Engine Troubleshoot

Idle Issues: Diagnosing an Engine That Runs Rich at Idle

Engine Running Rich At Idle

An engine that runs rich at idle can be caused by a bad coolant temperature sensor, a faulty throttle position sensor, a bad mass air flow sensor, a leaking fuel injector, or an engine vacuum leak.

When your car is running but you are not moving then the engine must continue running at idle speed to keep from cutting out. When idling, it should be using very little fuel and air, but sometimes there is too much fuel entering the engine cylinders and this causes the engine to run rich.

An engine will run rich when first started up to compensate for the colder fuel that is not as combustible as vaporized fuel. Once the engine gets up to operating temperature then it should start to use less fuel when idling.

If the engine continues to run rich, then this will cause a rough idle and the idle speed will fluctuate. This is usually a sign that there is something wrong with one or more engine components involved in the fuel delivery system.

What Causes an engine to run rich at idle? (5 Common Causes)

An engine that runs rich when idling is caused by a problem with the over-delivery of fuel to the engine cylinders.

This is usually caused by a problem with an engine sensor such as the mass air flow sensor or by too much fuel getting into the engine because of a leaky fuel injector.

The ECU controls the fuel supply and the amount of fuel needed is dependent on many factors including the temperature of the engine, the viscosity of the engine oil, and the amount of air entering the cylinders.

Therefore a rich idle can often be caused by inaccurate information being received by the ECU, assuming there isn’t extra air or fuel leaking into the engine cylinders.

Here are a few of the most common causes of a rich idle in more detail:

Cause 1. Bad coolant temperature sensor

A faulty coolant temperature sensor is a very common cause of an engine running rich after it has reached operating temperature and when idling.

faulty engine coolant temperature sensor
A faulty engine coolant temperature sensor can cause an engine to run rich when idling

The engine coolant temperature sensor is often located near the engine thermostat and in constant contact with the engine coolant. The temperature readings it produces are used by the ECU to keep track of the engine temperature and to adjust the timing and volume of the fuel delivery.

If the coolant temperature sensor is faulty it won’t be able to accurately measure the temperature of the coolant and the engine. This will fool the ECU into thinking that the engine is colder than it really is and this can lead to it running rich, especially when idling.

Cause 2. A bad oxygen sensor

Another reason why your car can run rich when idling is because of a bad oxygen sensor misinterpreting the unburnt oxygen levels in the exhaust fumes.

A bad oxygen sensor can cause an engine to run rich
A bad oxygen sensor can cause an engine to run rich

The oxygen sensors that are fitted to the exhaust, before and after the catalytic converter are there to keep track of how efficiently the engine is burning fuel. They do this by measuring the amount of oxygen in the exhaust fumes.

If there is too much oxygen in the exhaust gas then the engine is running lean and more fuel is pumped in by the ECU to correct the deficiency. If the oxygen sensor is faulty then it may not be able to accurately measure the oxygen level and this can lead to an engine running rich when idling.

Cause 3. A Faulty Mass Airflow Sensor

A bad mass airflow sensor is another common cause of a rich engine idle, especially at startup when the engine is cold.

bad mass air flow sensor
A mass air flow meter can cause an engine to run rich

The airflow sensor is involved in keeping track of the amount of air that is entering the intake manifold. It does this by generating an electrical signal whose strength is directly related to the temperature difference between the two heat-sensing wires within the sensor.

If the mass airflow sensor is overestimating the amount of air that the engine is receiving and therefore sends the wrong information to the ECU, then this will lead to the engine running rich.

Cause 4. A bad fuel injector

Bad fuel injectors are another common cause of an engine running rich when idling. Fuel injectors are designed to deliver a precise amount of fuel into the engine cylinder in a controlled spray pattern at a precise time.

bad fuel injectors
Some bad fuel injectors

Faulty fuel injectors that cause an engine to run rich are usually dirty, have a worn or partially blocked spray tip, or are leaking fuel into the cylinders when they should be shut off.

Fuel injectors are responsible for atomizing the fuel as it is forced through a tiny jet opening in the injector tip.

This creates a fuel mist that mixes easier with the air in the combustion chamber and therefore produces a more efficient and thorough ignition. Bad fuel injectors can often fail to atomize the fuel sufficiently and this can give the impression of an engine that is running rich.

Cause 5. A faulty MAP sensor

Another sensor involved in calculating the fuel and air mixture within the engine is the manifold absolute pressure sensor (MAP). If it is faulty then this can result in the engine running rich when idle.

The MAP sensor is located on the intake manifold and its job is to keep track of the internal air pressure of the engine manifold and to pass this information to the ECU. 

Problems with the air intake pressure measurements will cause the idle speed to fluctuate and this can cause more fuel usage in response to the extra air that is perceived to be getting into the engine.

A faulty MAP sensor can result in the ECU increasing the amount of fuel that is in the fuel mixture because of inaccurate air pressure measurements. An idling

What Next – How To Fix An Engine Running Rich At Idle?

If your engine is running rich after the car has warmed up and when you are idling then you will need to start by checking for error codes that may be stored in the ECU.

Starting with an engine diagnostic readout is a good idea because most engine idle problems are caused by faulty sensors or engine components that are directly monitored by the ECU. Any problems will almost always trigger an error code and sometimes a check engine light too.

It’s also a good idea to review the vehicle’s service history. If the engine oil or air filter has not been changed recently this can contribute to engine idle issues.

  1. Perform an engine diagnostic using an OBD-II reader. Look for error codes that are related to problems with sensors that manage airflow, air pressure, or fuel delivery. A bad mass airflow sensor reading will almost always trigger codes in the range of P0100 to P0104. A bad MAP sensor reading will give codes in the range of P0105 to P0109. These codes are not always triggered by bad sensors and other issues such as a dirty air filter and intake manifold leaks are some of the issues that can also cause engine air intake-related codes.
  2. Check for faults with injectors. If you see engine error codes such as P0171, P0172, or P0175, these can point to injector problems that are causing the rich fuel mixture to occur. Other symptoms of bad injectors or injectors that don’t shut off completely include really bad fuel economy, a rough erratic idle speed, a smell of gas from the exhaust, engine cylinder misfires, and a check engine light. To conclusively diagnose a bad fuel injector it must be removed from the engine for inspection and testing.
  3. Check the oxygen sensors. Faulty oxygen sensors will also trigger fuel mixture error codes such as P0171 and specific oxygen sensor circuitry codes in the range of P0160 to P0164. If you see any of these codes then it may point to a fault with the oxygen sensor or the wiring that feeds the oxygen sensor. Removing and testing the oxygen sensor is the best way to diagnose a bad oxygen sensor and if they are old then replacing them is usually recommended as they lose their sensitivity over time.

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About the author

The Motor Guy

The Motor Guy is a passionate car enthusiast with a love for troubleshooting and diagnosing all sorts of vehicle problems.

With years of experience in OBD diagnostics, he has become an expert in identifying and solving complex automotive issues.

Through TheMotorGuy.com, he shares his knowledge and expertise with others, providing valuable insights and tips on how to keep your vehicle running smoothly.

- 12 years experience in the automotive industry
- ASE Master Automobile Technician
- A Series: Automobile and Light Truck Certification, A9 Light Vehicle Diesel Engine Certification
- Bachelor's Degree in Information Systems

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  • If I’ve noticed my car running rich at idle, specifically with symptoms like a rough idle and fluctuating idle speed, but I don’t have an OBD-II reader, what are some simple checks or steps I can take at home before deciding to take it to a professional mechanic?