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Where is my gas going?

Tiny
RUNNING_GUY08
  • MEMBER
  • FORD TAURUS
I have a 1998 Taurus 3.0 V6 with just over 117,000 miles. Within the past week I have changed the spark plugs and the oil, following the recommended manufacturer's specs. Over the past month or so, I have noticed a slight decrease in gas mileage (which was previously above 20mpg average) and I was beginning to think maybe it was the gauge in the gas tank giving weird readings. I filled up again yesterday and figured my mileage dropped about 4mpg to about 15mpg. I cannot smell any gas fumes, there are no wet places anywhere in the engine or on the ground under the engine, and the the hoses look fine. Where could the gas be going? I am not really even sure where to start, and being a college student, I am reluctant to take it to a mechanic mainly because of cost.
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Thursday, September 13th, 2007 AT 11:47 AM

8 Replies

Tiny
SERVICE WRITER
  • MECHANIC
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HAve you set your tire pressures correct?

How is the air filter?

The Oxygen sensors can be checked for weakness as well as the engine coolant temperature sensor.

A bad thermostat can also hurt mileage.

Is the check engine light on?
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Thursday, September 13th, 2007 AT 12:11 PM
Tiny
RUNNING_GUY08
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I check tire pressure at least once a week so they are correct, balanced, and rotated. The air filter is very clean, I checked that this morning. The check engine light is not on and I have never seen it come on.
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Thursday, September 13th, 2007 AT 1:25 PM
Tiny
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A 20% drop in mileage is significant.

The sensors above I mentioned can be checked with a quality scan tool.

If the engine is taking too long to warm up it may be related to those or the t-stat.

A dragging brake (s) can affect it also. A change of driving, from highway to more of a city driving pattern are other possiblities.
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Thursday, September 13th, 2007 AT 1:39 PM
Tiny
RUNNING_GUY08
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  • 9 POSTS
I have suspected for some time now that there is something up with the mass air flow sensor. From time to time, the engine will rev up to at least 4,000RPM when shifting into park. And it does take a while to warm up at times and idles high. As far as driving patterns, I have been doing a LITTLE less highway driving but I do not accelerate hard in city driving so I don't burn through gas. Would it be a good idea to go through a few tanks of gas and see how the mileage changes from tank to tank? I have also used a fuel system cleaner within 3 months if that makes any difference. The car has been running smoothly, although when the engine gets hot from driving or if I am accelerating up at hill of any kind (Ex. Driveway, highway on-ramp, etc.) It makes a relatively loud knocking/lugging sound when the car is running at lower RPM's. Does the thermostat control the engine cooling system? I have heard the fan turning on in the engine compartment and the car has never overheated.
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Thursday, September 13th, 2007 AT 2:02 PM
Tiny
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The auto manufactures (pushed by the EPA), want the best fuel economy possible or the least amount of emissions anyways. The most emissions emitted are when the engine is not at operating temperature. So the goal is to get that engine up to temperature as quickly as possible when you start it. When the engine is started, a richer fuel/air mix is used, thus a higher idle. The engine being the heat hog, doesn't want to heat up anymore coolant than necessary. The thermostat is closed at this point, restricting full circulation thereby heating the least amount of coolant and keeping the temperature in a rising situation. When the temperature reaches about 195 farenheit, the thermostat opens and allows coolant to start moving throughout the system. When the temperature reachs are 220 or so the coolant temperature sensor sends a signal to the computer and the fan kicks on bringing the temperature back down, thus preventing overheating.

At this point I think we should start evaulating the thermostat.

The knocking noise your hearing may be pinging, it is like a marbles rattling in a tin can. I agree with the possiblility of a mass air flow sensor problem. I waould gain access to it and spray the internal electrical portion of the sensor with Electrical cleaner, I'm not comfortable with anything else for that.
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Thursday, September 13th, 2007 AT 7:34 PM
Tiny
RUNNING_GUY08
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Thank you so much for your help! I will definitely check out the thermostat first thing tomorrow morning. As for the Mass air flow sensor, I will find it and check it's condition out. If need be, I will try to replace it and see what happens then since I've seen that it is a relatively inexpensive part. I will definitely plan on posting back and explain my progress and any solutions!
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Thursday, September 13th, 2007 AT 9:44 PM
Tiny
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Don't replace the MAF, try cleaning it, it will probably work.$3 cleaner.$100 maybe for the sensor.
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Friday, September 14th, 2007 AT 6:07 AM
Tiny
RUNNING_GUY08
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Alright, here's the status right now. I got my tires rotated and balanced and found that my front brakes were 99% worn, so I installed new front brakes. I also cleaned out the mass air flow sensor with "Mass Air Flow Sensor Cleaner." The high idle problem has been a little better, but it still idles high and has revved up a couple of times. The air filter looks a little dirty so I am going to pick one up and put it in this evening. The gas mileage has significantly improved but still isn't as good as it was. Do you think it has anything to do with the fuel filter of the fuel pump?
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Monday, September 17th, 2007 AT 6:57 PM

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