2002 Mercury Villager--mysterious rapid decline in gas millage

My 2002 Mercury Villager used to get 23+ MPG. However, just in the last few months, it has been getting about 14 MPG. There has been no significant change in my driving habits.

I took it in to the dealership, and they did all their “diagnostics” which were inconclusive. They said it was the best looking Villager they had seen in a while and couldn’t find any reason why it should be getting such low millage all of a sudden. They charged my $90 just to look at it and tell me they had no idea what the problem was! : )

Does anyone have ideas about what may be causing this?

If it’s never been done, have the thermostat replaced.

If the thermostat is stuck open the coolant doesn’t get to the proper operating temperature and the engine uses more fuel.


Along with checking the thermostat, check the tire pressure, although if the tires were this low to cause the problem, you should notice it.
Also, did you buy new tires before the drop in mileage occurred. If so and the tire dealer put on the wrong sized tire, that would throw off the odometer reading and if the tires were oversized, fewer miles would be registered than you actually traveled making you believe you are getting less mpg.

Hopefully you have a temp gauge and can monitor temps. New tires recently? No check engine lights I assume. The dealer answer leaves me mystified but a dose of seafoam or techron can’t hurt. Check wheel temps after a short drive, one wheel particularly hotter than others could indicate a caliper problem causing the brakes to drag. Do you use the parking brake, is it stuck on, so many options.

Perhaps one of the fuel injectors is flowing too much

An injector balance test should help diagnose that

Perhaps an oxygen sensor is skewed/contaminated/sluggish

MAP sensor could be faulty

Low mpg is very often associated w/an operating temp problem. Is the dash guage showing below normal temps all the time? If it is a temp problem, either the coolant is never reaching the correct temp (which would usually be a bad thermostat), or the ECM thinks it isn’t. If the ECM thinks the coolant is always cold, it will inject more gas than it should. So ask the shop to check both the thermostat and the engine coolant temp sensor. There are usually two sensors. Not the one used for the dash guage, the one used by the ECM.

It could be other things too, but unless there is reason to suspect otherwise, me, I’d do this first.

I had a 92 Plymouth minivan and my mileage dropped from 18-20 to 12-14 in a couple of months before my check engine light came on and the cars self diagnostic indicated a bad O2 sensor. I replaced it and the mileage went right back up. So they can cause a drop in mileage long before they set a light.

At the dealership yesterday, they had told me that if it was using too much fuel, the spark plugs would probably be dark in color (and they said these were not dark when they looked at them).

Does this change any of the advice above?

Update: I called them back and asked about a possible temperature or oxygen sensor problem. The guy said that if a sensor was malfunctioning, that this would send a code (which it didn’t).

Lazy O2 sensors do not throw a code. They can monitor the temp sensor with the computer and at least check it.

If the temperature sensor was malfunctioning it would send a code? Not necessarily. If the wire fell off or it just stopped functioning completely, that would send a code, but if it was registering the temperature inaccurately, that wouldn’t throw a code. Testing that sensor is usually a pretty simple task, including the time to take it out and put it back in, and do the test, takes like 15-20 minutes. Just make sure they test the ECM sensor, not the guage sensor.

Do you have a temp guage on your dash? If so, do you notice anything unusual with the temperature guage readings? Compared to when the car got better mpg I mean?

Despite the appearance of the plugs, it must be using more gas than it should be, otherwise you’d not have the mpg drop. Right? The gas has to be going somewhere, and it isn’t going into powering the car down the road as efficiently as it used to. One idea besides the others above: It could be a leak in the exhaust system. This allows O2 to enter the exhaust stream, and cause the O2 sensor to think the mixture is lean, so the ECM compensates by injecting more gas than is actaully needed. In that situation the O2 sensor is working correctly, so it wouldn’t throw a code. You’d just get poor mpg’s.

Stuck EGR valves, or malfunctioning PCV valves can cause mpg loss too. And locked brakes. Have you verified that each of the wheels turn freely; i.e. that the brakes are not locked up?