i just got a 94 f150 5.0 and it barely idles and just idles up and down.and seems to be running rich. it gave the codes 556, 542, 157, 158. the 157 and 158 code was for the mass airflow sensor and i replaced it and it had no effect still the same problem. but the 556 and 542 code is for the fuel pump and whoever had the truck b4 had run a ground wire from battery to a power wire on the relay i dont understant this but pump wont run without the wire. and it runs when the switch is on. would this affect the running of the truck? i was also thinkin maybe the fuel pressure regulator im gettin a fuel pressure gauge 2day what range should the pressure be at?

 

Anytime you replace a sensor, you need to clear the KAM (disconnect the battery for 5 mins.) All I can tell you about the P/F relay is the PCM relay next to it, has to function for the F/P relay to function. (W/o the b*asardized rewire anyway). The F/P relay & PCM relay are not interchangeable. The PCM relay must be replaced w/ a PCM relay only.

 

i replaced the computer and the fuel pump still will not turn on the only way i can get it to turn on is to run a ground wire to the light blue/orange wire on the fuel pump relay and this causes it to run all the time. ive studied the wiring diagrams and the light blue/orange wire on relay is getting 8volts but i thought the fuel pump run off a negative trigger? is there nething else that controls the fuel pump on and off? its still giving code 556 and 542

 

What does replacing the computer have to do w/ replacing the PCM relay or Fuel pump relay?

 

i replaced both the fuel pump and pcm relay and the fuel pump still wouldnt come on. so i replaced the computer after going throught the wiring and checking all the grounds. i was told there was a bad diode in the computer that trigers the fuel pump to come on because its supposed to give a negative trigger to the relay but its giving a positive instead

 

Diode? The ECU controls injection triggering something like that. Checked Fuseable links?

 

Blue/orange is the negative trigger wire. Yellow always has power and is on maxi fuse O. Check the fuse. Green/yellow is power to the fuel pump. Red is the trigger power supply, but not triggered by the computer. When the computer relay is energized the red wire on that relay gives power to the fuel pump relay red wire.

 

the terminals where the fuel pump relay goes three of them are getting 11-12 volts with the ignition switch on and the lightblue/orange wire is getting a slight positive charge instead of negative.

I was thinking that maybe fuel pressure regulator is bad causing 02sensor to read rich and telling the computer not to trigger the fuel pump relay is this possible?.

as said with a ground wire run from negative side of battery to the lightblue/orange wire the truck will crank and run but the pump runs constantly causing it to flood.

also with a ground run to the relay i had it running it was flooding horrible where the pump runs all the time but i could pull vacuum of the regulator and plug it and it didnt effect the running of the engine at all.

 

the truck shouldn't flood if there is no vacuum, and the injectors aren't opening.

 

So do a F/P test.