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Cold Off-idle Stutter 280Z


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Good afternoon All,


I'm at a loss for what could be causing an issue I currently have with my '75 280z. The car starts fine when cold and idles fine as well, but if I blip the throttle when the engine is cold it'll hesitate and sputter, sometimes with a popping sound. Plugs, wires and cap/rotor is new. The thermotime switch is new, as is the cold start injector and AFM. Cold idle AFRs are between 10~11. Timing is set at 7' BTDC, per the FSM, and vacuum is between 18~19 with a steady needle. The issue completely disappears once the engine starts to warm up. I have also noticed that the engine does not idle up from a cold start, and the auxiliary air regulator and connector are both new and I've verified that it's receiving battery voltage when running, so it should be working fine. 


Is it possible that my cold AFRs are too rich and causing the issue? I have the AFM adjusted a few teeth from stock to compensate for a full exhaust system and the ethanol we have in our fuel here.  Idle AFR when warm is 13-13.5. Any advice will be greatly appreciated!




I rechecked my ECU connector and found a bent pin for the throttle switch circuit ( pin #18) that basically meant the car has been running on a part-throttle circuit the entire time, and that the stumble was likely due to not having the full-throttle circuit connected. Sorry for the post!

Edited by NC280z
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