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I am new to this board. Since 1973, I have had 5 Z cars; 73, 77, 81, 85 Turbo and 93. Still got the 85 Turbo and 93. The last non-Z car I bought for me was a new 1971 Mustang. Driving Z cars for 32 years.

I just got finished with a full off restoration on my 85 Z31 Turbo. I also had the engine rebuild (it had 294,000 miles on it and still ran great). The engine was rebuilt using genuine Nissan parts. Virtually all sensors, hoses, regulators, etc were replaced. Here is my problem. When driving the car and slowing down, when the tach gets down to 2400 RPM, the car starts to surge and the tach will not go below 2400 until I press the brakes and force the engine to slow down. Sometimes, if I put in the clutch and come to a stop, the engine stays at 2400 RPM. The restoration garage spent 2 months trying to fix this but gave up. The Nissan dealer in Boston said it was the engine wiring harness $(1000) and installed a new one. Still got the problem.

If you stopped the car and rev the engine, it behaves correctly. The return to idle problem only happens when moving.

When the car is cold (first 2 minutes after starting), the engine will idle down fine and there is no 2400 RPM surging. Once the car warms up, the problem is noticed.

This symptom was similar to one I had 15 years ago when the speedometer got disconnected. The car would not idle down properly when moving until the speedometer cable was re-connected.

Anyone out there got any ideas?
 

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Discussion Starter #5
I have had this car for over 20 years. In the last 3 years, I have gone completely through the entire body of the car and replaced everything that was rusted. New front fenders, new rear quarter panels, new rocker panels, new inner fenders, front sub frame, patched floors, passenger door, ground effects, new chrome wheels, new carpets, headliner, new leather seat covers, new audio system, a 3 inch mandrel bent exhaust from the exhaust manifold back, a K+N cold air induction system, rebuilt transmission, rebuilt rear differential, new half shafts, brakes, shocks, urethane suspension bushings, steering rack. etc.......

The engine had 294,000 miles on it and in 20 years, I NEVER had to add even 1 quart. The engine was running fine but since it was out of the car, I had it rebuilt with all genuine Nissan parts. I have always used Mobile 1 Synthetic oil. The first turbo was replaced at 190,000 miles and was not smoking or making any noise. The engine shop said the inside of the engine looked very clean and was in very good shape. I swear by the Mobile 1.

The car looks brand new inside and out and even underneath. The engine runs fine except for the idle down / surge problem. Other than that, the car wants to scream.

I have had 5 Z cars in 32 years: 73Z, 77Z, 81ZX, 85ZX Turbo and a 93ZX. I have never drive a 350Z or a 300 ZXTT but the 85 is the fastest Z I have ever driven.
 

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I'd start by blocking off some of the things that are supposed to let metered air in, like the PCV, the air regualtor, the FICD/AAC unit (especially the "vacuum control valve" portion). The vacuum control valve seems to be just a manifold vacuum limiter.
The airflow diagram in the EF&EC section of the FSM suggests that the vacuum control valve is a separate part but I suspect it is integral with the FICD/AAC unit. The FSM does not go into an significant detail about the VCV.
If the engine runs correctly otherwise then for now we can assume that the air leak keeping the revs up is a metered air leak.
Unless the tune is stupidly rich, an unmetered air leak should reduce the revs.
I'd like to hear the dealer's explanation of how the harness was supposed to be affecting this problem.
 
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