Having little experience with ailing EFI cars, I searched the Internet and quickly found out that everybody has an opinion. It's the computer, it's the knock sensor, it's the mass airflow meter (MAF), the catalytic converter is clogged, the fuel injectors are bad, it's the oxygen sensor, it's the EGR valve, it's the fuel pump, it's Duntov's ghost. But the thing I heard most was nightmare stories of how TPI cars can nickel and dime you to death. If I followed all the advice, I'd join the parts changers, spend more than a grand on dead-end leads, and end up in Gulchville. I braced myself for the decision to either cut my losses and sell the thing as a fatally flawed parts car or dig in and make the needed repairs . . . regardless of the cost. Having come so far, I dug in. My first task was to read the trouble codes stored in the ECM.
Doing the TPI paper-clip trick, I jumped the A and B terminals of the assembly-line data link (ALDL) under the dash. The SES flashed a 21 (throttle position sensor voltage high), a 34 (MAF sensor reading too low), and a 43 (knock sensor). Wow, how easy is this? I whipped out my wallet and dutifully replaced the knock sensor ($51.95), checked TPS voltage (it was spot on at 0.54 volts), and hoped the MAF sensor was OK, but priced replacements just in case ($500 new, $250 remanufactured). Dry dive, here I come.
When I turned the Vette's plastic-coated key, the L98 sprang to life with no SES activity. That changed as soon as I mashed the gas. Hello, SES! This time the paper-clip trick revealed only a Code 43 knock-sensor problem, the other two codes were not seen again. It was toying with me. I felt like a gambler on a losing streak. Lacking the experience to go any further, I sought professional help from Rich Sjoberg at Corvette Specialists.
I told Sjoberg about the rod-bearing hassle and convinced him that my mechanical work was not at fault. Connecting his Snap-On scan tool, he noted a Code 42 (electronic timing-control problem). Naturally, all the codes I had witnessed refused to reappear. This got Sjoberg curious about the many electrical connections to be found under the C4's clamshell hood. After a thorough examination, nothing seemed out of place, and all 11 ground wires were present and connected. I felt so proud. But the Code 42 persisted. Relying on his instinct to take customer claims like mine with a grain of salt, Sjoberg went back to basics.
On any TPI car, checking the basics consists of confirming minimum idle speed, TPS voltage, and base ignition timing. The Vette's idle speed was correct at 550 rpm, TPS voltage was still spot on, but when Sjoberg checked the ignition timing, he discovered the distributor was installed 10 degrees retarded after top dead center (ATDC) instead of the factory-specified 6 degrees advanced before top dead center (BTDC). The thing was waaay retarded.
The throttle-position sensor is a potentiometer that translates throttle-blade movement into an electrical signal the computer can read. It must register between 0.5 and 0.6 volts with the key "ON" (engine not running). The Thexton harness connector (PN 375) shown here allows the voltmeter probes to read voltage without the harmful effects of piercing any insulation. The pen points to the probe access openings at the end of each lead.
To check base timing, unplug the electronic spark timing (EST) connector at the distributor and start the engine. With the engine idling, use a timing light to confirm or establish a reading of 6 degrees BTDC. Reconnect the EST wire after this operation.
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