kylew707

Alright I have a 97 Z28 automatic. When I bought it I fixed a broken fuel line (2 places) and two different exhaust leaks along with a misfire cause by burnt wire from touching the header. I was getting this kind of gas mileage but then replaced the coolant temp. sensor in the water pump. For a couple of days I got normal gas mileage, mid twenties on the highway and 15-20 in town, but now I'm back down, WAY down. When I took out the original sensor one of the prongs where you insert the plug was rusted and bad, so today I replaced the plug-in too just in case but no luck, still getting bad mileage. The only code I have is P1351, igniton control module if I remember right. Any ideas on why I'm getting this bad of mileage? Also before I replaced the sensor the car would have warm start problems and was running really rich. Now its still running a little rich and bad gas mileage but no starting problems. HELP!

Injuneer

Any idea how old the O2 sensors are? Any exhaust leaks before the O2 sensors? Any misfires? All those can cause it to run rich.

Check the fuel pressure. Check the vacuum compensation line to the fuel pressure regulator for wet fuel.

Have you pulled the plugs to confirm the rich condition? Perhaps they are fouled and causing misfires, which will simply make the rich condition worse.

kylew707

No idea how old they are but now im even more stumped. I put $10 in it and got around 40 miles (12.7 mpg) on the highway, when i got to my destination the gas gauge had went from 1/2 tank to between empty and 1/4th. Now the strange part. Before my 35 mile trip back home i put another $10 in it but by the time i got home i only used about 1/8th of a tank. Car didnt act any different just got better mileage. I have no idea whats going on!

Injuneer

The only accurate way to check mileage is by filling it up, and dividing the miles travelled by the gallons. The gas gauge does not work in a "linear" fashion. Typically, it will show above "F" for the first 5 gallons, drop to "1/2" for the next 5 gallons, and drop like a rock from "1/2" to "1/4". When it shows "1/4" you're lucky if you have a gallon left. Do NOT run it that low, or you will run out of fuel, and you'll be back asking why your fuel pump died.

http://shbox.com/1/4th_gen_tech2.html#fuel_gauge

kylew707

I dont usually run it low but when i have, i have been on E and still went a couple if miles. I was just getting a rough estimate. Either way i got a lot better mileage on the way home.