Hi all, I've been beating myself up and replacing parts for way to long and it's time to ask for help. I have an '87 F-250 that was a stick six. I picked up a '90 Bronco. 351, with E4OD, and put the Bronco engine, transmission, computer, harnesses, etc. in the F-250. Everything was fine for weeks and then this intermittent nightmare began. When the ignition is turned to the ON position, the fuel pump should run for a second or two and then turn off if the ignition isn't turned to the START position. Sometimes everything works as it should, but other times the fuel pump doesn't stop running, and the engine won't start. Sometimes the fuel pump doesn't stop running and the engine does start. The truck is from California and it's in excellent condition so I didn't hesitate to start replacing things, looking at it as a partial restoration, thinking that I would have the truck for a long time. So far I've replaced fuel injectors, TPS, MAP sensor, distributor and PIP sensor, fuel pump relay and pigtail, power relay and pigtail, ignition switch and lock cylinder, gas tank,( it's front pump only) both fuel pumps, fuel pressure regulator, plugs, plug wires, new capacitors in the ECM, and some other non-fuel delivery/ignition related items. It doesn't throw any usable codes and I don't know what to do next. I have an OTC breakout box and a Ford Emissions-Diagnostics manual for 1990. Anybody got any ideas on what I should check for next and how to go about it ?? I'm thinking it's a short, or maybe something in the computer went bad, the computer is over 30 years old. HELP, PLEASE!!

 

Disconnect battery, locate and remove computer. Take its cover off. If you see any of the three electrolytic capacitors bulging or leaking, that likely is your problem.If you have soldering skills, you can easily replace them( do all three) or you will have to replace computer.. I believe the caps are are 10 and 47 micro farad, 16v....I would put 35v units in.

 

Hi raski, I have already replaced the capacitors, it didn't do anything for the intermittent fuel pump/start issue, but I think the truck is shifting better. I just ordered a Rotunda monitor (007-00018) from ebay, that unit is supposed to be good at finding intermittent problems, we'll see. Thanks for the reply.

 

You already know the path power takes to get to the fuel pump.
All you need to do is pick a few places on the path , tie in a small type test light. Now when power is being sent to the fuel pump the lights should lite.
When the pump keeps running, separate the wire coming to that point. If the power is coming before that point the lites on that part of the circuit will stay illuminated.
If the points after that separation stay illuminated , you know the power is not coming from the usual source. If you keep proceeding on that path you will eventually pin down the source of unwanted power. Personally I would consider this a fun challenge because you will eventually find where it is coming from.

 

hanky, I will have a Rotunda monitor in the next couple of days, this monitor is supposed to be good at finding intermittent problems. What you suggested works, I've actually used that method before on a different vehicle, but that electrical problem wasn't intermittent. With the problem I'm having, moving the harnesses and wiggling the wires might make and break the bad contact point I'm trying to find, and I still wouldn't know where the problem is, maybe the monitor will at least tell me what circuit the problem is in. Intermittent problems are by far the most frustrating. Thanks very much for your input, if the monitor doesn't tell me anything, I'll give your suggestion a try.

 

Thanks for getting back to us.
I'm sure a lot of forum members would like to know what you find.

 

I'm looking forward to testing with the monitor, and I would be happy to share whatever info I come up with. Thanks again for the input.

 

Just like placing "ears" on a vehicle to help locate a source of vibration or noise,

I placed small lites in the path to provide a visual idea of where the problem might be originating. It is a little bit of work , but you can see the result on the spot. I'm not sure how any other tester can tell you the exact location as good as watching the series of lights.

 

I think I've got the problem narrowed down to an intermittent ground, so I plan to check the grounds before I do anything else, thanks for the input.