Our 1990 240 has been getting poor gas mileage lately. Last checked it was getting 18 mpg in city driving. My experience with our other 240s is that we should be getting around 23 mpg in this sort of driving conditions.

So, on we go to check out the oxygen sensor. After warm-up during idle, we see the voltage swing between 0.075-0.825v. This seems a littler lower than the 0.1-0.9v that is spec. I do notice a little exhaust smell around the engine and suspect that we have a leak. Will an exhaust leak upstream of the oxygen sensor cause these lower voltage readings?

Also, can anyone suggest a method for detecting exhaust leaks around the exhaust manifold and/or downpipe? Just feeling around with my hand for puffs of air have not yielded results.

 

You also have to look at how rapidly the 02 sensor responds. It should go high/low at least once a second. An exhaust leak won't lose you 5mpg, nor will an 02 sensor. However, an old 02 sensor will reduce your mileage a little, maybe 1-2 mpg and if the 02 sensor is old, they're worth replacing for general improvements in mileage and running anyway.
I test for exhaust leaks by using a length of hose, hold one end tightly to your ear and direct the other end around all exhaust joints.
I would start with a thorough tune up, including air filter.

 

Tune-up has been done. Air filter is clean. I like to keep my cars running well. I'll try the hose trick to see if I have exhaust leaks.

If the exhaust leaks nor O2 sensor would reduce the mileage, then what other options? Temp sensor? I kinda thought I would get a code thrown if it was bad but I get no codes from the diagnostic sensor.

 

Keep in mind, exhaust leaks introduce oxygen into the stream and fools the O2 sensor. It thinks the engine's running lean so the ecu throws more fuel at the injectors making the car run rich. That hurts mileage and destroys your converter.
Mileage on these cars varies depending on transmission, rear end and manner of driving as well as state of tune. Still, I am amazed at the number of owners getting much better mileage than the EPA predicted norm. These were never particularly fuel efficient vehicles and 18 mpg seems ok to me...19 and 25 were the original mpg est. for a M47 245.

 

My expectations on mileage are based on the other 240s I have owned - all automatics. Our '89 wagon always would get around 24-25 on the highway (the only time I actually drove it was on the highway so I don't have city readings).

My daughter's '91 240 sedan gets slightly better mileage on the highway and around 22-23 in the city.

My '90 starts, idles and runs really well, has good performance, etc. Just gets poor mileage. The exhaust definitely smells like it is running rich.

I guess I will chase down the exhaust leaks and re-check.