Evening. I own a 1989 Ford F250 with a 5.8 (351w). A few months ago it lost spark as i was driving it, got it home and replaced the TFI (thick film ignition) on the distributor. After I replaced that it fired right up! Although it's not the same as before. Now, it has a longer crank before starting. I verified the timing, that's perfectly fine. It has a weird shutter(?)/stumble. When it shutters, the whole trucks shakes, and the RPM drops but comes back up and everything smoothens out. (It all happens in seconds at the same time, it's tough to explain). Another thing it does is when I roll to a stop light or sign in neutral (it's a 5 speed), the RPM drops really low, almost to the point of stalling, but come right back up and idles. It's only shut off twice. I ran a scanner and got code 18, which is IDM circuit failure or SPOUT circuit grounded. I checked all the wiring and it seems fine. Could i have just gotten a bad TFI out of box? Maybe bad distributor? Or did something else conveniently go wrong at same time as the TFI? Could've something else caused the TFI to go bad originally and it's continuing to be an issue? The mileage has also been TERRIBLE, i get 8 mpg on a good day and has gotten as low as 7. I understand old trucks don't have a good record for fuel mileage but 8 seems low. Sorry for this being long winded, Im just making sure i give all the details 😂. Thanks!!

 

There may be more than one problem.

Remove one or two spark plugs and tell us what they look like. Grey, black and sooty ?

Where did you obtain the TFI module?

Did you put the heat sink compound on the back of the replacement module before installation ? If not, heat can destroy the module.
There is a yellow spark output wire that should be checked for contacting a shield ground. Check that wire carefully..

 

The spark plugs seemed more on the black side. I got the TFI from advance auto (not the best choice but i needed it in a pinch). I checked the yellow wire, that seemed fine. And as for the TFI I put dielectric grease on it. Again, probably not the best idea but I was working with what I had.

 

When checking that yellow wire , don't be afraid to check it back as far as you can.