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idle surges between 700 and 900 rpm, and then it levels out to 850 rpm and holds steady for a short while. then it returns back to surging again. it just does this all the time at any temperature. and only at idle. all vacuum lines are ok. any suggestions?

1983 911 3.0L cis

Edited by 83QualityQuest911
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PCA Tech Specialist Allan Caldwell answered this once before. Here is his reply:

Surges at idle with CIS are often caused by either the warm-up regulator not providing the correct fuel control pressure, a problem with the vacuum hose system used on the later CIS cars, a problem with the auxiliary air regulator, or possibly an incorrectly set idle mixture control, rpm and oxygen sensor interaction. The fuel distributors are usually not a factor. In 1980/1981, there were a series of owner problems related to erratic idle in which the variation would be much greater than what you have experienced, like +/- 500 rpm or more. There was a tech bulletin released on the problem and one of the things they recommended at that time was a check list of first carefully setting all the basic mixture control and idle rpm settings and then, if the condition still existed, to remove the deceleration valve from the pneumatic circuit and compress it in a vise by 1 mm (a socket was used to protect one of the pressure taps in the middle of one side). Supposedly it was fixed after that, and the pneumatic systems on the 82 and 83 models were changed, but I have received a few owner complaints on even the later models.

It is really important to first make sure there are no vacuum leaks in the intake manifold or connecting hoses (this includes the injector seals and even the oil tank cap seal). The auxiliary air regulator has to be working correctly and fully closed when the adjustments are made. The warm-up regulator should be providing the correct fuel control pressure for both the warm-up phase and when the engine is fully warm. The ignition timing at idle has to be right at 5 deg. BTDC. The mixture control has to be adjusted 0.6 +/- 0.2% with the oxygen sensor disconnected using an exhaust gas analyzer, and the idle set to 850 rpm. Then the oxygen sensor is reconnected and the idle should be stable. If not, there may be a problem with either the warm up regulator or auxiliary air valve.

Allan Caldwell 1-10-02

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