The established process for setting static TPI ignition timing after engine disassembly involves positioning the harmonic balancer at 6 degrees BTDC then stabbing the HEI distributor with the rotor and No. 1 tower perfectly aligned. The computer takes it from there once the engine is running and imposes a preset timing curve that's factory calibrated to suit the engine.
Even though I had taken time to make sure the harmonic balancer mark, No. 1 piston, and TDC notch on the timing tab were in harmony-easily done when the engine was out of the car with the heads off-my error was a botched effort at maintaining proper rotor and cap alignment when installing the HEI. I must have bumped the distributor and screwed up the rotor-to-cap alignment during reassembly.
Fortunately, Sjoberg's scan tool detected the ignition problem in real time, and with a quick twist of the distributor housing, baseline timing was corrected. This gave the ECU the proper starting point from which to establish a workable ignition-timing curve. The amazing thing is how the ECU was able to make the best of my lousy distributor installation and that the car ran so well (at part throttle) despite my screw-up. In fact, it ran so well, I totally discounted the possibility of ignition trouble in my description of the problem to anybody who'd listen. It had me fooled. But now the car runs great; no trouble codes flash and life is good. It's time to get back to the job of reclaiming the C4 from the yo-baby gold chain crowd and enjoying it for what it is ... the ultimate budget supercar for the Car Craft masses.
Troubleshooting Tips
* A classic TPI goof is mixing the wires for the knock sensor and coolant-temperature/fan-switch sending unit on the passenger side of the engine. The blue wire goes to the knock sensor, the green wire goes to the temperature/fan sensor. Cross 'em and you'll trigger a Code 42 electronic timing-control problem and/or a Code 43 knock-sensor alert and bring on major driveability problems.
* Sjoberg said, "These computer-controlled cars will do funny things if you don't have clean, tight electrical ground connections." After major surgery like engine removal, make certain all ground wires are intact and properly routed.
* Lots of mysterious TPI problems are caused by old batteries that can't maintain a full charge. Check the battery before you dig in. If the battery cables are bad (checked with a volt-drop test), replace the whole cable. Don't cut the ends off and use clamp-on replacements because the strands deep inside the cable may have resistance-causing corrosion you can't see.
* Scan tools are not the same as code readers. While inexpensive code readers eliminate the hassle of counting the flashes on the SES light, they don't read and display TPS voltage, oxygen-sensor cross counts, manifold absolute pressure, manifold air temperature, coolant temperature, and other helpful, real-time diagnostic data. Code readers don't always identify the root cause of the problem. Take care not to jump to conclusions and randomly start changing parts.
Without checking the basics, even the most exotic scan tool can lead you astray. Incorrectly advised that the base timing was set correctly, Sjoberg said, "The car threw me a curve ball. With a Code 42 showing, the scanner told me to check for 6 volts at the B7 connector on the car's ECM. It read 9 volts. I could have easily misdiagnosed the problem as a faulty ECM, a $250 (remanufactured) mistake that would not have solved the problem. It all goes back to confirming the three basics: minimum idle speed, TPS voltage, and base ignition timing before you replace any parts."
Sjoberg said, "This is the first I've seen with this type of an issue, where the timing was so retarded that it triggered a code." Another Car Craft first! A counterclockwise twist of the misadjusted distributor restored correct base ignition timing. Sjoberg spent 1.5 hours diagnosing our problem. At the shop rate of $75 per hour, the diagnosis cost $112.50. Cheap compared with a dive into TPI Gulch, and it solved our problem.
Base idle speed is adjusted very simply by turning a set screw on the driver side of the throttle body with the engine at idle. The idle air control motor (IAC) at the base of the throttle body must be unplugged first so it doesn't hold the throttle blades open. Once the engine idles at 550 to 575 rpm, reconnect the IAC and drive the car for a few miles to re-establish proper IAC function.