I first want to appreciate everyone here - i have read many post while attempting to repair my vehicle. My truck will start if the fuel pump primes but it is not priming each time. I've changed the relay and made no difference. I jumpered the inertia switch. When the truck starts the voltage at the inertia switch is approximately 13 volts. When is does not start the voltage reads around 10 to 10.5 volts. Why would that voltage vary from one attempt to start to the next attempt. Any help is truly appreciated.

 

When an electric motor is starting or trying to start it uses much more current than when running. That could explain the voltage being lower when it doesn't run. I would make a wild guess that you need a new fuel pump.

 

The voltage at the inertia switch is only valid for 1-2 seconds after the key is switched from OFF to RUN until the engine fires. If you are checking after that brief window and before the engine starts, it's an invalid reading and should be ignored.

 

Thanks for the feedback. I thought after my post that I may not have been totally clear so if I can, I want to make sure. I had the inertia switch jumped (bypassed). I had my voltmeter red lead on the outbound wire and black to ground. As i turned the key, I watch the readout on it as well as listened for the pump to prime. When I heard the pump prime the voltage showed around 13 volts for a few seconds (while priming). The next few tries, the pump did not prime and the voltage on the meter was only 10 - 10.5 volts. So my question is why is the voltage going to the fuel pump varying that much. The pump has no issues running while driving, which may or may not mean anything. Seems to me that there just isn't voltage going to the pump to start it priming, but I don't understand why that voltage is changing coming to the inertia switch. Also, when the pump primes I have 40 psi at the rail every time and zero psi when it

 

Is the fuel pump relay energizing when the priming cycle fails? (Listen or feel for the click.)

 

The relay is working properly. I replaced it early in this process, and it made no difference in terms of the crank/no start. I still find it odd that the voltmeter connected to the outbound wire from the inertia switch (with the switch bypassed) is ~10.5 when the pump does not prime. Then with no other changes, when I hear the pump prime the voltage at that same location is 12+ volts. It appears that the voltage is dropping off periodically which in turn prevents the pump from starting. If that were the case, what condition would cause it?

Thanks again for your help!

 

have you checked the fuel pressure? It's easy to do with a gauge.

 

There's probably no need to measure pressure given the alternate data regarding the voltage measurement. If the pump doesn't get proper power, it won't run and there will be no pressure.

It depends.

The correct observation has to be "Is it working correctly EVERY TIME or only when the fuel pump primes?" There is a profound difference.

Run this test:
1. Connect your meter to measure power at the inertia switch relative to a good chassis ground.
2. Cycle the key to RUN and observe the measured voltage both during a valid priming cycle (battery voltage) and post-priming cycle. Is that post-priming cycle voltage the same or is it different from the voltage measured during a "failed" priming cycle (one where the pump does not run).

It is completely normal to read some voltage when the relay is not energized and the ignition switch is in the RUN position due to the circuit in the PCM that is supposed to monitor the output of the PCM relay (pump power). What that voltage actually reads depends on a number of variables.

 

I just bought a 2000 F150 4.2 liter and I'm trying to figure out why it takes a couple seconds longer to start if anyone has any advice what would make it not turn over faster

 

Maybe because it's missing two cylinders.

This is the V8 section. You need to start your own original thread in the correct area instead of hijacking someone else's thread.