Some drivers tend to worry too much about the rough idle when AC is on, while others find it normal. So, what is it?
To put it simply, the rough idle is a vehicle’s “sickness” related to engines. In other words, this problem prevents the motor from running at its peak level of performance.
Detecting it means that you need to get your vehicle taken care of with the mechanical work.
It is often easy to identify if the car idles rough when ac is on. The vehicle starts operating with a bouncing and shaking sensation.
Also, you could hear strange sounds and the RPM-counting inconsistency.
In general, we would like to list typical causes below.
- A faulty IAC valve (Carbon buildup)
- A throttle body that gets too much dirt
- A problem found in the cooling fan
- Other components based on different models like rough idle when AC is on Honda
Now, let’s dig into them.
What Causes Rough Idle When AC Is On?
Whenever you find the rough idle when the car’s AC is on, there must be a problem stemming from one of these four main parts: the IAC valve, the throttle body, the cooling fan, and the ignition & distributor system.
Check these elements one by one to find the root.
A faulty IAC Valve (Carbon buildup)
The Problem
The PCM standing for the powertrain control module, is a part of your car. It needs an idle air control or IAC valve to do the idle-speed controlling function.
In other words, there is an amount of air that will go through the throttle blade thanks to this valve.
An extra airflow enriches the mixture of air and fuel while the cold engine is starting. Moreover, it helps push the engine speed as the extra load of air comes into the engine.
However, a faulty idle air control valve could keep the speed with the car idling rough when AC is on.
And the most common cause to mention is the carbon buildup around the valve. It may lead to a miscalculation or an increase in the idling speed.
Why Does The Carbon Keep Building Up?
The buildup often occurs in the EGR valve, IAC valve, and the throttle body for several reasons below.
- A lack of refrigerant for the air control system: Unless the AC system is rich in the refrigerant, the compressor cycle and the engine load will increase.
- Failure of the air control system: The failure of the AC compressor could surge the carbon buildup. As a consequence, it can lead to a rough idle.
- A faulty belt: It is a culprit that many people tend to overlook. The surging car problem could result from a worn belt of the compressor. The worn or stretched belt often slips while the engine is operating easily.
- A bad switch of the air control cycling: The switch of the air control cycling manages the pattern of the compressor. Over time, this part goes bad and causes trouble.
- Overfilled air control system: As mentioned above, a lack of refrigerant is a common root of engine surging and air control system problems. That said, an overfilled amount is also not a good idea. It can bring about the same issue instead.
A Throttle Body That Gets Too Much Dirt
In addition to carbon buildup, a dirty throttle body is also a big stimulus affecting the idle with the AC operation.
The throttle blade always requires a regular calibration that will keep it closed. At the same time, the computer does its function of regulating the airflow into that plate.
If the plate gets dirty with insufficient air, the problem will come when the AC is on.
Problems Found In Cooling Fan
Any errors with the cooling fan could be linked to high pressure and engine idle rise. In terms of the fan having variable speeds, a proper operation plays a more important role.
Doing a quick check related to the fan circuit also helps detect the malfunction and find practical troubleshooting.
Ignition & Distributor System Problems
They often occur in the older models of vehicles featuring a distributor. So, we suggest checking both the rotor and cap if you own those.
For years, the external terminals and the cap center tend to wear with the carbon buildup. As a result, this change could prevent the strong operation of your vehicle’s spark plug.
Besides, it would be better to do a thorough examination to remove the carbon traces.
A Bad IAC Valve Is The Big Cause
The IAC valve has a connection with the vehicle’s manifold, where the intake comes into the engine. And the control unit must manage this valve by tracking the information from the oxygen sensors.
It knows about the electrical load and temperature of the engine to control the valve properly. Hence, the control unit ensures that the speed will not go down or up too much.
The following is a list of the top 3 signs that can tell you about a bad IAC valve.
Engine Warning Light
The IAC valve constitutes an indispensable part of the whole vehicle engine. It means that the control system monitors the valve’s status regularly.
So, if the valve goes bad or shows any malfunction, the warning engine light will get activated. In other words, the function of engine checking starts working.
But it is not that everyone knows what happens with the engine when seeing the light. The sign could also point out that there is something related to the engine going bad.
Engine Stalling
The engine stalling can happen by various causes. And one of them is the faulty IAC valve. An inconsistent air amount inside the engine could prevent the idle from stopping.
Through a valve with a crazy malfunction, the engine cannot get the air properly. Then, the idle speed gets out of whack and stalls the engine.
Fluctuation with Idle Speed
If you detect the engine’s fluctuating or irregular idle speed, the cause is often the bad IAC valve. This valve is the unit to control the idle speed of the engine.
Hence, the engine cannot run at the normal rate of revolutions per minute without a good valve.
In other words, an IAC valve could fail to do its function. And at the same time, the idle speed becomes all over the place.
For example, the speed could jump above 1,000 revolutions per minute. Or sometimes, it goes back lower than 500 revolutions per minute.
How to Fix the Problems
We have already learned about the reasons. So now, how to fix rough idle when AC is on? For help, we would like to recommend three common ways to solve this problem.
Cleaning the IAC Valve
We are so sure about the importance of cleaning the IAC valve. Periodic care can help get this part with its most life and fix the trailblazer rough idle when AC is on.
Plus, the maintenance is not complicated at all.
First, you need to determine where the valve is after lifting the vehicle hood. Next, you disconnect the hose of the air cleaner and remove the valve.
Using a soft cloth is a good way to wipe away all the carbon buildup.
Cleaning the Dirty Throttle Body
There is no doubt that the throttle plate needs to get calibrated to stay closed. It is a must-have condition so that the computer can regulate the airflow going through the throttle plate.
But a dirty throttle bore and control can prevent the process from operating properly.
Thus, it is essential to keep both of them clean. So, how to do it?
Throttle Bore
- Open the hood.
- Remove the duct of the air cleaner connecting to the throttle body. The structure could be different based on the vehicle models.
- Take the snorkel of the air cleaner away (if necessary).
- Use the carburetor cleaner and spray it onto a clean rag.
- Remove any deposits on the throttle body with that rag.
- Open the throttle plate by hand instead of screwdrivers because they can leave scratches.
Throttle Control
- Open the hood.
- Make access to its throttle body.
- Turn on the key to choose the “On position” without starting the engine.
- Have the accelerator pedal pressed by a helper.
- Spray a clean rag with some cleaner.
- Clean the throttle control with that rag.
During the process, there are some notes to bear in mind.
One is about the throttle plate. You should move this part by hand when cleaning. Besides, we suggest spraying the throttle plate and body only. The cleaner could destroy other parts sometimes.
The other relates to the carbon-buildup removal. In addition to the throttle body cleaner, it is not good to use any other solvents that can damage the whole system.
Conclusion
It is all about the rough idle when AC is on. The cause could be the carbon buildup, a faulty valve of the controlling air system, or a dirty throttle body.
Whatever the reason is, it would be better to have your vehicle checked regularly. Proper care holds the key to a great engine operation for sure.