How To Diagnose A High Idle RPM

A high idle can have a minor impact on fuel efficiency and engine wear and is a potential safety risk because the vehicle can take off when you release the brakes.

This problem can also cause more difficulty starting when the engine is warm.

Here is an in-depth explanation of how certain components affect the engine and possible ways to solve them.

Causes And Fixes For High Idle RPM

1. Ignition Timing is Too Advanced

A cross-section of an internal combustion engine with two cylinders combusting.

Adjusting the ignition timing too advanced will increase idle speed, and if the timing is too retarded, it will drop the idle RPM.

If the ignition timing is too advanced, you can also experience minor knocking sounds when accelerating or driving up a steep incline. This is sometimes referred to as pinging.

Over-advanced (or retarded) timing can also cause an increase in engine temperature due to the burning air/fuel mixture while the exhaust valves are opening.

Incorrect adjustment in either direction can also cause poor performance or damage to the engine.

You can get your local automotive technician to adjust the ignition timing for you, invest in a relatively inexpensive timing light (for the same or slightly more than the repair cost), and download a copy of your vehicle’s manual.

It usually involves connecting the timing light up to the battery unless it’s a self-powered timing light. The inductive pickup clamp around the first cylinder’s high-tension wire to the spark plug.

Warm up your engine (taking your car for a quick spin is best), disconnect the vacuum advance hose if your vehicle has one, and block the open end of the hose.

Loosen the distributor and rotate it to alter the ignition timing. While pointing the timing light at the harmonic balancer and the timing tab, the light strobes in time with the firing time of cylinder one, which allows you to see the position of the notch on the harmonic balancer as you adjust the timing by rotating the distributor.

The marking on the timing tab or marker plate will often be the number of degrees before the top dead center (BTDC). So you must align the harmonic balancer’s marking with the correct amount of degrees as per the manufacturer’s recommendations.

You may have to pull a trigger or push a button to activate the timing light depending on your type of light.

2. Misadjusted Accelerator Cable

A throttle cable tension adjustor.

Some accelerator cables have an adjustment on the end of them to help deal with too much slack, which can cause delays when depressing the accelerator pedal.

However, when it is over-tensioned, it can cause the throttle to be pulled open slightly, causing the increased idle RPM.

You can quickly check to identify if this is the problem by loosening the accelerator cable adjustor while the engine is running to see if the idle RPM drops afterward.

Adjust the cable so that there’s very little slack and not too tight that the accelerator mechanism is affected.

3. Sticking Accelerator Cable

When an accelerator cable develops sticking points, it can happen anywhere along the accelerator path, whether at the beginning, halfway, or at the end of the range of motion.

In some instances, the cable can be stuck near the beginning of the throttle range preventing the accelerator from returning to the idle preset position.

Replacing the cable is relatively easy, but to properly inspect it for internal damage, you must disconnect it from the accelerator pully by loosening the tension adjustor and swinging the cable around to the proper position to remove the end.

Be sure to check the return spring by the throttle pully to ensure enough tension pulling the throttle closed to the normal idle position.

Luckily, these cables aren’t expensive, and replacing them isn’t too labor-intensive. So it should be a reasonably cheap repair.

4. Faulty Accelerator Pedal Sensor

A faulty accelerator pedal sensor can cause a high RPM and prevent the engine from revving past a certain RPM.

It can also cause the vehicle to go into limp mode, which will prevent you from driving the car usually and could cause some safety concerns.

This problem is best solved by sending the car into an automotive repair shop where all the necessary equipment is at hand to conduct the repair effectively.

The sensors themselves can cost anywhere between fifty and two hundred fifty dollars or more.

5. The Electronic Throttle Control

As throttle cables have been phased out over the years, we are now left with an electronic throttle control that operates a car’s throttle in most cases.

It works with a small motor operated by the ECU depending on where the position of the accelerator pedal is based on the data from the sensor discussed in the fault before this one.

Some electronic throttle controls can be a bit pricey, but overall, it will not break the bank either.

The ECM needs recalibrating, and logs and checks must be cleared after replacing the component.

So, it is best to let an auto repair technician deal with this unless you have the necessary equipment to handle the repair.

6. Malfunctioning ECU (Engine Control Unit)

Sometimes, the ECU can go faulty in mysterious ways. For example, the malfunction could affect the idle speed, perhaps due to improper values interpreted by the unit.

Sometimes, a specific procedure can be followed using pedals, ignition, or other controls to reset the idle speed.

The other way would be for a technician to connect a diagnostic tool to the ECM and perform some changes.

Unfortunately, if all else fails, it may be a replacement that is the only cure for the problem. At that point, further issues are on their way anyway.

If replacement is the only way out, you could be looking at one hundred dollars for the ECU itself to three thousand dollars or more, depending on the make and model of the vehicle.

7. Broken Alternator

If the alternator in your vehicle fails completely, you are headed for a flat battery. But before that happens, you may experience an increase in idle speed.

This is because an alternator usually generates a load on the engine when it functions correctly.

It does so when a current load for charging is imposed upon the alternator’s output and, as a result, becomes harder to rotate from the pulley.

The law of conservation of energy comes into play as it takes more power to turn the alternator compared to the alternator’s total energy output.

So, if this process is somehow disrupted due to a fault, the alternator spins all the time freely, and you could experience an increase in RPM at idle.

It may be more likely to happen to smaller engines where the alternator load on the engine can significantly impact the engine RPM. Especially one without idle air control or electronic throttle control.

Alternators commonly cost between one hundred and four hundred dollars. So, having it replaced means that you could be looking at an extra hundred dollars or more added to the price of the part.

You can add an extra fifty dollars if you need a new belt added to the repair. However, you can sometimes get away with a bit less than that.

