Occasional No Start

[rob2337]

I have a 95 Explorer 4x4 4.0L. Every so often when I go to start the truck, it just keeps on cranking and doesn't catch. I have tracked the problem down as far as I can go and now I am stuck. When it doesn't start, I know that the fuel pump doesn't work. This is because the fuel pump relay does not engage. I did some checking with a voltmeter and when the problem occurs, the voltage that is supposed to be 12v from the pcm relay to the fuel pump relay is only like 3.5v....not enought to engage the relay(the -12v signal from the pcm is present). All the wires from the pcm relay to the fuel pump relay are good. The pcm relay is also used to give +12v to the MAF, injectors, pcm, etc. I am suspecting that there is somthing wrong with one of these things and it is shorting to ground. That would explain my voltage drop. One last thing, when the voltage is at 3 volts then jumps back to 12 volts, I can hear a slight click somewhere in the engine bay. Not as loud as a regular relay. Can't figure out what the heck it is. Injector sticking and shorting to ground, bad PCM, ...???? Pain in the but problem.... Any sugestions where to look before I start ripping apart the wiring harness and probing wires?

 

[MrShorty]

Being intermittant, it's going to be a challenge to find. Next time it won't start (3.5 V coming to the fuel pump relay), unplug the other items fed by the EEC relay one at a time, and see which one causes the voltage to jump up to +12 V. That would indicate which component has the problem.

 

[performancenut]

The EEC can occasionally be the failure item. It can fail internally with the part that generates the signal to drive the fuel pump relay. But before thinking of that, make sure all the relays are functional and doing well as they are a more frequent failure item.

 

[rob2337]

All the relays are good, and the pcm is sending the -12v signal to the fuel pump relay. It is just that I am getting a freaking voltage drop of 9 volts!! Then I hear a faint click, and the voltage goes back up to +12V!! It usually doesn't stay like that for more than a minute so it's hard to start unplugging things to see if it goes back to +12V. What sensors, that draw power from the PCM relay would i hear a very small click from??? Would a faint click come from the PCM? I'm going to take it out and put it to my ear. What about bad grounds, and the click I hear is a small arc? Any places to check for bad grounds?? Oh and when the pump doesn't work, it throws a 543.. well that's expected.