I have a 1996 T&C LXi. Recently noticed highway mileage dropped to 19-20 mpg from 22-23 mpg. I replaced thermostat, checked the temperature sensor OK. No fault codes. While cruising on a highway at ~60 mph, the instanteneous gas consumption on my computer cycles between 22-24 mpg down to 18-19 mpg - this happens every 30 seconds or so. Looks like the computer is trying to adjust the gas/air mixture. Any idea what to look for?

 

I'll assume you've done all the obvious things like remove dead weight, inflate tires to at least door sticker pressure, tried different gas stations, etc.
"Looks like the computer is trying to adjust the gas/air mixture." Key component in that mix is the upstream O2 sensor. If you want to start changing parts that's the one I'd start with. Just change it. To troubleshoot it you need a garage with a scan scope to check fuel trims. There goes an easy $100. Spark plugs are scheduled for every 100k miles. Getting close?

 

Wouldn't the map sensor have anything to do with t he issue?

 

Sure could. MAP would be a close second choice. The O2 sensor is the one I read about the most. The air/fuel mix is adjusted in direct proportion to exhaust oxygen levels. The MAP sensor adjusts air fuel mix by detecting engine loads. Of the two the O2 sensor takes more punishment.

 

Thanks Raptor 07 for the explaining

 

Thanks, guys. I've done the obvious things (tire pressure, good gas, spark plugs, etc.) - I will take your recommendations and check out the O2 sensor, then the MAP. Other then a code scanner or a multimeter I don't have fancy diagnostic tools, but will try to improvise. BTW, the car has about 128,000 mls and I have not changed O2 or MAP sensors yet. Thanks again for the help.

 

One more question before I jump in and troubleshoot/replace the sensors: wouldn't bad O2 or MAP sensor throw a code? I see none.