I recently replaced the engine in a 2006 Liberty due to a valve head dropping into cyl #4. The engine was a low mileage (27000) unit coming from a wrecker in florida. The engine came complete with fuel pump, turbo, injectors, rail, and all fuel system components attached to the engine. I put a new timing belt on it before installing it just to be safe.

After all was said and done I have one lingering issue. After a cold night outside, the engine starts fine but gives a CEL, with code P1202 Fuel System Overpressure, which puts the engine into a limp mode with little to no boost. If the engine is warm, restarting it will result in no CEL, restored power, which lasts the rest of the day until another night outside.

I have the entire fuel rail from the old engine with the pressure control unit and the sensor still attached, also the entire high pressure fuel pump from the old engine, and 3 of the fuel injectors.

Any suggestions on what to do to diagnose this problem? Luckily shotgunning parts at it doesn't cost money for parts, but I am limited on access to this vehicle, it belongs to a friend who paid me to do the swap for him.

1st or 2nd gen fuel head?

Try the other sensor, might even try the rail.

Is the ECU you're using coded for those 4 injectors? Did you flash it with the new information? That may be why it's throwing over pressure codes.

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Silver 2006 KJ 2.8L CRD Limited:
GDE ECO Tune / Fumoto Valve / E-Tecno GX3123 7v Glow Plugs / 2nd Gen Fuel Head / Mishimoto RED Hoses / Rockers/Lifters @ 114k / Hayden FC + 11 Blade Nylon / Sears P1 Battery / Transgo 45RFE-HD2 Reprogramming Kit / Timken Front Bearings

Do you have an Android device? You can get the torque app and a Bluetooth obd reader and read the value of rail pressure from the ecu. Maybe see if it's acting funny.

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05CRD: GDE Hot ECU & TCM tunes, Provent, Cat filter, Facet lift pump, TransGo kit, Florida TC, Samcos, stainless brake lines, HDS thermostat, Renegade light bar,
RL super sliders, Bilstein adjustables, Al's Gen 4.5 Arms, 235/85-16 Duratracs, DTT rear, Elocker front, EVIC+TPMS, Turbo timer, McNally pillar gauges, Weeks Stage II kit.

The codings of the injectors won't have any bearing on the overpressure setting.

Either the MPROP on the pump is stuck open, or the PCV is jammed shut.
I would make sure that you dont have the cyl 4 injector and the PCV connector swapped since they are the same.

Otherwise, i would swap the MPROP from your old running engine to this one and see if the issue clears up.

It is a second gen fuel filter head, I changed that with the engine. I'm fairly certain that injector 4 plug is correct, otherwise we would have power problems all the time. I am thinking that if the engine only has 27000 miles on it then it must have been sitting for a long time. I am going to try to run some diesel purge through it to see if it will clear it up. I will report back after this is done.

After spending the night in a warm garage the jeep started great and ran great all day. I had my friend bring it over and we ran a full can of diesel purge through it. Tonight he is going to park it outside again and we'll see how it works tomorrow morning.