Advice on poor mileage 04 Corolla 5-spd
It has about 200K miles now, I drive about 550 miles/ week average, and almost all hwy miles. Originally it got about 38-39 mpg hwy when I bought it nearly 5 yrs ago. Now I get only 32 mpg hwy. Of course since I got it, Valero has switched to a "up to 10% ethanol blend". Maybe a small part of decrease is due to this. But I hear that other Corollas, and even ones with auto trans, are still getting 38 MPG! I suspect I should still be getting somewhat better MPG than just 32! At the high current cost of unleaded, if I could just do something to get it back up to around 35 MPG, over the course of a year it would be some significant bucks saved. At abt 180K, the check engine light came on, but it is the common cat converter efficiency below threshold" code. I changed both the upstream and downstream O2 sensors with sensors bought from Rockauto, but this did not fix light, or improve MPG. About the same time, I put in new platinum spark plugs, and sprayed out the MAF sensor with MAF sensor cleaner. It was very dirty....but MPG did not improve. The intake filter is new. My understanding is that the "cat converter efficiency down" code is set if the efficiency is just down by as little as 5%, but if its getting major plugged, and there is abnormal back-pressure, would this decrease MPG? How to test for this? (Can't very well drive around with exhaust disconnected). Suggestions as to what to check for decreased gas mileage?