Q: Engine idle fluctuates

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When traveling above 20MPH, if I use the breaks and take the load off the engine (press clutch/shift to neutral) the engine RPM drops instantly to 1,000 and bounces between that and 1,500 until either my speed falls below 20MPH, or I let off the break pedal. My engine has a high idle problem I'm also working to diagnose and the two may be related. Normally with no load on the engine it idles at around 2K. Sometimes this can be adjusted by revving, which drops it by 500-1K. The idle fluctuation when breaking problem is somewhat intermittent. I have an after market K&N filter on it. I've cleaned the MAF and throttle body. Replaced the idle air control valve. Changed spark plugs.

My car has 175000 miles.
My car has a manual transmission.

This may need to have the throttle stop screw adjusted. The throttle stop screw’s primary function is to prevent the throttle blade from closing too far and getting wedged/stuck in the throttle bore. However, it serves as a secondary function to adjust the minimum air setting. The “minimum air setting” is what is used to describe the amount of air that is allowed to enter the engine thru a “closed” throttle. Because the throttle valve cannot be allowed to completely close (because this would result in it getting wedged/stuck closed in the throttle bore), some air will always be allowed to enter the engine around the throttle valve. On a 100% factory stock engine, you should never need to adjust the throttle stop screw. But there are times when the adjustment of this screw is necessary. One example of this is when the engine is modified, or a different throttle body is being used than what originally came with the engine. Larger displacement engines require more air to maintain a set idle speed. Aftermarket camshafts with lots of duration or lots of overlap tend to lower the amount of vacuum an engine can generate at idle. Lower vacuum levels translate to less pressure differential between the intake manifold and outside (ambient) air. This means there isn’t as much pressure difference to force air into the engine around the throttle blade or thru the IAC passage at idle. Either the IAC needs to open up or the throttle blade must be opened more to allow more air to enter the engine. Engines that have higher compression or are new/rebuilt can have higher internal loads/friction which can also result in a drop of idle vacuum levels. In short, any condition that increases load on the engine will result in the vacuum level to drop at idle, which will require the IAC or throttle blade to be opened up to compensate.

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