I have a real puzzler for you!
I have a very well maintained 2005 honda odyssey with 150,000 miles on it. It typically gets 22 m.p.g. It started using an unusual amount of gas - was getting 12 m.p.g. Took it to the mechanic and turned out the gas pump was cracked. We replaced the gas pump and the gas line, and it was getting 14 m.p.g. Took it to another mechanic, and he said that the spark plugs were the problem as they were not official Honda spark plugs (we replaced them at around 110,000 miles). So we installed the official Honda spark plugs, but still no good. I have visited several mechanics and no one seems to know.
The first place I’d look is the ECU; check it for stored fault codes, even if th elight isn’t on.
The second place I’d look is the temp sensor. That can dramatically hurt gas mileage by tellling the ECU the engine is cold even when it isn’t and it can do it without tripping the Check Engine Light.
The third place I’d look is the upstream oxygen sensor. Although that should trip a fault code.
Fourth place: a bad injector (any wet sparkplugs?).
Fifth place, a dragging brake. You can check for a hot wheel or jack up each corner and try spinnning the wheel by hand.
Sixth place, a plugged exhaust.
Oh, yeah, and I assume you’ve checked your tire pressure?
A simple test that I like to do when someone complains of bad fuel economy is to park on level parking spot. I turn off the engine and then put the vehicle in neutral. I step out the door and move the vehicle back and forth to see if the wheels roll freely. I find a lot of vehicles have sticking brakes either parking or regular. I have found mechanical problems like binding driveshafts and bad bearings. Good luck.
The thermostat is an excellent suggestion, but I just wanted to mention to the OP not to confuse that with the temp sensor. The T-stat of it’s stuck open won’t allow the engine to reach operating temp. The temp sensor will tell the ECU that the engine is cold even if it’s actually AT operating temp. Either can cause poor mileage.
We haven’t heard from Tommy in 5 years. I seriously doubt if he’ll respond.
Try describing your problem as clearly and comprehensively as you can in a new thread and we’ll try to help. Be sure to include in your thread your car’s
year,
model,
mileage,
engine (if there are options)
manual or automatic (if applicable)
mileage when the car was new (if the car is new to you, say so)
current mileage
maintenance
type of driving (city vs. rural)
recent work.
Knowing what region you live in might help too, as bad weather kills mileage.
With this info, we’ll see what we can do.
Yes I found the problem. The oxygen sensors in the engine were wrong. I first replaced some of them, and it temporarily helped but then the problem came back. After another checkup, we replaced ALL of the O2 sensors and it’s finally fixed.
On a separate issue (found this a few years later), we discovered the right rear brake was dragging because it was much more worn out than the other breaks. I don’t know if this played into the fuel usage, which did not change after we fixed it.
My honda drank double the gas for 7 years, did everything nothing fixed it. Anyways last month my Isle air control valve got stuck, so my husband replaced it and after a week of driving we realized are car is not drinking gas, the gas mileage is fixed! I drive a 2001 honda accord. Just thought I would let people know because we tried everything and gave up, spark plugs, tires, filters took it to Honda and nothing fixed it.
And thank you for your excellent list of suggestions.