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Old 08-21-2011, 11:11 PM   #5
shorod
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Re: 94 SHO intermittent start problem

The loss of tachometer is most likely the "Cylinder identification" issue, and is probably due to either the CMP sensor or possibly the ignition module.

If your TPS sensor is non-linear, it could be giving a very odd reading when the throttle is closed, contributing to the 121 and 122 diagnostic codes. As you open the throttle, the TPS voltage gets back to the expected range and therefore your car starts fine. You can also try cleaning the throttle body, throttle plate, Idle Air Control (IAC) servo, and IAC passages. Issues with the IAC or passages could contribute to the problem starting as well as why it may start when the throttle is slightly depressed, but would not explain the 121 and 122 diagnostic codes.

The TPS is pretty easy to test if you have a multimeter. Testing the CMP correctly really will require an oscilloscope.

-Rod
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