what's causing very poor gas mileage

[penwah114]

I purchased my 2008 vibe, not gt, in 2011. put k&n air filter, new michelin hydroedge tires and was getting 27-30 mpg in mostly city driving. I make short trips as well. I use mobile one or amsoil oil with mobile one filter for oil changes. My engine has no rough idle, nothing sounds wrong at all. alignment ok, only 10k on the air filter and oil change fresh, tires only 10k as well. tire pressure is kept at 35lbs and yet my mpg have dropped over the last 4 months to 25 - 24- 21- and now 19mpg. no codes. I cleaned my maf sensor and no change. I only have 46K miles on the car so I am stumped. I did not check the plugs, but figure no rough ride and only 46k miles on it. no brake drag or at least i don't feel anything. no gas smell, no rotten egg smell, nothing. Help! I bought this car to save on gas and I should have kept my explorer as the difference is not much. I am thinking about changing the gaskets anyway, will have to wait for school as i let the high school do the work for $10.00 as I don't work on cars. but there is no idle issues or driving issues at all.I live in MI and the mileage dropped spring/summer. The shop wants me to come and put it on the diagnostic for $40 - $80 to see if sensors are all within range but I was hoping for some help here. if there is no code it seems that is a waste of money.

[Doug14]

You've done lots of good troubleshooting already. only other things that I can suggest (other members are more knowledgeable) is check and clean/replace PCV valve and clean throttle body. then jack up the care and make absolutely sure there's no brake drag. Best of luck!

[SeattleJeremy]

Does the car make more noise that normal at highway speeds? Does it struggle to get up to speed? Are you able to pull the spark plugs to see what shape they are in?

[triz]

Get a OBD reader and get some 02 sensor readings. You might have lazy O2 sensor.

[JohnO]

I'd say to check the front O2 sensor and the thermostat. Both can be checked with a good OBD reader. The engine should stay up near 200° while it is running. Thermostat tend to fail slowly and over time will keep the temperature too low (under 180° or so) for proper operation of the engine.

[penwah114]

Thanks, I will make sure the shop checks with the odb? meter the o2 sensor, although they didn't mention that - said can't test for slow o2 sensors, they just said put it on the diagnosis I will mention to do the others first. I do check my temperature gauge and it heats up normally. I don't feel any drag on the brakes, but will check. I feel absolutely no performance issues whatsoever. I am going to check hoses and look for leaks today. I am not sure if I can get to plugs or not will try today as well. Thanks all. I am totally stumped

[penwah114]

what will the obd reader show if the o2 sensor is lazy

[JohnO]

I do check my temperature gauge and it heats up normally.

You can't really tell from the temperature gauge on the car whether the thermostat is good or not. The gauges are designed so there is a wide range of temperatures over which it will stay right in the middle. The easiest way to monitor the actual temperature is with the OBDII. 4-6 years is a typical useful life for a thermostat after which they won't maintain proper temperature.

[penwah114]

thanks, will have all checked.