Cadillac Repair: surging idle, seville sts, egr valve

Question
QUESTION: I own a mint condition 1994 Seville STS northstar with 140,000 miles. Recently it developed an idle problem. At stop lights the engine will idle down to aprox.700 rpm and hold for about 10 seconds then it will drop down to 500 rpm and immediately surge back to 750 rpm. This is intermittent as it only happens about 75 % of the time. Recently it went 2 days with no idle issues and went back to the surging rpm on the third. I have gone to 3 repair shops to no avail. Help


ANSWER: IS or HAS the CHECK ENGINE light ever illuminated on the dash when this occurs ?.

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QUESTION: No, never and my mechanic ask me the same question, he thought that was strange and told me there were no codes in the computer either. The throttle body was cleaned but that did nothing to correct problem. What are your thoughts? Where should we check next?

Answer
Being an intermittent fault it could be any number of things from a sticking EGR valve , intermittent fault in the Idle control circuit to an intermittent wire connection to an intermittent fault in the computer to an intermittent fuel pump fault.

This is goin to require a VERY GOOD technician to be able to have the vehicle for several days so he/she can experience this intermittent fault in order to make a diagnosis.

Try cleaning out the EGR passages of carbon deposits and cleaning the EGR valve.