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Discussion Starter #1
Hey mates, I'm having idle problems with my 1991 Nissan GT-R.

I can start up the car just fine and it will idle normal. However, once I rev the engine to pull out of the garage or more recently to check if the problem is fixed, the motor begins to suffer rough idle. Revving higher means the idle dips lower before returning to normal (such as to 3000 RPM, it drops to 800 before returning to 1100 / rev to 5000 drops it to 500 when I let off the throttle, then revving to 7000+ drops it to 100-300 and the motor stalls). When driving, the car will stall if I hold the clutch down too long between gears, or while waiting at a light. The car spits black smoke

I have checked

Timing Belt (to see if it skipped a tooth)
MAFs are replaced
IACM (cleaned the grease and gunk out)
Spark Plugs (all replaced)
Fuel Filter (brand new G35)
Catalytic Converter (Now straight-piped, but I removed the thermocouple - could this be causing the problem now?)

I have received some advice that it may be TPS messing with the idle or that it may be stuck fuel injectors. I don't believe it is stuck injectors as every spark plug has blackened excess fuel on them, meaning each spark plug is getting fuel, just that they aren't burning all the fuel they receive.

If anyone has any insight, I'd be greatful if you could help me out. I'm more than happy to provide more information if needed; I just want to get my baby back on the road again.
 

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Things to check, vacuum leak or lack of to fuel pressure regulator, TPS voltage, and if it occurs mostly when the engine is cold the ECU coolant temp sensor.
 

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Discussion Starter #4
Could it be a vacuum leak? or a vacuum pipe connected to the wrong place?
Thank you for the advice. I checked for a vacuum leak using propane, and there was no change in idle.

After removing the TPS, it is heavily corroded at the sealed plugs, so I can't imagine how bad it must be where it isn't sealed. Also, the TPS was loose in the bay, and rotating one way made the idle 2000 RPMs, then it stalled. Starting it up, then wiggling the other way made the idle drop to 600 then stall. Rotating its direction the original way again made it drop to 400, so due to inconsistent results, I believe the TPS is faulty. Bringing this to my family led them to the same conclusion as well.

I will update with how it goes
 

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Discussion Starter #5
Things to check, vacuum leak or lack of to fuel pressure regulator, TPS voltage, and if it occurs mostly when the engine is cold the ECU coolant temp sensor.
Update: Throttle Position Sensor is replaced, and now the car doesn't stall, however it drives like the BOV is always open. (potential vacuum leak I missed?).

I have an HKS EVC 5, and if that went bad, could that cause these problems?
 

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So your saying that the motor is not making any boost? could be failed turbo, stuck waste gate, bad actuator, bad BOV or pipe leak, or the EVC solenoid is stuck, but it should still make some boost if that where the case.
 

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Discussion Starter #7
So your saying that the motor is not making any boost? could be failed turbo, stuck waste gate, bad actuator, bad BOV or pipe leak, or the EVC solenoid is stuck, but it should still make some boost if that where the case.
Now I have replaced the coolant temp sensor and intake temp sensor, and after some timing adjustments, the car drives really well; full boost, accelerates hard, but the RPMs will drop to 600 after a hard pull, followed by minor misfire feeling at first. Also to note, the turbo makes a whistling noise that shrills more than it used to.

Do these sound like any of the symptoms for a problem you listed?
 
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