I have a 1986 f 150 that I am having a lot of trouble with. I have recently put a new pcm in this truck via the old one was burnt up. The truck cranked and ran alright not great. The truck now will not crank it did however start on ether the other day and would continue to run, but would not take throttle under a load and would sputter and backfire through intake. The truck now will not crank at al,l i could not get to crank on ether today. The fuel pumps do cut on and i have proper fuel pressure. however to my understanding the fuel pumps are supposed to cut off after 2 seconds after key is on and left in run position. They do not cut off and continue to run if key is in run position. Any help will be appreciated!

 

Welcome to FTE! We are pretty good at diagnosing problems, but we need to know what the truck is. Like, engine, carb or EFI, modifications, etc.

Also, a definition check: Some people say "crank" when they mean "fire". To us, when you say "it won't crank" you are saying the starter won't engage and the engine will not turn over. I don't think that is what you mean, but you will have to be very precise when you tell us what is happening.

 

Welcome to the forum, draye98! Please do as Gary has suggested... have you carefully checked [rechecked] the electrical connections? The grounds are very important, too.

 

The truck is a 5.0 efi. And the engine has spark and will turn over but will not fire up

 

The truck also has the required fuel pressure coming from the Schaefer valve

 

Have you checked to see if the computer is throwing codes? There's a how-to on that in the stickies, if I remember correctly. I ask because if it has spark and fuel pressure it should fire - unless the computer has spotted a problem. But, even then it should start and run in limp-home mode, meaning it won't have much power. So, if it doesn't fire when you are feeding it ether then either the spark isn't coming at the right time or there isn't enough compression.

Why did you replace the computer?

 

The guy that had it burned it up by crossing wires and I have pulled codes most are egr codes but the egr has been bypassed on this vehicle

 

Most are EGR? But there are others? It should still run even when it is throwing codes, but just not well. Have you put a timing light on it to see if it is sparking at the right time?

 

No but I have spark just don't see what could cause the timing to change overnight

 

I don't either, but unless you have a better idea I suggest you check the timing to see when it is firing.

 

K I will try and get a light and get back with you. Any chance the injectors could be the problem?

 

Don't know much about the EFI system, but if there is only one injector then that could be the problem. But I don't think multiple injectors will go bad at the same time.

 

The truck is timed fine any ideas on what else this could be? Like you I am not a fan of the efi I like natural aspirated engines

 

That says you have fire and at the right time. Given that, there are only 4 things that could prevent it from starting:

1. Battery: If the battery voltage is low then it won't spin the engine fast enough to start. Make sure the batty is fully charged.
2. Compression: Low compression, like with worn rings, will prevent it from starting. But, it isn't likely that the compression went south that quickly.
3. Valve timing: If the valves are opening at the wrong time then you won't get fuel into the cylinders when needed. A slipped/jumped timing chain will do that, but it will also change the ignition timing, and you've found that the timing is correct.
4. Fuel: The most likely thing is that the fuel injectors are either not being told to open or aren't opening when told to do so. I would put a meter on the wires to it and see if they are getting pulsed as you crank the engine. Wiring is shown on Page 61 here: Electronic Engine Control - ???Gary's Garagemahal.

 

I'm going with the safe bet and going with Gary! ;-)


Have you pulled the plugs and verified fuel is getting into the cylinders?


I assume you are spraying ether into the intake of the EFI unit?


Has the truck set any time at all? We all know the love of Ethanol laced fuel, the injectors might be totally gummed up.


You could try spraying ether directly into each cylinder and seeing if the truck starts up for a few seconds. (we use this trick on ATVs to see if there is a carb/fuel injection problem)