1988 Bronco Fuel Problem

[elkhunter]

Our neighbors pulled a 1988 Bronco with V-8, 4 speed here to fix. Someone has erased all codes. By grounding the self test only the front fuel pump would run. I changed the in-tank pump and grounding makes them both run and build pressure. I put a new fuel pump solenoid in but it didn't help. The coolant sensor reads within the cold engine limits. It will run when I squirt fuel through the throttle body so for some reason the fuel isn't going to the cylinders. There are 5 prongs in the fuel solenoid but only 4 wires to it with voltage to three prongs. I don't know if it should have 5 or not. The inertia switch is good. There is power in and out of the MAP sensor. I have checked so many things that I hope I have given enough clues for someone to give me a hand. It is probably something simple but I definitely need your help...THANK YOU!!!

 

[MrShorty]

Not at all familiar with an '88 V8, but, based on what I know of EEC-IV in general, I would look to see if the computer is getting power. If the computer has power, makes sure the injectors have power and are firing (It is EFI, isn't it?)

 

[elkhunter]

Where do I check to see if the EEC is getting power? The owners manual is gone and the 'generic' fuse panel in my books are different than the Bronco. The fuel pumps don't run at all unless I jump it to ground. I turned the key to the ON position and pushed down on the Schrade valve and there was no pressure. Something somewhere is not allowing the pumps to start. When I did the 'forum search' I found someone had listed a site that shows a schematic so I will try that today. If I have power at the solenoid I need to know where it goes to start the pumps, through the computer next or fuse, fusible link, etc. I really wished whoever cleared the codes would have written them down.

 

[MrShorty]

Most sure place to check to see if the computer is getting power is at the computer. Once you get a wiring diagram, you should know which pins are power.

 

[elkhunter]

Thank you for the help. I really need assistance with this one and I hate to just keep throwing parts at it.

 

[410Fortune]

stop throwing parts at it!
You can check the red wire at all 8 injectors with a voltmeter while cranking the engine to see if the PCM (computer) is telling them to fire.

 

[elkhunter]

Okay, thank you. If it is then it must still be something in the fuel system? If it is not, where and what do I check as far as the computer goes? I will post what I find with the injectors.

 

[410Fortune]

Fuel/spark/air.

First eliminate the fuel as the issue, check fuel rail pressure when the pump has primed. Listen to the pump prime
Check the pulse to the injectors at the red wire.

Then check for spark at the plugs.

It is not likely the computer.

It is likely the TFI module if fuel checks out and you have no spark.

I have worked on a few FSB and older 5.0L's now days, so hopefully I can help.

 

[elkhunter]

The fuel pumps won't run unless I ground the test terminal. Then they both run and build pressure. I tried starting it with the ground but it still would not start. I am fairly sure it is fuel or computer related because I used a squirt can and shot gas through the throttle body and it would start and run as long as I supplied the fuel. I only did this for 30 seconds or less though. This tells me there is spark so that has been eliminated. I did put on a new fuel pump solenoid but I don't know what the TFI module is. Something is not starting the fuel pumps with the key in the on position. I will check the injector signal today if the rain stops. I don't know if it matters but, after changing the dead in-tank pump, the fuel guage reads past full with 5 gallons of gas in it. I don't know if it is a clue or not. Before it wouldn't build any fuel pressure but now it does and I really can't see any way of not getting the pump wires right.

 

[410Fortune]

OKay the TFI module is mounted on the distributor, TFI stands for Thick Film Integrated
This little POS controls the spark/timing. So if it runs with ether like you say, you can skip the spark related issues.

Your fuel pumps will not come on unless you ground which lead?
Sounds like you have already changed the fuel pump relay.

We can check at the relay with a multimeter, or test light to see if the computer is telling the fuel pumps to come on.

The problem with your fuel gage is in the intank sender unit, sounds like when you dropped the assembly back into the tank you messed up the positioning and or damaged the float.

What wire are you grounding to make the pumps prime?

 

[elkhunter]

There is a plug by the fuel relay for doing the self test. The book says to ground the one on the right and, sure enough, they run. With the key on, there is power to three wires on the fuel relay and the black wire has no power. I took the tank off twice because of the fuel guage-once to change the pump and once to make sure the wires could not be hooked up wrong.

 

[410Fortune]

the black wire is ground, try using a jumper wire to go from 12V+ (hot all times, one of the wires at the relay) to the wire that goes out to the fuel pumps (pink/black should be)

Here is how it works.

the relay (just like all relays in the truck) gets a 12V+, a ground, a signal wire, and a feed wire. Some 5 prong relays have two feed wires.

Now the fuel pump relay should get 12V+ (either at all times or only when key is in ON pos, cant remember which)
Of course it has a ground (black wire)

The signal wire comes in directly from the trucks computer (It tells the relay to go on, which basically switches the battery 12V through to the fuel pump(s))

Then wire then leaves the relay and goes to the inertia switch (switch under glov box that shuts off pumps if truck is rolled over/hit hard)

The wire goes through the inertia switch and on to the fuel pump(s)

You can use a jumper wire at the relay to send the 12V+ signal from the battery to the fuel pumps, in effect bypassing the computer. Do this, listen for the pumps, then see if it will run.

The next step would be to monitor the ON wire at the relay, the one that comes from the computer, to see if in fact the computer is telling the pumps to come on.

If all this checks out then your problem may be in the wiring at/to the fuel pump(s)

Your 88 does have dual pumps?
Have you tested the feul pressure at the rail once its pressurized?
Does the fuel pressure regulator have a vacuum line going to it?
Is the vacuum line leak free?

I have seen so many FSB, especially 86-93 or so with serious vaccuum leaks. Your 5.0L is VERY dependant upon vacuum, basically every system on the 5.0L is vacuum controlled with solenoids.

 

[elkhunter]

Okay, be right back

 

[elkhunter]

I tried that but nothing changed. There is 12 volts to all 3 prongs with the ignition on. There is continuity through the inertia switch, it has dual pumps, when I jumped that self-test thing and the pumps were running I stuck a nail in the Schrade valve and there was a lot of pressure (I don't have a guage), there is continuity through the coolant sensor and MAP sensor(with no vacuum and 20#), and still no pumps. Did you mean to check the vacuum of the whole system as one?

 

[410Fortune]

Let it sit for a few minutes. ground the jumper lead again, ensure BOTH fuel pumps are running and try to start it.

Will it turn over?

For vacuum leak test first do a good visual. the plastic lines like to break.
I wanted you to check specifically the vacuum line headed directly to the fuel pressure regulator.

 

[elkhunter]

Congratulations to you, 410Fortune, and a huge thank you for the assistance. Without it I would still be stumbling around but with your advice and patience the Bronco runs again! Thanks again.

 

[410Fortune]

FYI everyone, his 88 Bronco was getting a + 12 volt signal at 3 prongs at the fuel pump relay, this is not right. He was also getting 12v + at both fuel injector wires, no - pulse fromt he PCM.

This told me his PCM was fried.
He got a replacement from a junk yard, plugged it in, truck works.

This is the 5th EEC-IV PCM I have seen to date fail.
4 have been 91-94 Explorer
1 has been 88 FSB