For starters it is on a 91 f150 4x4 5.0 e4od. The problem is it started acting up a couple of weeks ago. The first thing I noticed was the idle would jump up and down randomly. It might go from normal to 2000 rpm stay there for a while then might drop to 450 rpm and try to die then right back to 2000rpm . I have replaced the iac more than once and had it come right back. It would also make the transmission not want to shift right, although I think that has to do more with the motor wanting to idle so high. The next thing that it did is it would lose fire randomly when trying to start it. If it did start it would stay running until you turned it off. Then when you went to start it sometimes it would start, sometimes it wouldn't at all, and sometimes it would try to start than lose fire again. The frustrating part is there seems to be no set pattern. It doesn't seem to matter if it has been running or if it is first thing in the morning. The only pattern is it seems to be happening more and more frequently. When I tried to run a scan on it the only thing that it really come up on was 512 eec keep alive memory, but from what I have read it leads me to believe that is from killing the battery from trying to start it so much. I am starting to lean toward either the computer going bad or maybe a bad ground, loose pin connector, etc. but am running out of ideas and patience! Any ideas? Please help, this rental car is starting to get expensive! FWIW I would drive my 93 5.0 thunderbird but it died the same exact way not even 6 months ago so I know this isn't just a truck thing!

 

Sounds like a throttle position sensor (TPS) to me. When they aren't getting the right voltage they start making the truck idle weird and the transmission do crazy stuff.

 

I will check that too, but I dont really think that would make it loose fire. True the drivability problems are annoying, but not as much as the not starting problem!

 

Could be a bad ground inside the Distributor. This ground provides the only ground for the Ignition system. If not you may want to try another TFI Module.

EDIT:
You might want to check and make sure you have a good body ground. There should be a smaller black wire running from your Neg (-) Battery post to the body and while you are looking at the body ground there should be a Black wire with a white (sometimes green) stripe hook to that point, this is your PCM ground. This will give all kinds of problems if that ground is not a good one.

Your Ignition Switch can also give the above problems.

 

On my ground from the battery there is only one small wire running to the core support. Do you have any other idea where the pcm ground could be such as the main harness? As far as the ignition switch goes if it turns on the fuel pump, do you think it could still be that? I mean is there a wire or circuit that goes to the ignition that is separate from the fuel pumps, etc.?

 

I think on a 91 the PCM ground wire is hooked to the same core support bolt as the small wire from the battery.
The Ignition switch does not turn on the fuel pump, the PCM grounds the fuel pump relay to turn it on.
But the wire form the Ignition switch that picks up the PCM (EEC) Power Relay also feeds the Ignition system also. The PCM power relay turns on the PCM and also provides the power for the Fuel Pump Relay coil so that the PCM can ground it.

Here is a link to a diagram of your engine wiring.




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