91 Explorer with WEIRD problem - Run problems

[int26]

Here's one to scratch your brain on...

My 91 EB Explorer has had an intermitant problem for the last two years; the fuel pump wouldn't turn on. About a year ago or so when it wouldn't start because of this problem I got out the multimeter and tested all of the leads going to the pump. Everything tested fine so I assumed it was them pump. Slapped and new pump on and still had the problem. After banging on the harness leading into the tank it started right up so I assumed it was a partially bad wire inside the plastic harness going into the tank.

So for the last year, anytime it's given me a problem I whacked the tank and it'd eventually start up.

3 days ago it decided it would not start again. All of the usual banging had no effect. Got the multimeter out again an noticed that there was no power going to the pump. In essence, the last time I metered it I messed up.

Long story short, I tracked down the fuel pump relay, whacked it and it worked. I replaced the relay today. The pump spins up perfectly everytime now. The first time I started it with the new relay it barley idled. I shut it off, restarted and it was a little better. It would at least hold an idle. I let it run for about 15 minutes in case I did something that made the computer loose its memory and relearn the air.

So now it idles fine but as soon as I start driving it the engine cuts out and the check engine soon lights comes on. The cut out is only for a split second but it makes the engine slug, then surge. If I'm very gentle, and work the rpms up very slowly this doesn't happen. Any snap acceleration causes the symptoms to occur.

So the question is. What's wrong with it now? It's never had this problem before and it only started once I replaced the relay.

I did notice that when the engine cuts, I could here a clicking noise, so I had a friend rev it while I had my hands on the 3 relays found at the LF wheelwell under the fuse box. One of the relays was clicking alot as the engine cut out.

I'm thinking its a bad ground somewhere but I have no idea where to start looking. Anyone have any good suggestions? At this point I'll try just about anything

Thanks alot,
Scott

 

[dogfriend]

I found this schematic for a 91 Explorer

Dead Link Removed

You can find other schematics on this page Dead Link Removed

I see from the schematic that one of the relays is the EEC Power Relay. If you are getting intermittent contact on this relay it would screw up a lot of stuff.

 

[Opera House]

Could be the relay you really wanted to replace was the EEC relay. If the engine surges after a stall that would indicate the computer is resetting and telling the IAC to open up just like when you start the vehicle. It may not even be the relay, but the connections in the relay box. Look for any difference in appearance of the plug in contacts. I'd get another relay and try that. Don't put the old fuel relay back in.

 

[bnichols04]

heay I have the same problem! I turn the key and there is no sound of the fuel pump comming on. So I opened the hood and banged on the fuel pump relay and it worked. I think there is a short or something but it hasn't given me a problem so far. (1 year). I did replace the fuel pump relay and it still had the problem.

 

[Tony H]

When I first got my 'X' (October) I was told it had an intermittent 'No-Start' problem. I was lucky. I replaced the three relays in hopes that it was one of them. I have never had the problem so I'm guessing one of the relays was going bad.

If you replaced the green one. That's the Fuel pump relay. I would try the EEC one next.

They are the same relays. The factory ones are keyed and color coded but the aftermarket ones will fit all three. You might want to put the original Fuel pump relay back and shift the new relay to the EEC spot.

 

[int26]

Good advice from all. I'm going to try and swap all of the relays around tonight and see if that does it.

I'll let you all know how it goes.

I've only got 1500 more miles till I run over 200k.

Scott

 

[int26]

Problem solved...

Here's one to remeber in case you or a friend every has the same problem.

To switch the fuel pump relay on, 12v must be applied to one of the realy input leads. This lead is the triggered output of the EEC relay. So in essence, if the EEC isn't working, the fuel pump won't work either.

Here's the fun part...

If the EEC relay is just kinda bad (i.e. correded on the inside) and is intermitantly bad it will cycle on and off rapidily from time to time.

No big problem because you'd never notice the split second drop in fuel pressure since when the EEC relay turns off, so does the fuel pump.

This rapid switching back and forth causes the fuel pump relay (which were assuming is good) to switch rapidly and leads to a burn out. The fuel pump relay will actually fail first because the EEC won't arc itself open or closed because of the corrosion.

So to make a long story short. If you find your pump won't turn on, and you track it to a bad fuel pump relay, replace the EEC relay while your at it.

Thanks to all those who had suggestions.

Scott

 

[Tony H]

Good going Scott!

When I was fooling with mine, I looked at the schemo and saw that same thing. I was wondering why they would go through one relay to go to another. Not smart.

The color coding wasn't the same on the car as it was on the schematics either. Close enough that I was able to figure out what relay is for what.

Congrats

 

[Ryan1]

Scott, you are talking about the Relays in the power distribution box correct??

Ryan

 

[Tony H]

Ryan, Under the Fuses

 

[Ryan1]

under the fuses in the Power disrtibution box in the engine compartment or the fuse box under the dash.

Ryan

 

[TheRookie]

The fuses are either in the box or under it. I believe the change to be from 92 to 93 but cant guarantee that. My 91 has them under and my 93 has them inside the box. They are different types of relays also.

 

[dogfriend]

Originally posted by Tony H
Good going Scott!

. I was wondering why they would go through one relay to go to another. Not smart.

Actually, the EEC Power Relay provides VPWR (vehicle power) to the Fuel Pump Relay and most of the other sensors and actuators on the engine including the PCM (powertrain control module). The PCM is what actually turns the Fuel Pump on. It turns on the Fuel Pump Relay by grounding Ckt 22 in the PCM.

The purpose of wiring it this way is to insure that the Fuel Pump is on during cranking and running, but to cut it off when the engine stops, even if the key is left on. The PCM does this by monitoring the PIP signal (tach signal) and shuts off the Fuel Pump if the engine speed drops below 100RPM.

 

[Tony H]

I get it.

I wonder why they don't do the Fuel Pump shut down through a Crank Sensor. If te Crank isn't turning, shut down the fuel.

That's just the way I've seen it in other cars. Potato/pototo

 

[dogfriend]

Ford does everything different when it comes to Engine control.