I recently installed new NGK TR-55 V-power plugs (gapped at 0.035" because the opti is high mileage and possibly weak) and Duralast replacement plug wires on my bone stock 1994 M6 LT1 TA, only to see fuel economy go from 13 mpg in city driving to 11 mpg.

My old 94 TA, with an aftermarket muffler, !cat, and CAI used to easily get 15mpg around hilly Fayetteville, AR, and 28 mpg on the highway cruising at 80-85 mph for the most part. I had retuned the PCM with Tunercat for a leaner part throttle tune though, but it was by no means radical.

I searched the boards and found that nearly everyone's LT1 gets at least 15 mpg under the same driving conditions. As if 13 mpg wasn't bad enough for me, it's gone down to 11 mpg !!

I have tried different driving styles but it doesn't seem to make any difference at all. It stays the same whether I shift up at 2k rpm or 2.5k. FWIW, it doesn't lug at all shifting up at 2k. The motor's got PLENTY of torque to pull along just fine.

How can I test the O2 sensors to determine if they are responsible for the poor fuel economy? Is there anything else I can do to improve the situation. It sucks to get SBC power for big block fuel consumption, especially when gas is nearly 3 bucks/gal around here?

 

If your O2 sensors have more than 50K miles on them, just replace them with new AC Delcos. It's possible the narrower spark plug gap isn't helping your gas mileage.

 

I had a bad stumble/hesitation on throttle tip-in at pretty much any rpm after installing the new plugs. After much research, I determined that the incoming fuel/air charge was blowing out the spark at higher plug gaps. The narrower gap cured this problem, and the motor is actually much more responsive than it has ever been. This will have to do until I can diagnose the weak link in my ignition system.

I too, have a suspicion that the O2 sensors are past their prime.

 

A stock engine will not blow out the stock gap of .050" unless you have a weak ignition system.

 

I get about 14-16 mpg in town and about 20-22 mpg on the highway..is that normal?

 

I'm in the same boat you are, and lets not even talk about milage with the air on If you end up replacing you o2's let us know if helped any.

 

Is it the front or the rears sensors that calculate the gas needed?

 

hahaha fuel mileage. I get less than 10. Damn my lead foot.

 

Front sensors, the rear 02 sensors on the 96-97 cars are only there to monitor the efficiency of the catalytic converters.

 

I was hoping I wouldn't have to be the O2 sensor guinea pig. Hopefully someone else who has tried it already will chime in.

I know my ignition system is weak. Hence, the retardedly narrow gap. However, the improvement in throttle response should be an indication of more complete combustion, if anything. Theoretically, the motor shouldn't be wasting any gas now. Then why the drop in fuel economy?

 

So you believe you have a weak ignition system and you compensating by reducing your spark plug gap, and then you wonder why you're fuel mileage is down GM didn't set the gap at .050" for the hell of it. I run a gap of .038" but I have a capacitive discharge ignition system and a higher voltage coil to make sure I have a hot spark to manage my 10# of boost.