Sudden drop in gas mileage

I have a 96 Saturn (5 speed) that for its lifetime has gotten between 34-38 mpg (about 370-400 miles per tank). Recently (in the past 3 tanks of gas, or so) the mileage has plummeted. I’ll be lucky if I get 320 miles out of this most recent tank of gas. Within the past 4-5 tanks of gas, I have given it a dose of fuel injector cleaner so I don’t think it’s too gunked up.

Does anyone have any idea why it has experienced such a sudden and precipitous drop in its gas efficiency?

When’s the last time you changed the spark plugs? Is the check engine light on (or burned out)?

When a vehicle displays a sudden drop in fuel mileage, the first two thing to check are the thermostat, or the coolant temperature sensor for the computer.


I think I’m probably due for new spark plugs.

The check engine light is not on, I don’t think it’s burned out. . . but not sure how to check on that?

It [check engine light] should come on when you turn the key on, and then go out when the engine starts. If it doesn’t, then it is burned out. Was it on before?

No it wasn’t on before. I think it does turn on when i turn on the car.

Outside temperatures have gone down in recent weeks throughout the northern hemisphere and it is typical to lose a few mpg in colder temps. Check your tires air pressure and your air filter.

Yep, agree. In addition, I have had a severe mileage problem from a bad O2 sensor. From about 17 to 25+ with a new sensor and never sets a code or engine light. If you take it in for the plugs, get a diagnostic for $100 to check the O2, and the other sensors mentioned. Also check your RPMs to make sure transmission is functioning as before and not slipping or failing to go into high gear.

Changed the sparkplugs at home and will see how that works first.

Thanks for all the helpful suggestions.

So far, the check engine light hasn’t come on for anything, so I am guessing (hoping) the sensors are all working fine? Or is that a bad assumption?

A code will not be set for a thermostat that does not allow the engine to warm up sufficiently or a Coolant Temperature Sensor that is out of calibration that fools the computer into thinking the engine is still warming up.

In that case, what is a good indicator of/simple method for determine whether the thermostat is out of wack?

I always check the quickest, most inexpensive things first. I would check the air pressure in the tires.

I agree with all the above, but Saturns have a unique problem, the intake manifold gasket. It is not supported well around the #1 cylinder and has a tendency to get sucked in there.

If none of the above work AND your idle is running around 1100 rpm, spray a little WD-40 or carb cleaner around the gasket at #1 cylinder and see if it affects the idle. If it does, you need a gasket.

Get an after-market Felpro instead of the OEM. it is a little stiffer and will last longer.