94 ford mustang 3.8 engine why is no. 5 cylendre running too rich

lots of causes , injector leaks sure. is one of many.
this is EFI and EFI only controls injection.(and spark)

it and all its inputs. do. called sensors.
1998 car in usa the scan tool tells you what all sensors are doing,called PID scans and DTC errors of any kind
25 year old car. and engine and all parts on it.

ALL fuses good?
NEXT:
scan the PCM see P03xx error or others? (scan it)
p0171 And P0174? P03xx codes and more do the scans.
engine good first. for sure this OLD.
we do engine tests.
compression over 150psi on all 8 cylinders.????
vacuum gauge at 20" HG at idle and not BOUNCING
we use tools to find cause. gue$$ing can be endless and super expensive or worse adding on wrong parts.
SPARK:
8 spark plugs all new, and all 8 COP boots new
boots never every lasted, 25years, nor spark plugs
and last fueling , cant burn fuel correctly with bad engine or bad spark CAN not.

SENSORs and stuff:
MAF SENSOR BAD OR WEAK, OR NEVER CLEAN IN 25 YEARS WILL RUN LEAN .
FPR 25 YEARS OLD ARE BAD. THEY ALLOW FUEL PRESSURE TO BE CRAZY TOO HIGH OR LOW
CALLED OUT OF SPEC. (or gets stuck at max PSI)
THE VACUUM HOSE TO FPR MISSING FUEL PRESSURE WILL BE WAY WAY TOO HIGH and super RICH BURN.!
IF FPR VAC NIPPILLE IS LEAKING FUEL IT IS BAD.
Fuel Pressure Engine Running kPa (psi) (28-45)
Key On Engine Off kPa (psi) 35-45)
THIS SPECS ARE DECEIVING,
EFI LAWS, KEY ON PRESSURE IS ALWAYS HIGHEST PSI. (DUE TO ZERO VACUUM)
AND RUNNING ENG AND VACUUM MAX INCHES HG, IS LOWEST PSI FUEL;. IF NOT RUNS RICH AS A PIG.
KEY ON PRESSURE FPR IS 5 TO 6 PSI FUEL LOWER THAN AT IDLE. <THAT IS ITS A#1 TOP JOB THIS.

EXAMPLE 40 PSI KEYON PRESSURE.
THE IDLE WILL BE ABOUT 35.(20 "HG IDLE)
THE FPR DOES ALL THAT.
IF NOT ALL INJECTOR , OVER OR UNDER INJECT.
PERSONALLY I NEVER EVER TRUST ANY OLD FPR. I HAVE HAND VACUUM PUMP TO TEST IT FULLY. AND HAVE FOR 4DECADES.MID 1980S

INJECTOR LEAK DOWN TESTS FAIL OR INJECTOR BALANCE TEST FAILS ,TOLD IN SM= SERVICE MANUAL.
BASICS BAD: IS THIS:
THERMOSTAT BAD. OR MISSING, OR NOT AT 180F TO 200F TEMP STUCK AT 150F BAD BAD BAD.

ECT BAD, SHOWS 150F WHEN COOLANT IS 180F
CAUSING:SUPER RICH INJECTIONS.
SO PCM IS STUCK IN COLD STARTUP MODE FOR EVER AND BURNS FUEL LIKE MAD.

SURE O2 SENSORS DEAD OR EXH CRACKS NEAR BY THEM MAKES THEM LIE AND EFI GOES HOG WILD RICK. SURE.

WHY NOT SCAN THE PCM FIRST.
AND SEE WHAT IS WRONG FIRST.
SAVE TONS OF GUESSING

LAST AND NOT THE LEAST
YOU CAN SCAN ANY PCM AND SEE IF IT IS IN LIMPHOME MODE AND IF IS, IT BURNS FUEL LIKE CRAZY
AND SEE TOO ,CLOSE LOOP MODE IS DEAD
FULLY HOT ENGINE.
YOU CAN SEE LTFT EFI IS STUCK FULL RICH.
AND DTC ERRORS ,LOTS?
THIS IS NOT GUESS IT IS TESTING USES REAL TOOLS. TO FIND CAUSE.

No the electronics will be different. IF you want a replacement engine use only the identical engine from another 94 model mustang or you will have numerous expensive problems.

Its always a good idea to double check codes especially with sensors, if its the heater circut replace o2 if its out of range reset and see if returns

Sounds like a dead coil pack and needs replaced cause you're getting no fire to those plugs and causing a rough idle. Another thing is, that catalytic converter will only turn red because it's plugged up and needs replaced. Have that checked out friend.

Make,year,model,engine? don-ohio

Leaking fuel pressure regulator , or leaking injector's ,depends on injection system . ! it's only running rich on right bank. whatever that means . Your vehicle is a v - 6 has two banks . right side of the engine an left side of the engine . Bank 1 is where #1 spark plug is . bank 1 is odd numbers 1-3-5 Bank 2 is even firing #'s 2-4-6 . What fuel injection system does it have ?
Your best bet mite be to take your vehicle to a qualified repair shop.

The coolant temp. sensor you replaced - was the new one plastic or metal? There is a reported very high failure rate in the plastic sensors, and it is recommended that you go back to the metal style. Check the signal wire for the sensor - it should be less than .7 volts. If it is out of range, replace the sensor with a metal one.

It could also be caused by excessively high fuel pressure - caused by a sticking fuel pressure regulator, or a restricted fuel return line.
If you are able to check fuel pressure, it should have 35 - 45psi with key on, pump running - and 30 - 45 psi idling

Let me know how you make out!

REPLACING THE PCM WILL BE YOUR BEST BET. HAVING IT FLASHED OR RE-PROGRAMED WILL PROBABLY CAUSE MORE PROBLEMS THAN ITS WORTH. YOU MIGHT CONSIDER GOING BACK TO OEM PARTS,ALOT MORE SIMPLE SOLUTION.

I would hate to say it may the ecm, I would swap with a known good ecm.
see if that makes any difference. good luck!

May have to run in line fuel pressure gauge to determine if the fuel is losing pressure and causing this fault or wether you need to look else where like constant injector pulse,air flow meter,faulty distributor etc.