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Dropping idle, surging idle

1990 Maxima…245,000 miles…car idles fine except when sitting at stoplight for more than 15 seconds, then idle starts to drop slowly and become unstable. Any suggestions? Then there is a seperate issue: After putting the car in Park (after warming up) the car will drop/surge from approximately 1500 to 1800 rpms three times and then go back down to normal (I am guessing transmission clutch)…Thanks for your help
-Matt

Sounds like an idle valve problem. Hondas are notorious for this. Google for “nissan idle valve,” see if that helps. Regardless, it’s probably a vacuum leak somewhere.

You describe the symtoms of a defective Idle Air Control valve. This valve controls the amount of air that enters the engine at idle conditions. This is an electrical component mounted on the throttle body. And as they get old they can start to what is called “Hunt” for the proper position for the idle condition. This then causes the idle to be erratic or the engine can stall while idling.

Tester

This might be something as simple as a dirty throttle plate or a dirty throttle assembly. Before you try anything else, take the air filter off and spray some throttle/carburetor cleaner into the throttle. You should do this with the engine running and and someone using the throttle to keep the engine running. This just might clear up the problem.

If this doesn’t clear up the problem, look to what the others recommend.