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75 280z - Rough surging idle when cold


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My son's 75 280z is having the following issue. When we start it up cold (sitting overnight) the car starts with no problems however the idle surges between 600 and 1500 rpm with about a 1 second frequency. This goes on for about 1-2 minutes. After the car starts to warm up slightly the surging goes away and idles smooth as glass. If we shut the car off and start up warm there are no problems. We've tried using throttle body cleaner to spray around and look for vacuum leaks but have found none. The one clue that we discovered this weekend was that vacuum is low and variable when we start the car cold (12 - 15). When the car warms up and the idle gets smooth the vacuum goes up to 19 and is steady. We have checked all of the sensors (water temp, air temp, etc...) and they seem to be fine. We just pulled and cleaned the BCDD and that seems to be ok as well. The throttle plate works smoothly and is clean. Does anyone have ideas as to what we can look at to find an answer to this problem? Any pointers would be greatly appreciated.

 

Thank you,

Mike Nycz

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