Poor Gas Mileage Focus Wagon

I had an oil change a few months ago and my gas mile has dropped by almost 10MPG city. Focus 2003 wagon. What happened was the oil cap came off after the change. I discovered it missing the next day. Needless to say there was a lot of oil on the engine. I had the fuel injection system cleaned. Nothing found on the computer. spark plug wires clean. Air filter clean. Gas filter OK. Driving habits same. Same gas station. Could the oil have sprayed onto some electrical wiring to cause this?

Anytime there’s a sudden drop in fuel economy, the first two things to have checked are the engine thermostat and the engine Coolant Temperature Sensor for the computer.

If the thermostat is stuck partially open, it takes longer for the engine to come up to the proper operating temperature for the computer go into the closed loop mode. If it ever does. So have the thermostat checked out.

If a defective engine coolant temp sensor is telling the computer that coolant doesn’t get up to full operating temperature when in fact it does, the computer won’t go into the closed loop mode.

If the computer never goes into the closed loop mode, you’ll use more fuel.


What does’ “…gas filter ok”, mean? The old spark plugs and air filter can contribute to poor mileage, as well as the things Tester spoke of.

spark plug wires clean

They can be clean and still be a problem. How old are they (miles and time) if over 10,000 miles I would consider them a target for replacement. If fact If they were covered with oil, I would consider replacing them anyway. Frankly I believe your will find out that Tester has nailed it in the end however.


I’ll have those checked.
The mechanic said the spark plugs were good. There was no oil on the cables either. Is it just a coincidence that the problem started after the oil change or can oil affect the temp Sensor? I have no idea what it looks like.

This engine is a double overhead and does not have an engine Coolant Temperature Sensor. There is nothing else to indicate a problem with the thermostat, i.e. check engine light, etc. And the mechanic says there is no way to check if it is stuck open unless he takes the whole assembly apart which is a lot of labor.
One other thing though is that the brake light has been coming on for the past few months but the brakes are in good shape. I still think oil has gotten into a cable connection somewhere.

Here you go:


I find it very difficult to believe there is no coolant temp sensor. Is there a temperature gauge on the dashboard? If so there’s a temp sensor for it, and there may be a separate sensor for the computer. The computer needs information about the engine temperature to adjust the fuel mixture. There has to be at least one coolant temperature sensor, or sender if you prefer. A faulty temp sensor, or a bad electrical connection at the sensor, can cause problems.

There should also be a sensor in the radiator to operate the cooling fans.

The check engine light will not come on for a stuck thermostat, but a stuck thermostat can significantly reduce gas mileage. The only way to be sure it to remove the thermostat and check it. Labor is involved, yes.

The brake light coming on once in a while usually means the brake fluid in the reservoir is low.