Cadillac Repair: 1991 Cadillac Deville 4.9 Fuel Problems, poor gas mileage

Question
Dear Sir, I have a 1991 Cadillac deville. I just got the fuel pump replaced and now their seems to be an extreme amount of pressure streaming out when I undo the cap. I am worried that the car could blow up. Also the car tops out at 10 miles to the gallon in the city, although I live in Las Vegas and the driving conditions are extreme, and the air is always on in stop and go traffic: I used to get about 13 to 15 miles to the gallon in the city


Answer
Hi Richard,

        Since you noticed that the gas cap released alot more pressure since the pump was replaced I would have the shop recheck their work to make sure that a vent line wasn't pinched or bent closed. Otherwise up at the radiator there is a vent valve that can get plugged and need to be cleaned or replaced.

         As for the mileage that really is something that a person would need to check firsthand with a scan tool or onboard diagnostics to see if there are any sensors reading to the high side that would cause more fuel to be used. For example if the oxygen sensor is always between 0 and .45 volts then it will use more gas. If the MAP sensor reads a higher number than 30 at idle then look for a bad sensor or cracked rubber vacuum hose at the sensor or throttlebody end. The throttle position sensor could be sticking in the open position at idle or be adjusted too high. Or another one to look out for is a leaking fuel pressure regulator. Look on the fuel rail and you will see a valve with a vacuum hose. If you remove the hose and gas comes out replace the reg. If you start the engine and run it for 2-3 minuits with the hose off and tap litely with a screwdriver handle that can show that the diaphram is leaking and need to be replace. Of course if the engine hasn't had a tune-up in 30,000 miles then that can help. Especially if the engine has original cap, rotor and wires with the car having 100,000 miles or more. Then I would replace it all. Fuel injectors can act up and short out. What the computor will do is send more fuel through the remaining injectors that are still good and that will cause the engine to use more gas.

      As you can see there are alot of things to check and adjustments to verify so if you plan to do some of the things yourself you will need a good factory shop manual or go to alldata.com and subscribe to the manual for your car and then you will have all the info you need available to do the checks. You probably will need some special tools so do what you can and have your mechanic do the rest. Hope that helps raise the fuel mileage. Bill