I have a 1988 f150 302 5.0L. I'll be cruising down the road and the truck will shut off just like i turned the key off. All the dash lights come on and all the gauges drop. No warning, it just shuts off. I have to turn the key off and back on to start it again. I don't have to wait for the truck to cool down, it'll start right back up. I had this problem, intermittent spark, loss of power, misfire, weak spark, hesitation and then the truck wouldn't start, no fire. I replaced the ICM and the truck ran better than when i bought it. It fixed every problem but the shutting off. I have replaced fuel pumps, fuel filter, oxygen sensor, MAF, EGR, ignition coil, and ignition switch. New distributor cap, spark plugs, and spark plug wires. Have shook all the wires going from the ICM to the coil and from the ICM to the computer while the truck is running and it didn't shut off. Checked the TPS and had a good reading. It's extremely intermittent. It might happen ten times one day and then maybe not once for several days. I might be crazy but it seems to get worse when it's humid. I was thinking bad computer but I'm done just throwing money at it. You guys have any ideas?

 

Sounds like a wiring issue. Check your engine grounds and your battery cables for tightness, corrosion, ect. Check your connections at your alternator, make sure everything is clean and tight. Have you pulled the codes or are there any?, has your engine light come on?

 

I have pulled the codes and it said the EGR opening wasn't detected. It was really clogged up so i just went ahead and replaced it. My battery connections are new and I replaced the alternator about two months ago. I checked the connections on that yesterday believe it or not. Engine light has not come one but like i said wouldn't the truck shut off if i shook the wires?

 

Shaking the wires is different from driving the truck and hitting bumps and pot holes. Have you pulled the main harness connector at the ecu and looked at the pins for corrosion or melting of the connector? Maybe it's a simple case if it being loose or dirty.

 

I'm not home right now but I'll check them when I get the chance and it doesn't shut off while it's bumpy, it has shut off on the freeway before.

 

If you're sure everything is good in the engine compartment then how about your ignition switch? Could it be rolling back enough to kill it? Is it worn, sloppy?. Id almost say pull the ecu and open it, look for bad caps but you say it restarts right away so next logical step is look at ignition switch and work back to engine.

 

I replaced the ignition switch a week ago and it didn't change anything. And before i recycle the key if I try to start it the motor just turns over and there is no spark but if i recycle the key i gain spark back.

 

I pulled the computer out, looks like it's not the original but has been remanufactured more than once. The pins all look good. But I have seen these things go bad on only one capacitor and might take it apart but I don't think I will as this is my only vehicle and if I break the computer I won't have a vehicle until it is fixed

 

Well unless someone else has an idea i think I'd put the old coil back on if you have it, just to rule out that the new coil might also be causing the problem.

 

Already tried that, ordered a new computer I'll let you know if it worked

 

REPLACE the ECU power relay.

I had the exact same issue with my 89 F250. Drove me nuts and it would always started right back up. About a year later of that PITA problem, the ECU relay hard faulted enough for me to diagnose it!

After many years of useful service the relay contacts will degrade and will intermittently break contact even though you hear it click. When you cycle the ignition switch, it will reseat the contacts and every thing will be good till it fails again.

 

Well Ive already done that as well

 

Check the wiring under the ECU Relay. As most of the time it is the wiring going into the relay socket and not the relay itself.

Most with the 86-91 trucks will up grade the EEC & Fuel pump relays to the newer stile.
There is a lot of write ups on doing this mod.

 

You know what I did notice that the plug was a little broken. I'll take a look at it when i get home. Oh and a new symptom, seems to want to take a while to start. Hard starting.