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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Doesn't appear to be a factory wire. Maybe someone had a nitrous kit, or a thermostatic fan switch. Are the ends of both wires not connected to anything? Where do they originate?

(This response was typed BEFORE your second post/picture)
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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14,975 Posts
Does the A/C work? There's a wrapped wire with a diode in it for the A/C clutch circuitry. If I recall correctly, that is routed around the alternator. The wires with the diode are dark green and black. If you have the factory service manual, look at Page 8A-64-1.

Free 1994 factory service manual downoad:

 
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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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When does the idle surge - cold start, during engine warm up, engine fully warmed up, all of the above? What is the range of the surge, low RPM to high RPM? Changes slowly or rapidly?

Is this an auto or manual trans? The affects idle programming.

What brand is the Opti? I've diagnosed three cases in the past of huge swings in idle speed (700 - 2,700+ RPM) caused by defective MSD Optis. Those were units produced at least 5 years ago.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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How do you know it's the Opti?

The stock PCM programming for an auto trans, fully warmed up, is 550 RPM in gear and 650 RPM in neutral. It's not unusual for the stock tach to read 100-200 RPM above actual RPM. The only way to see the true RPM is with a real time OBD-1 scanner, or data logging software.

For coolant temperatures below 154°F the PCM elevates the idle speed. It can be as high as 1,200 RPM on cold start on sub-zero days.

Idle speed at closed throrrle, vehicle not moving is controlled by the idle air control (IAC) valve in the base of the throttle body. That can be related to erratic RPM. As can loose accelerator linkage or cable, or faulty throttle position sensor. The Opti can cause the problem if the dielectric in the cap that separates the internal cylinder wire routing is failing. The MSD is the only brand I have found the problem with the cap. Most of these problems are very difficult to detect.

To repeat my earlier question, what brand is the Opti? Does it have a red cap? Appears the throttle body is stock. Are there any significant engine mods like a more aggressive camshaft that would require custom PCM tuning? Aftermarket camshafts without good tuning can produce surge.
 
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