1998 Grand Cherokee Laredo P0171 code

[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.

[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.

[ken729]

I had exact same problem with my 1996 Laredo about 3 weeks ago. It was the pre cat o2 sensor that failed.

[alaskagoldman]

I changed the O2 Sensor, problem persists... looks like I'll have to check the fuel system for sure. I just hate the idea of taking the fuel tank off the car.

[alaskagoldman]

Originally Posted by alaskagoldman

I changed the O2 Sensor, problem persists... looks like I'll have to check the fuel system for sure. I just hate the idea of taking the fuel tank off the car.

I spoke before I checked the code reader! The P0171 code is gone and is now replaced with a new code.

The new code is a misfire in cylinder 5. So I will now investigate that. My gas mileage is much better now with the replacement of the 02 sensor. I can actually feel the miss in the engine now though.

[alaskagoldman]

Had a carbon fouled plug on number 5 that occurred when the O2 sensor was bad and the engine was running rich. Cleaned the plug and Everything is Perfect! I'm getting better gas mileage now than I ever have since I have been the owner of this jeep.

I hope this experience of mine being posted here helps the next person out.

Thanks to those that responded to my request for suggestions.

[Funcamper3969]

I also have this code, with the similar problems. Mine is actually better in lower gears. I have the rich black smoke, in the exhaust as well as the plugs. I have the P0171 code. It sounds like more gas then less. I changed the plugs, cap and rotor as well as the plug wires. I was thinking of the fuel/air sensor? Any suggestions?