Skip the auto shop - Our mechanics make house calls

Q: Engine idle fluctuates

asked by on

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: rear turn signal is out brake light works just not the turn signal?

If the front turn signals work but the rear does not, then it is not the fuse of the turn signal switch. You do have a trailer adapter module in left side of luggage compartment that may not be sending...

Q: Why does my van always stay at a high idle 2000 Chevrolet Express

Hello. There are a lot of reasons that would cause a high idle, but most of the time it is caused by a vacuum leak. When there is an excessive amount of air entering the engine the computer will compensate...

Q: I have an 07 FJ cruiser with about 175,000 miles and I recently noticed there's a constant clicking when driving straight and turn

Hi there. My initial thought was that this noise would be caused by damaged CV axles, but since you just replaced them, it's more than likely that this clicking noise is a brake or wheel bearing issue. However, with so...

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...