I’ve got an 01 Honda Civic MT with 208k miles that used to easily get 38 mpg highway. Over the last 6-8 months it’s been steadily dropping and I’m now in the high twenties. I’ve replaced the upstream O2 sensor, coolant temp sensor, thermostat, checked the spark plugs (only about 20k miles on them) looked for obstructions in the intake and exhaust and have a sparkling clean K&N air filter. Tire pressure is holding steady at 32 psi. I checked with the Honda dealer and the only fuel filter is the screen inside the fuel pump which they say they’ve never seen get clogged enough to cause a low mpg problem. I don’t think the cold weather would have this big of an impact (I’ve had the car 5 years and have not had this kind of drop in the winter before). Does anyone have any more ideas? I’m pretty stumped.
How long ago was the K&N added? If these get over oiled even by a little they are known to mess with the MAF sensor. Obviously one would hope that this would toss a code, but there can be issues without it. If there is any question at all I’d be tempted to try an “experiment” involving cleaning the MAF and going temporarily to a regular paper filter. If you do that and nothing changes then you can likely rule out an oil fouled MAF.
How is the alignment?
Check for any kind of even mild sticking in calipers and wheel cylinders (if you have drums in the rear) that could create a little brake drag. It wouldn’t take much.
I also wonder where you live and whether or not you could have a “perfect storm” of coincidences: 1) winter blend gasoline + 2) cold winter temps + 3) maybe a recent change in where you get gas or in gas stations going from 0% ethanol to 10% ethanol blends. No one of those things would make a huge change, but 1 + 2 + 3 ?
I’m in Lex, KY. I’ve had the K&N for several years as well. Could the MAF sensor toss a code without turning on the CEL? I might try removing the MAF sensor and cleaning it. I hear there are cleaners for that. Thanks