Bringing a classic Nissan 300zx back to life is a rewarding experience, but sometimes restoration projects can throw unexpected curveballs. Imagine the satisfaction of reviving a 300zx, only to be met with a frustratingly rough idle. This is a common issue faced by car enthusiasts, and it’s often related to systems disturbed during the restoration process. If you’re experiencing a similar problem with your 300zx after its revival, this guide is for you.
One 300zx owner recently shared their experience after an extensive restoration. Having rescued a neglected 300zx Twin Turbo (TT) that had been sitting for a decade, they undertook a comprehensive restoration. This involved stripping the car down, addressing rust in critical areas like the floor, roof, sills, and engine bay, and completing a full respray. The engine was removed and meticulously reinstalled with new components such as the water pump, timing belt, and auxiliary belts. The intake manifold, throttle bodies, and IACV were also thoroughly cleaned. Upon reassembly, the 300zx fired up smoothly, initially boosting and driving well. However, an idling issue surfaced, becoming a persistent headache.
The owner describes the problem as a consistently rich running condition, requiring constant throttle input to prevent stalling. A slight misfire is also noticeable around 1000rpm. To address this, they’ve already systematically checked several potential culprits:
- Spark Plugs: Confirmed to be sparking correctly.
- Vacuum Leaks: Smoke tested, with no leaks detected.
- ECU Codes: No fault codes are present when flashing the ECU.
- TPS (Throttle Position Sensor): Functionality appears normal.
- IACV (Idle Air Control Valve): Confirmed to be working.
- PTU (Power Transistor Unit): Replaced with a new unit.
- MAF (Mass Air Flow) Sensor: Replaced with a brand new sensor.
Despite these efforts, the 300zx idle issue persists, leaving the owner understandably “exhausted” and seeking solutions to enjoy their newly restored summer driver.
If you’re facing similar 300zx idle problems after a restoration, consider these additional troubleshooting steps:
Fuel System Checks:
- Fuel Pressure Regulator: A faulty fuel pressure regulator can cause excessively high fuel pressure, leading to a rich running condition. Test the fuel pressure to ensure it’s within the 300zx specifications.
- Fuel Injectors: While less common after a restoration unless injectors were removed and potentially damaged, leaking or improperly functioning fuel injectors can also cause a rich mixture. Consider injector testing or replacement if other fuel system components check out.
Sensor Checks (Beyond TPS & MAF):
- Coolant Temperature Sensor (CTS): The CTS provides crucial temperature data to the ECU for fuel mixture calculations. A failing CTS that incorrectly reads a cold engine can cause the ECU to over-fuel, resulting in a rich condition, even when the engine is warm. Check the CTS resistance and signal voltage.
- Oxygen Sensors (O2 Sensors): While the owner hasn’t mentioned O2 sensor codes, if the 300zx has been sitting for a long period, the O2 sensors could be degraded or contaminated. Faulty O2 sensors can provide incorrect feedback to the ECU, potentially leading to improper fuel trims and a rich condition.
Mechanical Timing and Compression:
- Timing Belt Alignment: Although the timing belt was replaced, double-check the timing marks to ensure the belt is correctly aligned. Even a slight misalignment can affect engine performance and potentially contribute to idle issues and rich running.
- Compression Test: While not directly related to richness, low or uneven compression can cause misfires and rough idle, which might be exacerbated by a slightly rich condition. Perform a compression test to rule out any underlying engine mechanical problems.
Intake and Exhaust System:
- Exhaust Leaks (Pre-O2 Sensor): Exhaust leaks before the O2 sensors can introduce extra oxygen into the exhaust stream, causing the O2 sensors to read lean and the ECU to compensate by adding more fuel, leading to a rich condition. Inspect for exhaust leaks, particularly around manifold gaskets and joints.
- Intake Air Leaks (Post-MAF): While vacuum leaks were ruled out with a smoke test, meticulously check all intake connections and hoses after the MAF sensor. Unmetered air entering the intake system after the MAF can lean out the mixture, but sometimes the ECU overcompensates in other areas, potentially creating a rich condition in specific scenarios or at idle.
ECU and Wiring:
- ECU Issues: While less likely, an ECU malfunction could be contributing to the problem. If all mechanical and sensor checks are exhausted, ECU diagnostics or even testing/replacement might be considered.
- Wiring and Connectors: During a restoration, wiring and connectors can be disturbed. Carefully inspect all wiring harnesses and connectors related to the engine management system for damage, corrosion, or loose connections. Pay particular attention to grounds.
Restoring a 300zx is a labor of love. Don’t let a persistent idle issue diminish your enjoyment. By systematically working through these troubleshooting steps, you can pinpoint the cause of the rich running and rough idle and get your 300zx purring smoothly once again, ready to be enjoyed as the summer driver it was always meant to be.
