragging brakes, vehicle load. Fuel composition, try another fueling station and retest, could be a problem with the pump you use, pretty rare, but of course, it could happen.

The computer is smart enough to catch engine related problems most of the time.

 

My 87 gas has always been E10. Even if it's seasonal, I'm comparing Aug 2012 to Aug 2011 to Aug 2010 to Aug 2009.

Good point on the alignment, in that "straight" means nothing. The tires are almost touching the wear indicators. So tires in a month or two is my plan.

That is also on the to-do list.

 

Worn tires will increase fuel efficiency because the rubber is softer than a new one, so if anything, your problem will get worse, minutely of course.

 

Check ECT sensor, a defective sensor would not allow ECU/ECM to adjust A/F ratio, you may be running rich
We have run into that on more then a few Honda/Acura. 94

 

O2 sensor should be your first item to replace, then move on to the MAF/MAP.

Also like slowcivic said, hook up an OBD datastream viewer and check your fuel trims.

 

The trims do not lie. Start by monitoring the trims, nothing else matters. If they are good, then the problem is environmental.