[fperri302]

Hello everyone I just came across the site today while searching the net for some help with my truck.. I have a 1990 F-150 4x4 5.8L .. one morning it decided not to start and the fun began... I put an ignition module on it right away having seen that be the problem before.. No dice... I noticed some green in the bottom of the EEC and Fuel Pump relays so i cut them out and replaced the connectors and the relays...still nothing... I noticed the fuel pump running constantly with the key on ... at this point i had spark and fuel pressure but no injector pulse... so between no pulse and constant running pump i took a chance on changing the PCM.... So i put the new computer in and she fires right up and runs for about a minute and a half ( im all happy at this point) until boom it cuts out and wont restart (not so happy anymore)... Now i have no spark and no injector pulse... i took the connector back off the computer and tested each power and ground point and they were all good so i hooked the computer back up.. The EGR and TPS both have a nice 5volt reference with the key on... Now to throw a curve ball in the mix she will start some times and runs great(the new comp actually fixed a tranny shift problem i was having) and if you get it to start it will continue to restart all day and night..however once she sits over night and i assume gets colder it will not start again in the morning.... if i wait till mid day when the suns hot she will start most of the time... I have had every connector apart on the drivers side fender well and also the large round one going through the firewall and still cant find a bad contact... I have also checked the pickup coil in the distributor and she is showing a nice square wave pattern .... Also when it has the no start condition it will not communicate with the scan tool but when it does start it will ... its driving me nuts and any suggestions would be much appreciated.. Thank You - Frank :mad5:

 

[Zephyrman]

How is your trucks electrical system grounded? Does it still have at least one ground strap from the engine to the frame? Fuel injected trucks need as good of grounds as possible for the computer to work correctly. A poor ground or a dead short somewhere will cause you all kinds of hell. Have you noticed any of your accesories acting up while the truck is running?

 

[Poppy]

Also when it has the no start condition it will not communicate with the scan tool but when it does start it will ...

This has me thinking ignition switch.
When it won't start I'll bet you don't have spark.

When it won't start, do you have 12 volts to the coil?

BTW... I agree that you may have grounding issues.
Imnportant grounds include the little ones to the top of the radiator support on both sides, and (I think there is one to the firewall for the PCM).

 

[fperri302]

well i just checked all the grounds ... both sides of upper radiator support..couple in the back by map sensor and one over by master cylinder that looks like goes right into computer harness and all look fine... also tested coil and have 12 volts on both wires .... also pulled out the spout connector and got spark but still no injector pulse....checked for power from ignition switch to main relay and then power leaving main relay and thats all good...im starting to wonder if i got a bad reman computer ?.....

 

[Poppy]

im starting to wonder if i got a bad reman computer ?

I'm not a mechanic, and never ran into this, so I don't ahve first hand experience with this, nor had any particular reason to research this so some details may not be completely accurate. Also I won't have access to additional information until I get home later. Here goes none-the-less.

PCMs in newer engines get info from crankshsat, and camshaft position sensors to set timing and injector pulse. Its the PCM that determines the pulse width of the injectors. Since your engine does not have either of the two sensors mentioned, the PCM (as far as I know) gets its info from the pickup coil inside the distributor, or hall effect sensor.

Since that seems to be working in that it is collapsing the coil to give spark, either there is a break in the wire that goes from there to the PCM, the PCM is bad, or there is a break in the (I'm thinking TWO wires that go from the PCM to the left and right banks of injectors)

I'm really reaching here, but I think that all of bank one injectors are open at the same time, and then all of bank two injectors are open at the same time. I might be completely wrong on that though. Did you check injectors on BOTH banks? If you get injector pulse on one bank and not the other, that might be helpful to know.

 

[Poppy]

I did get access to a wiring diagram for a 95 5.8 and it seems that the red wire from the PCM relay supplies power to all the injectors, and that the PCM sends grounding pulses to each of the injectors individually to complete the circuits.

 

[fperri302]

Well I messed around long enough to get it to start and to talk to a scanner and i was able to pull a code 95 - Fuel Pump Circuit Open - ECA to Ground not sure if this could cause the starting issue or not..

 

[Poppy]

I'm thinking that could be as simple as a bad fuel pump relay, or those crappy connections you mentioned earlier. Since your PCM relay was just as crappy that could cuase the no injector pulse.

Seems to me that your various findings are intermittant. Spark, no spark, pulse, no pulse so crappy connections could be a problem. I am thinking that the fuel pump relay gets energised by the PCM relay. So if you are getting no spark you won't have fuel either.

Did you say that your fuel pump is always running? Did you jumper the fuel pump test port in your OBD self test connector and leave it jumpered?