Re: Severe problems accelerating 2003 Grand Cherokee 4.0L
Welcome to the garage.
I know how you are feeling, it is hard to thrust garages nowadays.
After your friend replaced the parts, did you check for fault codes again? Do you remember which fault code you had before?
Eventhough there could be a problem with the cat, there could be several other things as well. For example a low fuel pressure due to a pump which is going out.
I don´t know the 4.0, but most probably there is a valve on the fuel rail, where you can check the fuel pressure. When you don´t have a pressure indicator, you could do a quick check as follows: with a cold engine (to prevent fire), switch the ignition in on and then press the schräder valve in on the fuel rail. With a good pump, fuel under high pressure will come out. Watch eyes and clothes and use a rag to catch the fuel. If the pressure is low, or the pump is bad, or a seal in the pump unit.