Why is my car engine revving higher than normal?

By Julia Freeman

As a general rule, your engine should usually reach about 700 RPM when you start it. If your engine seems to be revving at a few hundred RPM over that level then it is likely that there is a significant issue and you should try to diagnose, or seek a diagnosis, at your earliest convenience. If you ignore this problem it could cause significant damage to your engine, potentially even meaning the vehicle becomes unusable.

car engine revving higher

Consequences of over-revving

Over-revving can cause damage to your engine, even if it just lasts a few seconds. It can damage your valve train, throw a connecting rod, or damage your flywheel/clutch.

Causes of over-revving

There are several reasons that your car may be revving higher than normal. The main three are: stuck/dirty throttle plate, stuck/dirty idle air controller (IAC), or vacuum leak.

Stuck/dirty throttle plate

You don’t need to be a mechanic to fix this one! This can easily be dealt with by cleaning with a throttle body cleaner (commercially available). Spray the linkage and ensure that the throttle can open/close all the way smoothly. Also, remove any sludge that has built up on the plates, on the bore, or in the passages.

Stuck/dirty IAC

The idle air controller (or IAC) is a valve that is used to regulate the engine idle speed. You can check if this is causing your issue by following the steps outlined below:

  • Step 1: Locate and gain access to the IAC valve. Your service manual should show you where to find it.

  • Step 2: Disconnect the AC valve.

  • Step 3: Start the engine and see how the engine reacts to the IAC valve being disconnected.

  • Step 4: Turn off the ignition.

  • Step 5: Reconnect the IAC valve electrical connector.

  • Step 6: Start the engine again.

If your engine idle is now normal, you may have fixed the issue.
 
If it is still revving higher than normal, the next thing to do is to try cleaning the IAC valve. It is recommended to use a carbon cleaner/intake cleaner solvent to do this.

Vacuum leak

A vacuum leak is a leak found anywhere between the engine and the mass air flow sensor. One of the main signs of a vacuum leak is an unintended increase in RPM.
 
To discover if this is what is causing your issue, first you need to locate the vacuum hoses under the hood and inspect their condition. If there are any that look worn/broken/separated then you should replace them.
 
Please note that this requires some expertise so should ideally be left to professional mechanics to do; they also have specialist testing equipment for this.
 
If you would like to fix a revving issue, schedule a car service, book an MOT, or arrange a free safety check for your car you can book these through our website. Or you can contact your local Protyre garage by using the search function on our website.

Our garages are staffed by qualified fitters with years of experience who will always be happy to help. All our garages also stock a wide-range of tyres from manufacturers such as Pirelli, Bridgestone, and Falken.

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About the author

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By Julia Freeman
Julia is Head of Retail Marketing for Protyre and loves engaging with customers and the business as a whole to make sure Protyre is more than just a local garage.
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