Ok, 2000 civic EX.. still stock auto d16y8 motor.

When I travel at normal 65mph speed we will say around 2500 rpm... says the stock tach.. I have reciently been getting crappy gas mileage. From 15-23 mpg...

When I ride at 80 mph or so 33-3500rpm... it shoots wayyyy up... usually 37-42, but on this trip today, 53mpg... It has never gotten that good in the least... and no, the tank could not hold another gallon, tried...

What could be causing this? It used to be 32mpg city, and 36-38 highway... and that was summer, this is freezing winter.

---a perplexed and mezmorized person...

 

Do you run out a whole tank of fuel during your "trip"? Otherwise, you are mixing other driving conditions into the equation.

FWIW, I find my mileage alternates every other fill up. I tend to average out the calculation over 4 fill ups to get a true mpg figure.

 

It doesn't matter if you use the whole tank, you just always fill up and divide miles driven from the last fill up by gallons to fill up. But yeah, that's doesn't make any ******* sense.

 

maybe your o2 sensor is goin screwy

 

yeah I do the fill tank thing... and no not mixing, I do appraisals hundreds of miles away, so it is the drive there, to drive home, staight out highway, about as true to a constant speed like I said as it can be...

So far, O2 sensor, and I have heard maybe fuel maps? from a friend of mine... keep any ideas coming because a Honda dealership had no freaking idea at all. And said it was too much of a job to figure out the problem.. (slackers....)

As for as O2, I have noticed that more soot than usual had been coming and collecting on the exhaust tip.... (I clean it a lot so I know there was as difference) and with this last trip it barely had a haze on it... perhaps it is dumping gas and then at 3000 leaning out? this is so... wtf....

 

a fouled o2 will crack out your mpg. Usually when mpg issues start arising o2 is the first cause

 

still have to wonder how I was getting awesome mileage at both higher speed and RPM though... I've seen the dyno curves of this engine, and it doesnt have any large gain between 2500 and 3500... at least not compared to the resistance from wind it should have... I have heard it before... but... I should probably hear it again to believe it... about what is the cost of one of those expensive little buggers... just doesnt seem right though with only 92,000 miles...

 

bump for anymore ideas?

 

last bump then no more from me.

 

Replace O2.
Replace air filter.
Run some fuel system cleaner through with your next tank of gas.

Test with more driving.