Alright, this car has gotten 12mpg or worse since I purchased it a couple of months ago. It's an '84 Berlinetta.

In the time that I've had the car I've replaced:

oil/fliter
spark plugs
plug wires
distributor cap
rotor
pcv valve
fuel filter
air filter
oxygen sensor
set timing

I'm getting anywhere from 10-11 MPG now! I am shooting a 54 trouble code, which I'm told is for the mixture control solenoid.... how much could a bad mixture control solenoid throw the mileage off? And how would I obtain one of these, seeing as how they aren't carried by auto parts chains and it's discontinued by the dealer?

The car does run extremely rich, but it runs smooth. No bogs, hesitation, flat spots, it idles wonderfuly, etc. I'm at a loss now, what can I do to get my mileage up to a reasonable level?

 

I just typed that into ebay and there were several new ones for sale. I have no idea though what model you would need.

How is the alignment?

 

The alignment is good. I see the part you're talking about on Ebay, $50 buy it now seems reasonable. Could it effect mileage that much?

 

if its the (fuel) mixture control solenoid, I would say its your problem.