Cant fill more than 2 gallons w/o shutting off. Was running fine now hard to start. Changed fuel filter, pump, maf sensor, air filter. Where to start?
First things first. You should post codes with the definitions behind them. You are more likely to get help this way. Most members will not take the time to reply if you haven't taken the time to look up the codes for yourself.
Your PO171 and 174 codes are both lean codes. Most of the time, these are set when a vacuum leak is present in the intake system. You said the engine was recently replaced. I would check around the intake manifold, intake boot, air filter box etc. for leaks. The codes set when un-metered air is allowed to enter the intake system. Un-metered air is a vacuum leak.
PO191 is for the fuel rail pressure sensor. I cannot speak to the 4.0L SOHC engine, but on the 4.0L OHV engine the fuel pressure sensor/regulator is operated by vacuum. The rubber diaphragm inside the regulator would tear over time and cause erratic fuel pressure readings. This system may only have an actual pressure sensor that is attached to the fuel rail. It is possible that it has failed. To verify it's operation you would need to know the spec at which it ohms out. Then you can check that the sensor is in range. If it is in range then you have a harness issue. The insulation on these harnesses is known to deteriorate over time allowing the wires to become exposed. This could also be the cause of the code.
PO300 is a code for random/multiple misfires detected. This could be anything from a weak cylinder, head issue, spark plug, spark plug wire or defective coil pack. Unless you have access to a quality scan tool, you will not know which exact cylinder is causing the issue without some diagnostic work.
PO451 is a code for evap pressure. Chances are this is why you can only put 2 gallons of gas in the vehicle before the pump clicks off. I would bet that when the fuel pump was replaced the line to the evap pressure sensor was not hooked back up. Could also be a line going to the evap canister itself, which may be where the pressure sensor is located. Some pressure sensors are located at the top of the fuel tank and are only accessible after removing the tank. Either way, I bet the fuel pump replacement is what is causing the no fill issue. Ask yourself, did this problem start before or after the fuel pump replacement. I would get a schematic of the evap system and it's parts so you can identify where the sensor is, what lines are connected to it, etc. Then you can start troubleshooting the issue.
This information should get you started. Hope this helps.
