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Discussion Starter #1
As most of you may know, I recently bought a ’90 F-150 with a dead cyl and swapped in a well-running, known good motor in its place. The truck runs fine anytime I am on the throttle, and the problem happens mostly when I am trying to park the truck. It will die almost every time I am backing up trying to get into a parking space. The truck has a solid idle, around 650RPM, if I am stopped in park and after I shift it into reverse. It usually goes something like this: place into reverse, touch the gas pedal enough to get moving, let off the pedal, start to slow down and the RPM drops to almost zero, then it will jump up to about 1000RPM (as though the computer is trying to keep it from dying), then it dies. This only seems to be a problem if the truck is warmed up already. A less common occurrence is when the truck does this when I am moving forward. It doesn’t happen all the time like it does in reverse, but it does happen often enough for me not to trust it when I’m trying to make a left turn across traffic with just a green.

I do have a leak in the passenger side header/y-pipe connection, but I fail to see how this would cause this problem.

I replaced the throttle body complete with the TPS and IAC (idle motor valve) with known good parts that I had lying around. No change.

I just installed new (less than 500mi ago):
Cap, rotor, plugs, and plug wires.
MSD coil
Set timing to 13.5BTDC (it has much better pick-up after this nice little mod)
Ghetto FIPK
PCV valve
Rebuilt injectors (a shameless plug for Mr. Injector, good injector rebuild kits)

I don’t see how any of these codes could cause the problem, but here they are anyway.

KOEO (my CEL is never on for more than a few seconds, but sometimes it will come on for a few seconds after I start the truck or intermittently when I am driving it for a few seconds):
34-PFE or EVP above closed limit .67V (btw, what in the hell is the PFE? Does this mean my EGR may be stuck partially open?)

CM:
34-Intermittent PFE or EVP
51-ECT > 4.6V

KOER:
21-ECT out of range .3-3.7V (I think that this one was my fault, the engine wasn’t completely warmed up)
44-Thermactor air injection system inoperative
34-PFE or EVP above closed limit .67V
25- Knock not sensed during dynamic test (my timing was a little retarded when I did this KOER, so I’m not sure that this really means anything)

Could that Code 34 be something meaningful? Maybe my EGR valve is stuck partially open and the truck is starving for fuel when I let off the throttle. That’s my wild ass guess anyway.
 

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oh man i just came to post the same question.. i got an 88 doing EXACTLY the same thing.. and i've also replaced the PCV valve, cap, rotor, plugs, spark plug wires, oil change fuel filter, added some chevron techron, ran it forever hoping it would fix itself, but hasnt.. and like you said, i trust it but not enough when im stopped or parking, or making a quick turn at a light...

i was thinking IAC for mine, or the MAP sensor.. anything guys?
 

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Discussion Starter #4
Need to add something. I know it isin't a vacuum leak. I plugged all of the "branches" on the vacuum tree and ran new lines to the fuel rail, and MAP sensor. Still did it.

On the issue of the EVP sensor. I was thinking that maybe the sensor isin't bad, but the valve itself. If the valve were stuck slightly open, than the mixture would be lean all of the time, expecially when there is a sudden opening and closing of the throttle.

elemen0hpey, do a pull the codes and run a KOER and see what codes it gives you.
 

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Discussion Starter #6
Cleaned the IAC, seems to have solved the problem. I also had to replace the ECT because I broke the one in the motor, don't ask...
Here are the codes I've got now after resetting the computer:

KOEO (my CEL is now constantly on)
34-PFE or EVP above closed limit .67V

CM:
34-Intermittent PFE or EVP
51-ECT > 4.6V

KOER:
44-Thermactor air injection system inoperative
34-PFE or EVP above closed limit .67V
25- Knock not sensed during dynamic test

I didn't get the KOER Code 21 this time, but the CM still tells me that there is something wrong with the ECT and EVP or the wires going to them. I'm leaning toward the wiring now as the ECT is a new sensor. I'll do some tests to see if there is any continuity between the VRef side and the Return signal at the sensor plugs. I hate wiring problems.

BTW, does anyone know how many connections the engine harness has to the chassis harness? I've got a spare wiring harness that I might rebuild and swap in.
 

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Discussion Starter #7
Steve83 said:
650 is a low idle.
After I cleaned the IAC it now idles just slightly higher than before, about 700RPM now. If I remember correctly, I read somewhere on fordfuelinjection.com that the computer tries to set the idle at 642RPM. What RPM does your truck idle at?
 

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Discussion Starter #8
Update:
I drove it for the second time after cleaning the IAC. Any more ideas? The dying has returned. The CEL didn't turn on at all this time. At least I'm getting 16MPG, though. :thumbup I'll see if I can't find a wiring short in the EVP sensor tomorrow, though.
 

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Discussion Starter #9
I reset the computer and ran the codes again. Got Code 34 for KOEO, CM, and KOER. EVP out of range. I also got Code 44 for KOER. Thermactor. I backprobed the EVP and the sensor is working correctly, so that means I have a wiring problem somewhere between the EVP and the EEC.

I also backprobed the TPS and ECT and they check out good. I am completely stumped on this one. It almost sounds like a mechanical problem that the EEC dosen't have a sensor to tell me about. I have ruled out anything tranny related because it will happen in park and neutral. If I grab the throttle linkage and open it just slightly the truck will stumble and die just like it does when I am in reverse.
 

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The 302s have a power steering presure switch that is supposed to increase the idle when you start using the steering heavily to keep the truck from dying. This could be going bad and staying open/closed when it should be doing its job. Its on the presure line between the box and the pump.
 
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