Fuel Pump problem... or is it?


AJamesR

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I have a 1994 Ford Explorer that I put a 92 explorer engine into a while back. My problem is this: sometimes it starts and runs perfect, but other times it doesn't start at all, if I turn the key on and hear the "hum" of the fuel pump then I know it'll start, but if I turn it on and don't hear that it doesn't start.

I've tried a lot of things and figured stuff out that just confuses me more. first when I turn it to the "on" spot normally the check engine light will come on until it's started, but the times the pump doesn't turn on the light doesn't come on either. I bought a new Fuel pump Relay, and it didn't change anything, however I know now that if I have the key on and it won't start I can get out and tap the relay and it'll click and then I hear the fuel pump and it starts, however often when I do this I'll drive about a mile and it'll just die on me, until I do it again.

And the other thing the confuses me more, is I know if you unplug the fuel pump safety switch so the fuel pump doesn’t come on, the engine will start and run for a little while before it runs out of gas and dies, but when it's having this problem where I don't hear the fuel pump turn on, it won't start at all, like there's no spark or whatever.

I'm wondering if this could be a problem with the PCM but if it is, it doesn't make since to me why taping the fuel pump relay would cause it to start. Is there a ground wire for the PCM or fuel pump I can check. And does anyone know where the PCM is on the 94 explorer? The book says it's on the firewall on the left side and I can't find it there for the life of me. Thanks
 
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MAKG

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I thought 1994 PCMs were still behind the passenger side kick panel and that it's 1995 (OBD-II, EEC-V) that went under the hood.

If tapping the fuel pump relay turns it on, I have a strong suspicion you're really tapping the PCM relay (which is exactly the same and usually right next to it). And it might be a loose connection in the connector if replacing the relay didn't help. Or it may be a case of just trying again; it's easy to be fooled by that.

I'd check the PCM is getting power and ground when this problem occurs.
 

AJamesR

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Is the PCM relay the one labled EEC? When it stops raining so hard I might try switching the new relay over to that one. As far as just trying it again and having it work, I tested it problbly 30 times to make sure I got the same resualt each time and always did, I would turn the key off and on until I didn't hear the pump, then I would go out and hit the relay and every time that would make the pump click on as soon as I did it.

When I find the PCM, is there colors of wires that are power and ground that I can just test with a mutimeter? Thanks for the reply
 

MAKG

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Yup, EEC is another way of saying PCM. If you're Ford, at least. I don't like using it because it has another unrelated meaning in your truck (evaporative emission controls -- the charcoal cannister, purge valve, fuel cap and fuel vent line).

If you can reproduce this problem so easily, get a voltmeter and find the voltage drop in the fuel pump circuit. You can use it to pinpoint the problem, even telling the difference between a connector and the wires going into it.

You'll need an EVTM for this, but you apparently have one or its derivatives (Mitchell?) if you have the connector locations table.

The EEC-IV grounds are pins 20, 40, and 60. I think the power is pin 1 (but don't quote me on that). It's in the EVTM.
 

AllanD

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on a 1994 Explorer the PCM is inside the cab behind the passenger side kick panel.

On a 1993-94 Ranger it's in the firewall outboard and below the mastercylinder.

on a 1995-up Ranger OR Explorer it's in the firewall/Cowl just to the passengerside
of the vehicle centerline.

And the symptoms you are encountering sounds like either:
a)a Bad ground next to the PCM (look for the 7mm screw and several ring terninals)
b) a bad fuel pump relay inside the power distribution box mounted to the passenger
side inner fender.
c)a loose or dirty CONTACT to the fuel pump relay in the power distribution box.

It COULD be something else, but those are the first three things to check.

AD
 

AJamesR

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If tapping the fuel pump relay turns it on, I have a strong suspicion you're really tapping the PCM relay.
I think you might be right about the PCM relay, after doing more tests I figured that even tho I was tapping on the fuel pump relay it would jar the PCM relay enough to make it click. I put a new relay in the PCM spot and was unable to reproduce the problem, but that's not saying it's fixed becuase it's had good days in the past. So I guess right now only time will tell if the problem comes back or if I got lucky with a simple relay.

So thank you guys for all the help, and keep your fingers crossed that it's fixed.
 

woodyedmiston

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fuel pump

what aland said - and - as i understand it there is also the possiblility that your computer needs diagnostic work
 

Aubrey

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.... if I turn the key on and hear the "hum" of the fuel pump then I know it'll start, but if I turn it on and don't hear that it doesn't start.
My first thought after reading your post was that you might have a bad ignition switch since whenever your pump failed, so did your engine light test. If the ignition key return spring doesn't return the key all the way to run, I wonder if you're not simply losing fuel pump power right from the ignition switch? Not sure if that helps or not.
 


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