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.

A: This may need to have the throttle stop scr...

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

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Save up to 30%

Get a quote

What others are asking

Q: Possible starter issues as vehicle does not start.

It may very well be the starter. If you hear a slight click sound or nothing at all when turning the key but all lights and accessories still work, you may have a starter problem. If your battery checks out...

Q: Truck overheats

Hello. You did not specify the mileage of your vehicle but if the thermostat is not changed around every 60,000 miles it may cause overheating. Thermostat replacement is considered routined maintenance just like brakes or oil changes. I would recommend...

Q: Rumbling on acceleration and idle while using air conditioning.

This may be related to a dirty or failing idle air control valve. The idle air control valve monitors the air intake as it is mixed with fuel prior to being injected into the engine at low speeds and at...

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...
How to Renew Your Car Registration in Oklahoma(DELETED)
Having your car registered with the Oklahoma Tax Commission is an important part of driving legally in this state. You will need to do this within 30 days of moving to Oklahoma...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.