Thread: Loss of mileage

Over the past 6 months my mileage has significantly decreased (30 to 20 mpg). However, the car seems to be running fine...no issues except it does appear to be running fairly rich. I know clogged main cat can cause poor mileage, but could that also result in a rich condition ?

EDIT: Going to take some more knowledge before I state that.

What car, what mods if any, etc.?

Rich = Probably bad O2 sensor. They go out every 100k or so miles. Spark plugs and wires are a good replacement too.

O.K. 91 Ser, with all the bolt-ons, including Wolfe ECU. New O2 not too long ago. 240,000 Compression 155 to 158. Original cat

i agree. maybe a good cleaning of the maf sensor too would be good

and regrounding it lol

Any new parts since you noticed the drop in mileage?

None...I did have an episode about 6 months ago where the coolant temp. sensor went bad and it took me a while to figure it out. during that time I'm sure the fuel ratio was way rich,as the car didn't "think" it had gotten warmed up

tune up time IMO

The stock cat is pretty restrictive, and if clogged, can actually skew the o2 sensor reading. It's likely rich because of the stock cat.