poor gas mileage 258 fuel injection MOPAR

[nana]

I have a rebuilt 258 engine (Mopar EFI) with less than 3500miles on it. Usually I got 14-15mpg in the city and 19-20mpg on the highway. However just recently for the last 600miles the fuel economy dropped to 10mpg in the city, the highway mileage was almost the same. I didn't change my driving style. The check light gave me only the error code 55, which means the computer doesn't detect any problems.
Here my question:
What can cause this efficiency drop particular for city driving. Could be there any problem with the O2 sensor even the computer check light dosen"t complain. The power seems to be Ok it.It looks like the motor is just running too rich. I've check already the temperature sensor-- seems to be OK. Thank you for any help in advance

[Bounty__Hunter]

Is the catalytic converter original? It could become plugged and hurt mileage, especially if the old carbed engine dumped unburned fuel into it.

You'll notice the power loss if it's the converter and it gets worse.

[firemanharry]

What is you gauge reading? I've seen a rich burn problem occur when a FI motor does not get up to temp. Could be your thermostat is stuck open. Seems cheap enough to look into.

[BENT]

i have the same engine, and had the same mileage and now im down
to 10mpg also, it droped in like a couple days. ive replaced the exhaust manifold with a hedman header, replaced the o2 sensor, put k&n on, took off cat converter, and little other things. it ran better, more power, but the gas mileage stayed down. i dont know what it could be.

[sillsmack]

Sounds like you both are suffering from
>vacuum leaks
>timing off