So while I have done a few minor things to my new 91 240 (Stereo and interior stuff), I have done nothing under the hood save for changing the oil and airfilter. Yet some how it has gone from 25mpg down to 20-21.

The first tank I had her for she pulled in 25 (50/50 city/hwy), next tank was 23(70/30 C/H), and now she is in the low 20 range (80/20). What is going on? I don't run the AC, i just keep the windows down and the roof open. What can I do to get her back up to the mid 20's?

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85 240DL 189k; 91 240 169k

If you do plan on changing your O2 sensor you can go with a Bosch #13913 for a 86 mustang. Many people use these as a replacement but you'll need to splice the wires and seal them well. They are around $30-35 vs $75-150 for the Volvo Bosch part. Do a search, there's much information about the swap.

Your decline in mileage seems about right considering your increase in city stop and go driving. Last week I pulled 31.6 mpg and 32.1 mpg in my 92 on two trips with about 75/25 hwy/city. You can increase your mileage though. How long has it been since a complete tuneup? Has the O2 been changed in last 50K? If it's been awhile you can increase your mileage by a few mpg which will more than pay for itself considering the price of gas.

i gotta ask, i have been all over my '85 244DL, and, i know it has one, but where the heck is the 02 sensor?? must be relly close to the exhaust manifold, because when i was checking the inline pump and fuel filter, i had a good look around, and, it's not on the lower, or middle of the pipe... i don't have a good manual , yet, so ,i'm on an info gathering mission. if anybody else has changed it, and knows, i'd appreciate some help on findin this now! thanks, louis

It's in the exhaust manifold. Look at the manifold and follow the path of the exhaust, you'll see the O2 sensor. I changed mine from above the engine. Later LH systems have the sensor down near the catalytic converter.

O2 sensor could be on the exhaust pipe, right near the catalytic converter.
Sticking out horizontally towards the driver's side.

A few ideas:
1. How long since a tune-up? If it has been awhile, a replacement of plugs, dist. cap and rotor, inspection of the plug wires, and setting the mixture (AMM) may help, can't hurt.

2. How about tire pressure? Keep the tires on the hard side. I run about 34 psi.

3. At highway speeds, some cars get better mileage with the wndows closed and the AC on. Something to do with added wind resistance from the open windows. I don't know if this is true about the 240 though.

4. While it's an older 240, my daughter's '84 240 ,auto tranny, gets 20 to 22 in city driving. It has been years since I took it for a highway drive but I think I got about 28 then. Since you are doing mostly city driving, the low 20's seem about what to expect. I'm assuming you have the auto tranny. If you have the 5-speed, you should do better.

Well, you said it yourself. You're getting around 25 mpg when you split driving time between the city and highway, 23 mpg with 70% city, and low 20's with 80% city driving. Sounds about right to me. You can't expect to get the same gas mileage with an increase in stop and go driving.

I drove out to Austin this past weekend and only got 20 mpg. When I drove the car home from Dallas a month ago I got 360 miles to the tank. I filled up on Monday and only got 270 miles.

Even with 20-40 miles of traffic time, that does not make sense.
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85 240DL 189k; 91 240 169k

If you want to address fuel consumption, you need to measure miles driven and gallons bought when you fill the tank.

Zero the trip odometer every time you buy gas.
Next time you fill up, you have your actual miles available.
You can't judge properly if you judge by "tankful" when you decide to fill up.

I think I may have just gotten some improvement in my 245 (non-turbo, auto trans) by taking care of a couple bad vacuum hoses. It's a good thing to check.

The hose to manifold from Flame Trap was a problem. It seems to be metric, and a tough size to find. We had 2 cars out of 3 with a slightly-too-large US size hose there, apparently leaking vacuum.

Same goes for the little curved one from manifold to the thin hard plastic tube.

Of course they all need replacing now and then.

Just don't get into the habit of resetting your trip odometer, unless you have replaced/want to replace the plastic 25-tooth gear in your odometer.
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'89 244 GL -- 111,XXX miles (see profile for info on car)