wtf is up with my mpg?!?!?!

[lsmalley]

I have a 1990 190E 2.6L and I think that my fuel economy is declining, but I can't be sure because I have nothing to compare it too. My tank is about 14.5 gal and on a full tank in city driving I get about 280 miles till it runs down to R and the light comes on......But on the other hand, if I am on the highway I notice that if I reset the miles to 0 and drive on a full tank after about 50 or 60 miles the needle is just approaching the 3/4 mark. What could be the problem? and what does it mean for a car to "run rich?" I just picked that term up looking through the forum. Does the check engine light being on have anything to do with it? If I disconnect my battery for a min or 2 the CE light goes off and will stay off untill I turn on my A/C. Is that a separate issue?

[brewtoo]

Is there a problem?

try miles driven / gallons consumed = MPG

[alienman]

Perhaps I'm reading your first inquiry incorrectly, but if you get around 60 miles with less than a quarter tank and if we assume that you can drive 70 miles with a full quarter tank, then 4x70 = 280miles.

[G-Benz]

Running "rich" or "lean" are terms implying fuel-to-air ratio when the mixture is in the combustion chamber (where the pistons do their thing). The optimum ratio is 14.7:1 air-to-fuel, or 14.7 parts air to 1 part gasoline. The engine management system strives to maintain this ratio and makes all sorts of adjustments to do so. The emissions system (O2 sensor) monitors the exhaust output and tells the management system how well it's doing based on what is sniffs out.

A "rich" condition is when too much fuel is being used in the mixture. The symptoms are poor gas mileage, engine miss, and the smell of raw gas out of the exhaust pipe. This will foul spark plugs and the O2 sensor as well as ruining the catalytic converter over time. The check engine light comes on because the system is unable to adjust within parameters in order to get the mixture back to optimal.

A "lean" condition occurs when not enough fuel is being sent into the mixture. This manifests itself in poor performance, higher engine operating temperatures and if left unchecked, over time can burn pistons!!

Since you seem to notice a degredation or fuel economy, you are probably experiencing a "rich" condition. The check engine light confirms this, and since the engine works harder under load, switching on the A/C magnifies the problem.

Throrough diagnostics by an experienced tech can pinpoint the source of the problem. I don't recommend "throwing parts at the issue", as there are several things that can cause this...it can get expensive buying parts for these cars and installing to no avail.

Offhand though, I believe a new set of plug wires would be needed, as well as a new O2 sensor. If the bad plug wires caused the "rich" condition, the sensor is probably fouled as well...

[mpolli]

The only way to calculate MPG is: fill up the tank. Drive some distance. Fill up the tank again (preferably from the same pump). The gas you put in is how much you used. MPG = miles/gallons

Gas gauges are dreadfully inaccurate.