1991 F150 4X4 300 6 146,000 miles.
Truck wouldn't start. had a bad fuel pump. Replaced the fuel pump. When i turn the key on, the fuel pump would cycle on and wouldn't turn off, also wouldn't start. After research, i found capacitors on the ECU are known to fail causing this. Pulled the ECU and found 2 bad caps. Replaced the caps, put the ECU back in and it fired right up. Drove for about a week, then did the same thing. Threw some fuel in the intake to "prime" and it fired right up. Shut it off and it fired up again. Drove it for several days again with no problem. Parked for a couple days then went to start and did the same thing again. Turn the key on, fuel pump kicks on, does not click off, and truck won't start. I know there is a fuel pressure regulator, but that doesn't make sense to have an intermittent problem...i don't think. Anyone had this issue? Wondering if someone else on the ECU could have failed. They seem very simple and i replaced the caps that failed. All help is appreciated so i don't dump money in the wrong places.

 

is it possible that your solder joint failed ?
apart from that, i would check fuel pressure with a gauge and pull codes.

 

I agree it could be a bad solder joint or cracked trace. Since the the fuel pump runs all the time when the no start occurs you know it's a computer problem.

 

Are there any other relays in line after the computer? Like a fuel pump relay, or fuel bypass valve?

 

There's a fuel pump relay, but if the fuel pump is running all the time and the relay isn't stuck, the issue is your computer.

 

How can I tell if the relay is stuck?

 

swap it with a different one. But if the truck doesn't start and the fuel pump is running, it's probably the computer.

 

Thank you! Now the fun part...they don't seem to sell that ECU anymore, it has to be rebuilt...who does that?

 

SIA Electronics | ECU Repair for all Makes and Models

Rebuilt ECM, TCM, & ECU | Cummins, Dodge, Ford ... | Go ECM

Never used these so can't recommend. There may be others.

 

What are the numbers off the computer. There are plenty of sources of used computers. Some good, some not...

 

If you have an automatic, an 1990-1993 4.9 computer should work. If you have a manual, any ~87-93 4.9 computer should work.

 

Shouldn't you have replaced 3?

 

Hi fellas, first time posting here and in need of a bit of help.

I'm having the same issue, minus the bad pump. 2 tanks on my truck and you can hear both pumps run. If you listen real close you can hear gas draining back into the tank.

I've been researching hard start problems for a 91 f150 4.9 manual and found nothing about the ecu and caps going bad.. not even sure what "caps" are. Learned a lot about what to look for on youtube...but I saw no vids on this subject.

I experienced ALMOST the same thing.. I went up the road one night (2+ months ago) parked it get something to eat came back and hit the key and it cranked and cranked for a good 5 to 10 secs before I stopped. Tried hitting it again for 5~10 sec... nothing. Usually I hit the key for 2 or 3 sec before it fires. So I pumped the gas a few times... knowing its a efi and wouldn't do much good I was desperate to get home... thankfully it started after hitting the key a few more times, got home, shut it off... next day I got in it, worked fine.. drove it for a couple weeks then one day (2~3 weeks ago) I went to and friends house near my home.. talked a bit. Came back out it was doing the same thing .. this time I couldn't restarted .. Had a friend pull back to my house and I've been trying to diagnose it since.

So my diagnoses results have been this so far... Ditto on the fuel pump running all the time and not shutting off - while the key is on engine off position. Put my spark tester on the #1 plug and found 20K volts or more firing regularly. Found good spark so I pulled the air tubes to the intake and shot some carb cleaner in it while hitting the remote switch and BANG.. it fired right up... wasn't running great but it was running. This is were I'm hitting a wall trying to figure out why I cant get the key to start the engine but if I'm shooting carb cleaner in the intake while using the remote switch it will start and run (not good mind you). Next I pulled the vacuum hose on the regulator .. let the engine run to see if there was a hole in the diaphragm... after a few mins I didn't see or smell any gas coming from the vacuum port, GOOD. Next I back probed the MAP, TPS, coolant sensors.. and the air bypass valve. The TPS had good 5v in and the return signal was moving nicely up and down with throttle movement. MAP sensor had a good 5v in but 2.5v return..which sounds right but the signal wasn't moving as I was revving the engine. Coolant sensor had good resistant and went up and down with temp change. Didn't check the o2.. it's been replaced about a year ago, and should have no effect on a no-start condition nor did I check the ERG (think its fubared anyway) or knock sensor. The one last thing that I could think of was the air bypass valve... It's messing with my head cause the input voltage is reading 12v and return signal around 7.5v that seems WAYYY high to me.. but I've not found any specs yet telling me if those values are wrong or not.. anyone know?

Oh.. one other thing, I have a code reader and I went to check codes and when I hooked it up and hit test I'm getting nothing.. don't know if its the reader gone bad or computer??? I was going to do the old paper clip trick and count the flashing lights tomorrow ... see if I can get more info as to way it not starting without the aid of starter fluid.

Conclusion.. none

Only questions
1) if its a bad puter... how is it turning over?
2)can the bypass valve being out of wack cause a no-start condition using the key to start?

ok..thx for any input
DB

 

If the fuel pumps run continuously, the computer is bad. Simple as that. The ignition system is more or less independent of the computer, and can provide spark with the computer non functioning.

The computer also has a fail-safe limp-home mode that just fires the fuel injectors a fixed amount. The engine will generally only just barely run when operating in this mode.

The MAP sensor output wasn't changing, because it's a 50% duty-cycle variable frequency output. Not a variable voltage output. 2.5v is the correct voltage you should have read.

 

Hey Thx for the info Lead Head. I've been wondering if the obd-1 computers were antiquated in the way they operated. The last thoughts I had was..
1) I was forcing a bypass of the computers operations (so to speak) by shooting carb cleaner and firing the engine that way.. It just didn't sound right to me because all cars I've dealt with that had bad computers gave no spark AND no fuel as a diagnosis condition (no injector pulse. After all, how can injectors fire with out a brain telling it to..guess Ford made away) . I admit I haven't worked on many cars with bad ecu's..

2) It might have had something to do with the idle bypass valve.. that 12v in ..7.5 out still sounds high likely a faulty valve? Any thoughts? Anyone know the specs for the valve or where to find them?

Thx Again.