[reekor]

Check your fuel pressure with engine at idle should be around 50PSI. With engine and pump off should hold above 30psi for 5 minutes.

Originally Posted by alaskagoldman

I have a 1998 Grand Cherokee Laredo. I suddenly noticed that the engine was running rough and my fuel mileage dropped by about a third. BLACK smoke shoots out the exhaust when I accelerate. Runs better at high RPMs than low ones. I will manually shift my auto transmission because it runs better with higher RPM shift points. I have changed plugs, wires, dist cap and rotor. Plugs were BLACK and carbon fouled. Problem remains and Check engine light is on and trouble code still reads P0171.

I am constantly looking for a vacuum leak because the code is detecting a lean fuel mixture and pumps too much fuel in to try to compensate.

The engine is 4.0 L straight 6 with automatic transmission. I listened to the fuel pump underneath the car when it was running. High pitch whine coming from inside the fuel tank. Any ideas will be appreciated.

Oh, I have also purchased a new Oxygen sensor but have not installed it yet. Was hoping that maybe this might help out. I did remove the current one, the one before the catylitic converter and cleaned all the carbon off of it.

HELP! I can't afford to only get 150 miles to a tank of gas when it costs 90 dollars to fill it these days.

most recent maintenance BEFORE the problem started was i replaced a worn serpentine belt.