Reader Question: Erratic Idle?

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Here’s the latest reader question, along with my reply! 

Jay asks: I have a Nissan pickup that does a weird thing. The engine revs on its own. This happens intermittently. The engine will run up to around 2,00 RPM and then back down to its normal idle speed and then back again for awhile. It usually stops after a few minutes of this. By the way the truck has a manual transmission. What do you think could be causing this?

My reply: Lucky for you I have a Nissan pick-up, too – and the same thing happened to mine. Check the idle air controller. It’s probably faulty. As an aside, in the Olde Days, some GM vehicles with carbs had a mechanical device that kinda-sorta served the same purpose. A little plunger would shoot out to increase the engine idle speed by pushing on the throttle arm when the AC was turned on. Sometimes, these things stuck – and the idle would run high for that reason. It could be fixed with a screwdriver, too. Just remove the little plunger thing and screw in/out the idle speed screw to obtain the idle you liked.

In modern EFI cars, you get the idle the computer likes.

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  1. A few more things to check if a dirty lazy IAC isn’t the culprit…
    A faulty Throttle Position Sensor can cause erratic engine speed. You can check them with an ohm meter or scan tool with live data. Got any hesitation or surging? Ac commanded on with a faulty compressor clutch that doesn’t engage could also cause intermittent high idle. Check for vacuum leaks or flaky MAP/MAF sensors as well. And of course before anything else make sure the throttle body operates smoothly. If its the electronic type get a helper to operate the throttle as you observe the throttle plates operation. Do not force it by hand.

    Good luck, report back!