My 2002 Avalon XLS with 130K has had a sudden loss gas milage. I used to average about 27mpg on the interstate...now I am getting only 22-25mpg.....inconsistantly. Around town has dropped from low 20's to high teens. It started about a month ago during a long trip up north. A couple of weeks ago we had more serious problems with stalling, erratic shifting and idling, unable to start plus the milage problem. Spent $800 on analysis that ended up with codes showing IAC module needed replacement. Car runs much better, but milage problem still the same. Only major work was fluid flushes and induction cleaning, brake job and timing belt replacement one year ago. Tire inflation good and there are no other usage changes. Tried some Lucas a few days ago, but no significant change yet. Getting ready for another long trip, and don't feel good about it. Planning on taking it back to dealer next week. Any suggestions?
IAC module was replaced last week. Have filled tank with 93 octane along with Lucas additive. Plugs have not been replaced, and may even have 100K on them. I did have induction cleaning and air filter done last year. Have never had a tune up that I can remember.
Try clearing the adaptives by disconnecting the battery. Theory: You got a bad (low octane) tank of gas on that trip where it started. Your ECU started retarding timing to prevent knock. It can take a long time to "unlearn" the self protective adaptation, so make it forget.
I think Ted has a good chance of hitting it on the head. If you think you want to keep the car a few more years, I would suggest either buying a decent scanner or a if you are into PC's a device like Elm Scan that will give you the ability to get a whole lot of data about fuel usage/trim. At 130K a whole bunch of sensors can be slowing down and not giving the PCM the right voltage/info to adjust the air/fuel timing magic. Plugging into the PCM and learning what's going on, is worth the time and cost. You don't need to spend a lot on a scanner or the Auto-Can. I just replaced two air/fuel sensors, an O2 sensor and a Mass Air Flow sensor on my 2000 with 158K. The engine has not felt so robust in years. If you want to accelerate the fuel injection, spark plug cleaning etc al., then buy a two pack from the dealer or on-line, BG44K/Air Induction cleaner. Put the 44K in the tank and hold onto since you just had it done, the can of air induction cleaner, or spray it into your brake vacuum hose.