Hi,

I have two problems with my 91 F250 with the FI 3.9

1. I can't seem to read the codes with either a jumper wire or a code reader. I get no light flashed with the jumper (I have seen the light in the cab light up on start but not every time), and when using the code reader I get one of two things. Key on eng off, the reader sorta stalls and keeps "checking" forever without returning any codes. Engine on, reader revs engine and then pauses for quite a while, then says "can't communicate"

2. Truck runs OK over all, but it has a really annoying bog just off idle when you tap the throttle.

Disconecting the EGR vac line did not change it. New IAC did not change it (did help with high revving between shifts). Have checked the vac hoses, found and fixed one, no change in bog. I filled the intake with smoke to look for other vac leaks, none apparent. I checked fuel pressure, ok. I checked the voltage output of the throttle body, seemed to smoothly increase without a dead spot. The idle stop screw seems to have its thread locker intact.

Ideas?

 

I pulled the computer. Found one leaking capacitor. Going to fix it and see if that will allow me to read codes and maybe fix the driveability issues.

 

I had a similar problem with my 89 F150 4.9-- it would almost stutter when trying to start. But once it did it would idle fine and then get a little rough and die. I replaced the fuel filter as well as cleaning out the tank and replacing the fuel pump in the tank. Its been running great ever since.

So if the capacitor isn't the issue, maybe its something fuel related. Just food for thought

 

While you're in there.............you may as well replace the other two capacitors as well.
They'll start to fail soon.

Here's a little read for you : A9x ECM's (and same years ECM's) Failures Due to Age

Bob