Hello,
I have a 1992 F150 with the 4.9 Liter strait six. I have had it for almost two years. When I bought it, it had no oil pressure and a bad knock so I put a short block in it and it has run great until last week.
I only use it a couple of times a week. The last time I tried to use it, it was hard to start amd ran real crappy, missing and running rich. I started poking around under the hood and after a few minutes it started to run good all of a sudden.
I went ahead to run my errand and on way back it was running crappy again.
By the time I got home it was running good again. So it runs good for 10 minutes or so then crappy for five or ten minutes.
When its running crappy the fuel pump runs nonstop with the key in run. The check engine light stays lit and you cannot read out the error codes. After it clears up the check eng light goes out (without restarting thr engine) the fuel pump runs a couple of seconds with the key on and you can read out the error codes. The codes are 631 which refers to the overdrive light and 111 which is all clear.
Because the fuel pump is controlled directly by the PCM and that it doesn't store trouble codes even when the check eng light is on and that the light goes out without restarting the engine it must be the PCM itself.
Is there a way to definitly check the PCM and by the way, where is it located?

 

First thing is the fuel pump will run full time when the engine is running. That's not a problem. The pump has to run to feed the injectors.
The OD lamp fault and code should not be the cause of rough running engine in park.
Do the self testing KOER as many times as needed to get some sort of direction on the problem while the trouble is present..
At the moment you don't know if it is fuel or ignition related or even wire harness shorts.
Test fuel pressure for about 39 psi and leak down time after shut off.
How did you determine a rich condition?
A bad MAP hose or Map, open OX sensor harness can cause richness.
Normally the computer will try to compensate for richness and set a code in a short time telling you of that fact.
Hard starting can be a PIP sensor, TFI, coil, bad fuel pump, pump relay, inertia switch, even ignition switch.
It does take some detective work but I would not put any effort in the Computer at this time unless all else fails.
I suspect if the fault clears that fast it is electrical in nature.

 

Ok, when the truck is in the "bad" mode,
The fuel pump runs non stop with the engine off, key in run...
The check engine light stays on KOEO and when running....
The exhaust is dark and smells rich...
The check engine light doesn't flash out the codes, just stays lit.

When it's in the good mode,
The fuel pump runs about two seconds key in run, engine off..
The check engine light is out KOEO and when running....
The engine runs good...
The check engine light flashes out the codes normally.

I was thinking the computer was bad or had a loose connection but I'm not sure where it is. Where is it?
Thanks, Paul

 

Ok Paul.
The pump normally runs from a timer circuit in the ECM as long as it is pulsed by the dist PIP pickup of a running engine or pulsed on by the ignition switch power up that lasts a couple second till the timer runs out.
If the pump runs without the engine running then there is a ground on the same lead the ECM use to power up the (pump relay) or the relay has burnt contacts sticking closed and powereing the pump.
Find which condition you have for that problem then move on to the rich issue.
To find which fault for the pump, tap the relay hard to see if the pump stops. If not remove the relay when the pump is stuck running then measure for a ground from the ECM to the relay "winding". The ECM lead also appears on the test connector. If there is a ground present then unplug the ECM to see if it opens up.
Take it a section at a time to run the fault down.

 

check the small gound at the negative battery cable
if this is bad the pcm does not ground proper
and the relay timer function is affected also pcm ground
mitch

 

When the engine is off, and you turn the key to Run, and the fuel pump continuously runs (until you turn the key off), will it start? Or will it only start when operating normally?

 

Yes, it will start then but will run poorly. The check engine light will be on and the truck is barely driveable. After a while the check engine light will go out, and it will start running good. If you turn it off and try to read out the codes, there are none. Then the fuel pump operates normally untill it starts messing up again.
Thanks, Paul

 

Normally, I would go with the ECM, but since it starts, that may not be the problem. I'm suspicious of the distributor pick up.

 

Hi,
I just wanted to finish this thread. My local parts store had the EEC/PCM for $99.99 with exchange. I swapped out the EEC and that took care of the problem. I have worke on a lot of cars and only had to change one other PCM, so I think they must be fairly reliable. This one was bad then good then bad but probably rapidly going all bad.
Thanks for your help all,
Paul

 

When it comes to Ford systems, 95% of the time, or more, that the fuel pump stays on even with the key off, it's a bad ECM. Add to that the inability to pull codes and about 99% of the time it's a bad ECM. It was just unusual that you were able to start it when it would act up. Anyway, thanks for the update. I'm glad to know you got it fixed.


BTW, the way you can always tell a GM ECM is bad is when the fuel pump relay starts to sound like castanets.