Why Does my Car Idle High When Started?

We never like to hear our car's engine audibly struggling. It can be a concerning set of circumstances. Cars are not a cheap endeavor between gas and other running costs. The concern that our car may be about to break down can be scary.

In this post we will take a look at high idling upon starting and what if anything this could mean. Could it just be normal or does it indicate something is about to break?

What Is Idling?

If our engine is running but we are not physically moving the car this is known as idling. Essentially the engine is still running even if it isn’t moving the wheels and creating forward momentum. Generally speaking for cars, trucks and motorcycles idle speed is around 600 – 1000 revolutions per minute or (RPM).

These rpms refer to the number of times over a minute that the crankshaft turns during that time period. While idling these revolutions of the crankshaft are generally enough to operate things like the water pump, alternator, air conditioning and if applicable power steering.

Once we start to drive the RPMs should increase to supply the power required to accelerate as well. Theoretically then while idling we should not see more than a 1000 RPMs when we first start up the car in the morning.

What Constitutes High Idling?

Revolutions per minute above 1000 and certainly higher than 1500 when you have first started the engine or are not moving forward could be considered as high idling. Vehicles may vary but generally each vehicle has an ideal idling level so research this for your specific vehicle to be sure.

What Can Cause High Idling Without Being a Problem?

If you are in your car and the RPMs are between 1000 – 1200 do not immediately panic. First, ask yourself "Am I wearing a thick coat and gloves?" If you are then it’s probably cold outside and you're struggling a little to get started today yourself.

Cold weather can increase your normal idling RPMs because the system literally needs the increased power to get itself warmed up. Give your car a chance to warm up a little. You may be running a heater to get yourself warm as well; this all takes power from the vehicle.

After a few minutes the high idling will likely decrease to the normal 600 – 1000 rpms when you are in an idle state.

The main reasons for cold weather increase of idling would include

  • Dealing with emissions while the catalytic converter warms up. This device requires heat to function at optimum levels so the engine needs to work harder on cold days to supply this
  • Gasoline evaporates more slowly in the cold so more fuel is needed to the engine cylinders during cold weather start ups.

Problems in the Cold?

High over 1200 -1500 rpms in the cold is generally not a normal occurrence and may indicate an issue.

Secondary Air pump or Line

As mentioned when cold combustion is more difficult so a secondary injection system pumps air into the exhaust manifold. This helps the remaining fuel to continue burning as it makes its way to the catalytic converter.

A leak in the air pump or its line could lead to idling issues as the air needed to aid combustion is lower than needed. The engine therefore adjusts to push more air through increasing the rpms.

The Fast Idle Screw

This affects carbureted engines where the fast idle screw is designed to increase rpms to warm up the vehicle while the choke is closed. A poorly tuned screw might cause idling to be too high or even sometimes on the low side.

What if the Weather Is Not a Factor?

It might be a wonderful warm morning and there should be no idling issues related to a cold car. What could be the cause of high idling in this situation?

Electronic Control Unit Issues

The majority of modern vehicles are equipped with electronic control units or (ECUs). These are the brains of our cars and control all the bells and whistles we enjoy in the modern automobile. I was once advised the smarter the car the more things there are to go wrong with it.

An ECU for example controls the air fuel mixture and the ignition timing of your engine when you start up. If there is an issue with this control system it is possible that the idling may be off creating a high or low idle in comparison to normal.

Idle Air Control Issues

Activated by the ECU, the Idle Air Control or IAC helps control the air used in the combustion process. This operates the throttle butterfly valve and if not working correctly can lead to poor air flow and high idling upon starting.

Generally dirt or grime may be the cause of the issues with AIC and a simple cleaning might be enough to rectify the issue.

Vacuum Leaks

There are lines running from the intake manifold to various locations in your car such as windscreen wipers, fuel pressure sensors and the brakes. A leak in these lines can cause confusion with the manifold sensors. As a result it may incorrectly request more fuel causing the car to idle at a higher rate unnecessarily.

Mass Flow Sensor Issue

This sensor measures the rate of air flow into the engine sending this information to the ECU. If this sensor is malfunctioning it might cause the ECU to miscalculate how much fuel is needed to the pump. As a result too much fuel may be added to the system making the engine work harder upon start up.

Other Sensors That May Be At Fault

It doesn’t take much to confuse an ECU so sensors such as O2, throttle and air intake sensors can be the cause of high idling. If any of these is not recording correctly or is damaged it may be the cause of high idling.

The ECU relies heavily on these sensors to calculate the correct air to fuel ration to efficiently run the engine. If this ratio is off then it will cause high or low idling to occur.

Conclusion

There are a few things that can be the cause of high idling especially in newer vehicles that rely on high tech sensor systems. Although high idling may also be just an indication of cold weather and a car that needs to warm up.

On a cold morning RPMs at starting of up to 1200 are not unusual as long as they drop back down to 600 – 1000 once the engine warms up. If the weather is warm or if the rpms do not decrease while idling then there is likely another issue that you might want to investigate.

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