Automobile(s): 1992 Seville STS & 1991 Sedan Deville
My STS has new plugs, wires, cap, rotor, remanned bosch 3 injectors, o2 sensors, ECM, fuel filter, timing is set dead on 10 degrees BTDC, and EGR tubes are 100% clean as I can get them. Car still has a slight miss to it. Does have 20 inch rims and no cat but I can't see that = to that bad of gas mileage.
91 Deville I fear has a cat issue or injector issue. Has new plugs, caps, wires, rotor, and fuel filter. Also have timing set right. Feels like she is being held back when I get on it. Does not pick up anywhere near as good as the STS. Never seen the cat red hot but it has a small exhaust leak right in front of it a sign of possible backpressure. Also on initial startup it seems to run fine but after a few minutes it seems to start running rough.
AC Delco 91-402 I do believe is the number in both of em for plugs with top of the line parts for the rest. Neither get good gas mileage and it is driving me nuts. I can deal with 15mpg city but I am nowhere close right now.
Forgot to mention both fprs are good STS has a new one.
91 Deville, check your fuel pressure. My guess is fuel pump is going bad, had the same symptoms on a '92 Eldorado. You can rent a fuel pressure test gauge at Autozone, they refund all money when you return it.
You should be able to run diagnostics through the DIC in the dash...
The other systems that greatly effect gas mileage and power are the TPS, MAP, and EGR. Exhaust leak might not be signs of back pressure, could just be a bad gasket. If the leak is bad enough, it can throw off the O2 sensors...