I have a 90' F-150, I-6, Manual Transmission that I have owned for the past 20 years. For the past 6 years the fuel pump will occasionally run constantly in the "Prime Mode" and not shut off. I'm unable to start the truck when this happens.

The truck may begin running roughly as I'm driving. It may run well for 2000 miles or 2 miles when the fueling issue occurs.

I have checked the fuel pressure when it fails and when it is running correctly. It is within specs. Replaced pressure regulator anyway, fuel filter, checked fuel relay.

I'm inclined its the ECM. I haven't removed and opened it up to check for capacitor leakage.

Any thoughts?

 

I have a buddy with a '90 I6 and his did the same thing. He kept throwing parts at it, he had multiple relays he used, multiple parts store TFI modules, coils, and he eventually replaced the distributor to no avail. He finally bought a computer and he's been driving no problem for months.
You most likely have a computer issue, though. Here's what you need to do. Buy yourself a noid light set and when the truck does it's prime mode and not start, put the proper noid light in one of the fuel injector plug ins and turn it over. If you get no pulse from the light, that means you are getting no signal to pulse the injectors. The computer uses a PIP in the dizzy to know when to pulse the injectors. So it's either an intermittent PIP problem or a computer problem. It would be a good idea to dump the codes (while it is able to run) to see if there's any stored codes. You won't be able to dump codes while you're having the issue though, because the KOEO test powers up the fuel pump and shuts it down so if it can't shut it down, it isn't able to dump the codes. Hope this helps!

 

Thank you for your reply. I used NOID lights and I was getting a pulse, but at the time I used the lights I can't remember if the "Fuel Problem" was actually happening.

From your reply and what I've read in other posts, it is really looking like the computer. I will do as you suggest and dump the codes.

Thanks again.

 

You have a bad computer. The clock is not running in the computer is why the fuel pumps will run with it not starting or running.
If the capacitors do not have leakage and are OK you may have a bad solder joint on the board. The crystal sometimes brakes loose from the board and has caused this problem and some of the posters on here have re-soldered the leads of the crystal and that did fix their problem.

 

Thank you. When I get the compute pulled and opened, I'll take photos if I find anything obvious. I'm an aircraft electrician by trade, so the component removal / replacement shouldn't be an issue.

John B.

 

Some have even had to replace the crystal when the clock wont start.