I’ve been given the “opportunity” to help a friend with his 2008 Explorer Sport Trac. It has the 4.6L V8. In the beginning, he had a cel on and would sometime experience a hard start. Took it to a shop and was told that it was a fuel pressure code. The shop replaced the following:

Fuel pump

After replacing the fuel pump, has consistent hard start and would stall at idle and has very low power. He then replaced:

Fuel pump control module

Still experiencing the same issue. He then replaced:

ECM

Still experiencing the same issue. The shop gave up and had my friend tow this home. Since I’ve been helping, I suggested the fuel rail pressure sensor since the technician had already replaced everything else. Still no change. I’ve visually inspected wiring at the fuel rail and fuel pump module and all looks okay. I’ve determined that it is getting some type of fuel pressure as it will run when you hammer on the gas. There is no valve on the fuel rail for me to physically check the fuel pressure for some reason. Once it drops below 2k RPMs, it will stall. It sounds like it has a very bad misfire when it idles. I pulled the codes, and the two active are P0191 and P0642. Both seem to be fuel related. I’m not sure what my next steps should be here. Please let me know your thoughts.

 

There is most likely an electrical problem with the 5 volt reference voltage the ECM supplies to the fuel pressure sensor.
You will need at least a multimeter to pin the problem down. First you need to verify if the 5 volt reference voltage is present at the fuel pressure sensor.
It most likely is either low or absent. A schematic diagram is needed to confirm the path and pin # on the ECM to confirm the voltage is present to start with. You could also check another 3 wire sensor to see if that 5 volts is present there. You just work your way back to the supply OR you can unplug the other sensors that share that voltage supply. and see if the voltage comes up after you unplug sensors that share that supply. If there is a defective sensor in the line , the voltage will come up to 5 volts when you unplug the defective sensor. This is where a diagram needs to be consulted. If you can check for any corroded connections if you don't have the necessary tools to work on this. There could be several sensors sharing that voltage supply and a diagram is a necessity. Just replacing parts can get quite expensive and still not solve the problem as you already know by this time.
A wire that is carrying that 5 volt voltage could be making a partial contact to ground causing the voltage to drop below the required reading for everything on that line to function properly. You might want to do a good visual inspection and look for this possibility.

 

Thanks for the quick response. I believe I did check the 5v reference and from what I remember it was good. I will double check this week. Assuming the 5v reference to the fuel pressure sensor is good, where would I go from there?

 

You are jumping the gun a little.
One of the codes you passed on to us is telling us that voltage either isn't there or is lower than it should be.. We need to verify everything onestep at a time.

 

Okay, so I measured the power to the connector at the fuel rail pressure sensor. Below are images of where I measured and below each image shows the reading.



This looks to get about 2-3V

For image above.

This looks to get 1V

For image above.

This looks to get 1V

For image above.

This looks to get 1V when using ground in wiring.

For image above.

When measuring with battery ground / any other ground this gets battery voltage.

For image above.

 

You know you're cheating here , right ?

You need to check the system voltage with all the plugs connected ,under load.! You need to back probe the connections and this can be done if you use a T pin or straight pin.How about you try this again with all the plugs connected and see what you get ?

 

Supply voltage will not change whether it’s plugged in or not.

 

You may or may not be aware, a faulty component on the line , and there are several, can pull down that 5 volt reference to the point the PCM will not respond.
Trust me, would I lie to you, don't answer that !

If you do a little more reading up on how electrical and electronic things work and the proper way to use that multimeter in situations like this , it will be most beneficial and you can take that to the bank. I really am trying to help you out here.There are a few videos on U Tube that might clarify some of this, why not check them out?