With higher mileage comes engine wear which could cause lower MPG. As far as the coolant temp sensor, yes. If they are telling the computer the wrong ambient temp, it will over pr under compensate for the fuel mixture. For example, if it is 80 degrees F and the computer is told it is -30 degrees F, it will send too much fuel to the engine causing it to run rich, use too much fuel, and usually run rough.
What needs done is the computer needs to be connected to a scanner that provides real time information. Both the coolant temp sensor needs checked to see what temp it thinks it is outside. Also, fuel trims need checked as well as oxygen sensors.
Fuel trims are basically the amount of time the computer keeps an injector open based on need. They need checked to see if they are opening too long. Also, a bad oxygen sensor can cause the vehicle to run in what is called a closed loop. Basically, if the computer isn't getting a reading from an O2 sensor, it runs the engine rich to keep it going.
One last thing. Fuel pressure should be checked. If the regulator is bad, too much fuel pressure can build and cause a rich mixture.
Let me know if this helps.
Wednesday, July 29th, 2015 AT 7:42 PM