When the engine is cold it needs more fuel to run and from the description your not getting enough which could be due to a weak fuel pump or an injector problem. Try cleaning the injectors first and if this doesn't help then see what the fuel pressure is as it should be around 49 psi and if less than this replace the "new fuel pump" installed.
Also, check the throttle body and make sure it is not all carboned up and i highly recommend running a decard on the throttle body and engine to start with... the engine gets gummed up with carbon and causes all kinds of driveability issues. This is a common problem on the 4.0L Jeeps.
I have one, but is 5.2L
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Hi.
If fuel pressure is low and you have already replaced the fuel pump, then the problem is usually the fuel pressure regulator. The pressure regulator is in same assembly with fuel filter. The assembly is located to the fuel tank, bolted to the truck's frame. The pressure is tested connecting a gauge like this : Set Sealey VSE952 to the port on the fuel rail. The pressure on Grand Cherokee must be around 49 psi plus or minus 5 psi. If engine runs but pressure is below 44.2 psi , check for a blockage in fuel supply line between fuel rail and fuel pump module (e.g.kinked line). If the line is not blocked, replace fuel pressure regulator and filter. Test pressure with and without fuel cap on.
Another test to be done is reading pressure, then shutting the engine off with the gauge connected. If pressure goes down too quick, then the regulator is defective. After 5 minutes the pressure should not go below 24 psi. If it does replace the regulator.
If regulator is OK check for leaks at the injectors that can cause fuel pressure drops.To check for injectors leakage clamp off the rubber hose of the pressure gauge tool between fuel rail
and the port. If pressure staysr above
24 psi and before was dropping, then a fuel injector or the fuel rail is leaking.
Below a diagram representing the fuel rail and test port. The engines from same years like your six cylinder have the port and rail in the same position.
Regards.
Ginko
RE:"Also I found the solution on another forum: The upstream O2 sensor was
bad and trying to regulate the fuel mixture before the engine was warmed
up. Replacing the O2 sensor fixed the problem."
Try that. As I said there are many factors that may result in lean mixture. A defective O2 sensor is one of the possible faults. If the problem is the O2 sensor you will get an OBDII code. The problem is not necessarily the O2 sensor.
I have done my best to help you. I am sorry about your decision.
Regards.
Ginko.
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I would have the fuel pressure from the fuel injection checked first.
Check if removing the hi-tension cable one at a time would cause the engine idle to worsen or stay the same. If there is no change for a particular cable check if there is spark or if it is weaker than the others then you may need to have the cable replaced. For a pair or more cable with issues it may be a bad distributor coil.
Check also for leaks or worn out vacuum hoses that may need replacing.
Check also the Idle air control valve if items above are ok. A dirty or bad idle air control valve will cause the same issues. If you also have a spare carb cleaner see if a spray at the intake while the engine is revving would help. It may clear out dirt in the intake and in the MAF sensor.
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It seems the expert is not familiar with the engine in this vehicle.
I do not want to ask any more experts. Either I worded my questions poorly or they do not understand it.
Also I found the solution on another forum: The upstream O2 sensor was bad and trying to regulate the fuel mixture before the engine was warmed up. Replacing the O2 sensor fixed the problem.
The problem was the upstream O2 sensor. The temperature sensor on it had gone bad so it was trying to regulate the fuel mixture as soon as the engine started. Normally it does nothing until reaching 300 degrees. Replacing the O2 sensor fixed it.
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