1994 Geo Prizm Excessive Fuel Consumption

Hi,

I have 1994 model year Prizm (1.8 L engine, 4 speed auto transmission).


I do not ever remember riding more than 250 miles (highway/city mixed) on a full tank of gas. The previous owner installed a performance type exhaust/headers. The car failed emissions test the first time I took it there because of high NOx readings.

The car does not have an EGR system (factory original).

Failed test results:
HC 0.28 g/mi (13% greater than reference car)
CO 1.18 g/mi (73% less)
NOx 2.69 g/mi (135.8% greater than reference car)
1% Lean

The last passing test results:
HC 0.8 G/MI
CO 15 G/MI
NOx 2 G/MI

After adding some Lucas injector cleaner and with the help of tester (tried several times) it passed.

Since then I had a coolant flush done as a preventative measure.

What could cause this high NOx reading? What causes my poor gas mileage? Can both be related? What do you suggest I do to address the issue?

Thanks a lot :)