It has been proven numerous times that the Jeep 4.0L and Bosch O2 sensors do not play well together. It is very highly likely that the CEL trouble code will clear up by itself after you install an NTK or genuine Jeep O2 sensor.
Exhaust leaks and O2 sensors have no effect on the fuel pressure drop off. The 1997-2001 fuel pump check valve is supposed to hold pressure in the fuel line while the engine is off. The check valve is located on the fuel pump and located inside the gas tank. A failed check valve allows the fuel to drain back to the tank. Check valve failure is very common. This results in one or more of the following symptoms:
　
SYMPTOMS
　
• Longer than normal cranking times
• Never starts on first try, almost always starts on the second try
• Rough idle for a few moments of idling
• Rough running for first ½ block of driving
• Little or no fuel pressure at the fuel rail test valve similar to this -
　
1- Key in on position; fuel pump primes for 2 seconds then stops. 0 PSI
2- Cranking over the engine. 2-3 PSI
3- Engine starts and idles for 20 seconds. < 5 PSI
4- Engine idles and fuel pressure creeps up to 45-50 PSI.
　
　
POSSIBLE CAUSES
　
• Failed check valve on the fuel pump in the gas tank.
• Leaky fuel injector(s) may be the true cause of your problem, but more likely they are simply contributing to the check valve issue.
　
Testing of the fuel injectors should show if any are faulty and are allowing fuel to drain into the cylinder. Either problem can allow heat soak to vaporize the remaining fuel in the fuel rail, and you may be dealing with a bit of vapor lock as well, especially in warmer weather.
　
1. Connect the gauge at the fuel rail and start the engine. The gage should read 49 psi plus or minus 5 psi.
　
2. Turn the engine off and immediately clamp the fuel line just ahead of the fuel tank. Watch the pressure gage and see how long it takes to loose pressure.
　
If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - probably the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably one or more leaky injectors.
　
There are a few solutions to the problem –
　
1. Turn the key to ON for 5 seconds and let the fuel pump prime. Turn the key to OFF, turn the key back to ON for 5 seconds, start the engine.
　
2. Crank the engine for 3-5 seconds, pause, and crank the engine again. It will almost always start on the second try.
　
3. Replace the fuel pump assembly in the gas tank. The check valve is not a separate part and cannot be serviced. This is both expensive and time consuming because you have to remove any hitches and skid plates and then remove the gas tank from the vehicle.
　
You can use solutions 1 or 2 for as long as you want to, they do not harm anything and the fuel pump will continue to function for many more years.
　
　
If you do replace the fuel pump assembly, purchase a high quality OEM style pump assembly such as Bosch or Carter. Some cheap aftermarket fuel pump assemblies, like Airtex, and Precision don’t hold up and will fail completely within 12-18 months.
