2008 Grand Caravan, 3.8 V6, 22930 miles. Engine overheated in March, not bad just to where the light and buzzer came on and I immediately pulled over and had it towed to the dealership. Dealer replaced the coolant temperature sensor and flashed the PCM, Change Auth 18-15-10 Dealer Code 41573. Immediately my mileage started dropping, it went from a average of 17.2 down to 14.5. Dealer checked it out and can find nothing wrong with it. NPF diagnostic test: No Trouble Codes, Checked for updates, All Current, Addaptives Normal. 22000 Checked Over, No Codes, All Modules Up To Date, Drives Normal, Oil and Fuel is Clean, Addaptives are All Between 0 and 8.2. I even added fuel injector cleaner to the tank and ran a bottle of Sea Foam through a vacuum intake hose at the advice of Auto Zone, no change. What would cause the mileage to drop after the overheating, coolant sensor change and flashing the PCM?

TIA, Chris

 

are you ideling more? I know that sucks my mpg down,
any change in your driving routes,

and if not that, have you compared acuall fill up amounts of gas vs miles driven, just in case your trip cpu is off for some reason

 

Driving habits have not changed, trip cpu is right on, you can actually tell when the mpg drops that much.

Changed the upstream and downstream oxygen sensor, no change in MPG. Found out the downstream one only lets you know if the catalytic converter is bad. Auto Zone recommended I change both, guess they just wanted to make a sale. I will update this when I find out what the problem is.

 

Update. I have also changed the intake temperature sensor and the external temperature sensor located behind the front bumper. I picked up a OBDLink SX Scan Tool and ran the software and it shows Short Term Fuel Trim Bank 1 3.13% Lean (-) and Long Term Fuel Trim Bank 1 5.47% Rich (+). Took a trip that was mostly freeway about a hour and a half each way. The mileage at the end of the trip was 19.8 mpg which is about where it was at before this problem started. For some reason it's fine on the highway but the MPG really drops in the city. Here is a link to a picture of the sticker that was placed under the hood after my first trip to the dealer. http://www.avinstallations.com/flashpcm.html

TIA Chris

 

The city driving will always mileage down, thats because of the stop and go.

 

Understood but it never dropped that low before until it came back from the dealer. Average was about 17.2 between city and highway driving not 14.5. Could Long Term Fuel Trim Bank 1 5.47% Rich (+) be caused by a vacuum leak?

 

I would advise both O2 sensors too, if one was bad and your mileage was high...BUT...you shouldn't have needed one at 23K miles and the ones you have on it now are inferior by a landslide to the ones you had. I would recommend you not go by the trip computer to calculate your total mileage. Those stupid things should just have up and down arrows instead and give driver feedback when your mileage is better or worse. Do the math at fill up.

A guy came in the other day and said he was getting 11.8 MPG because he believed his trip computer. We filled it up and had it driven for 150 miles and did the math. It came out to 21.4 both city and highway driving 50/50.