8. Issues with the Idle Air Control Valve

The valve allows air to bypass the throttle’s butterfly, allowing a higher idle speed and introducing more air.

The ECU controls how much the idle air control valve opens depending on how much the RPM needs to be adjusted.

It is more commonly used in vehicles that don’t have an electronic throttle control unit to control engine idle RPM.

The symptoms can vary in a few ways when any problem arises with the idle air control valve.

  • Rough or fluctuating idle speeds.
  • Higher than normal idle RPM.
  • Lower than normal idle RPM.

Removing the unit and cleaning it can sometimes solve problems if the valve is clogged and not operating correctly because of buildup.

But other electronic faults may make this part non-repairable, and a new replacement would be the only way to fix it.

Once again, if the part is replaced, the ECM may need retraining to restore proper idling behavior.

The replacement value of one of these parts typically lies more than the cost of the part itself. Therefore, replacing it shouldn’t be difficult or time-intensive.

Some of these valves come in under one hundred dollars, while others can be around four hundred dollars or more.

9. A Fuse Has Gone Open Circuit

When mechanical operations rely on motors or solenoids, worn or clogged parts can impose a more significant load on the electrical supply, causing fuses to blow.

This is, of course, in addition to electrical faults that have short circuits or other malfunctioning electronic components also causing fuses to go open circuit.

When a fuse goes, that is in the same circuit that supplies power to a sensor, controller, or electronically operated valve. It could cause the idle RPM to increase.

The most common case would be the idle air control valve fuse. It may cause the valve to be in a position where too much air is introduced into the intake.

The cost to replace a fuse is negligible, but the faulty component that caused the fuse to go might not be.

So it would be difficult to predict the cost of this kind of repair.

10. Leaky Hoses, Tubes, Gaskets, or Housings

Specific hoses, tubes, gaskets, or housing leaks can cause various idling and performance-related issues.

Here are some of these items that can contribute to increased idle RPM:

  • Air cleaner housing: Quite often, the mass airflow sensor is located somewhere in this housing, and once this sensor reports improper amounts of airflow, wrong fuel mixture ratios are likely to be delivered to the intake because of the ECU’s decision based on the readings from the mass airflow sensor. The improper fuel-to-air ratio can increase idle RPM.
  • Vacuum hoses: Many components rely on a vacuum to determine the amount to alter the timing, fuel mixture ratios, and more. Any number of hoses that contain a leak can cause higher than usual idle RPM.
  • Fuel injector gaskets or seals: When a leak occurs between any fuel injection system parts or throttle units and their housing, you can introduce more air into the intake system. More air creates a leaner fuel mixture, which can increase the idle RPM.
  • Intake manifold gasket: A leaking intake manifold gasket can cause more air to enter the air-fuel mixture, making it leaner and thus increasing the idle RPM.
  • Carburetor gasket: If you still have a carburetor, it may be that the gasket is leaking air into the fuel mixture, much like the example mentioned about the intake manifold. As the fuel mixture gets drawn into the intake through the carburetor, extra air makes it into the intake, making the fuel mixture leaner and increasing the idle RPM.

Spend some time trying to find a leak or faulty gasket. Then, with anything with hoses, simply pull them off and listen to the idle speed.

If no difference is heard in RPM after disconnecting a hose, I recommend that you investigate further to find if there is a leak somewhere in the same line.

The cost of this repair may be difficult to predict but shouldn’t be expensive if it’s simply a broken or old hose.

On the other hand, a faulty gasket has a higher labor cost to repair than the price of the part. This involves removing a few parts to get the gasket replaced, and a torque wrench with proper tightening procedures will be required.

11. A Faulty Temperature Sensor

An ECU module uses temperature sensor readings to determine the air-fuel ratio best suited for the engine temperature.

If this sensor is faulty, the ECU will have the wrong engine temperature reading and make incorrect adjustments regarding the fuel mixture.

The wrong air-fuel mixture ratio can raise or lower the engine idle RPM and cause damage or a higher fuel bill at the end of each month.

So, checking that the correct engine temperature is being read by the temperature indicator on your vehicle’s dash is essential.

This is the easiest way to determine if the sensor is working correctly and, in most cases, end the investigation into the engine temperature sensor.

In some cars, these sensors only have one wire attached to them. This opens up the possibility that the wire might break off, so look out for that before replacing the sensor.

The part itself should be under one hundred dollars, but the total repair could be up to two hundred dollars if you take your vehicle to be repaired by a professional.

12. Faulty Fuel Pressure Regulator

A fuel pressure regulator helps maintain the fuel pressure in the supply line, ensuring that the fuel injector unit always has enough fuel to keep up with demands.

If this vital component goes faulty, you may have too much gasoline supplied to the injectors.

Some symptoms of a faulty fuel pressure gauge:

  • High engine RPM.
  • Rough idling.
  • Black smoke is coming out of the exhaust.
  • Stalling.
  • Black buildup on spark plugs.
  • Gas smell in engine oil.
  • Gas drips out the exhaust.

If just enough extra fuel starts reaching the fuel injectors, you could be in a situation where the idle RPM increases due to a richer fuel mixture.

Repair costs could be as high as a few hundred dollars, but it isn’t extremely expensive. If you can conduct the repair yourself, you could get away with under two hundred dollars for the part.

Conclusion

And that’s my list of the most common causes of higher-than-normal idle speeds.

The good news is that most of these repairs shouldn’t cost the earth, and you should be able to get away with a relatively cheap repair from your local auto repair shop.

Sometimes, it won’t be obvious which part is the culprit, but by spending time and patiently looking at each item listed in this article, something should reveal itself.

If you are ever in doubt about how a part works, perform a Google search to learn more about it and how it works and operates.

This is a very powerful way to learn more about engines and how to repair your vehicle.