Do It Yourself Auto Repair Case Study #2 - Author's Nightmare

I'm dredging up this case study about a 1993 Ford F-150 that appeared to have some serious engine performance issues, when in fact it turned out to have a simple problem most technicians rarely consider, let alone test for. I had maintained and repaired this particular vehicle for over 2 years, including doing extensive work to the front end, suspension, fuel delivery system and even an engine replacement. All recommended services were promptly addressed and completed. My rapport with this customer was very good and he trusted me explicitly for repairs to all his vehicles.

The truck came to my shop with the engine running very rough only when warmed up. It had been tuned up less than 6 months prior. I did notice a new alternator had been installed and not by me. I remembered the customer had told me he was on vacation a month ago and had it installed because it failed while traveling. I inspected both the primary and secondary ignition system for signs of failure. These systems checked out fine. I hooked up my scan tool to retrieve any service codes, there where none. I ran both a "key on engine off" and "key on engine running" self tests with my scanner. No codes where retrieved. I then went to data stream mode and observed the critical engine control inputs and outputs. All readings appeared normal except for the upstream O2 sensors; both indicating "fixed rich".

I immediately attached my lab scope and verified the condition of the upstream O2's. Both where at .75 - .9 volts and not "shifting". By the sound of the engine and the condition of the upstream O2 sensors, I was inclined to believe that the vehicle possibly had an ignition timing issue. I then attached my timing light, unplugged the "timing jumper" and checked base timing. I was dead on. I reconnected the timing jumper and checked the PCM's timing control. It was all over the map! I could not get a stable reading, even at idle. This lead me to conduct testing on the Ignition Module and Pick Up Coil Assemblies. These components also tested fine.