Car Idle Is Running Rough Inspection & Diagnosis

Porsche Cayman rough running idle diagnose

Car Idle Is Running Rough Inspection & Diagnosis

Car Runs Rough at Idle Inspection & Diagnosis

A rough idling engine will feel rough and bouncy. You may notice that the car feels extra shaky when stationary, like at a stop light. When looking at the odometer, you may notice that the engine will run slower or idle below normal. You may also notice that the needle on your odometer is not staying steady in a normal position below 800rpm, but instead it is bouncing up and down a bit and kind of erratic. The RPMs will be inconsistent.  You may have noticed that the check engine light or engine service light has illuminated. Sometimes a rough idle may seem like a minor annoyance, but you should get it checked out immediately. Small problems can often become large and expensive issues when not taken care of promptly.


An engine that has a rough idle can have many causes.  Some of them are minor while others are much more serious.


How the idle system of your car works?


All combustion engines by design and simple physics need to maintain a basic minimum rpm in order to run smoothly and stay running. Your engine made of many cylinders and each cylinder must be able to fire in a particular order. This is called your firing order. In addition, gasoline and diesel engine must rotate fast enough to allow the internal components to generate enough compression of the fuel and air mixture. Your engine also has many accessories that is must power, such as the AC Compressor, alternator, power steering pump, and water pump. The accessories create a load or a drag on the engine and the engine must be able to turn or rotate fast enough in order to overcome those loads while maintaining enough momentum to allow the next cylinder in its firing order to fire. Most vehicles have to maintain a steady idle speed somewhere between 600 RPMs and 900 RPMs.


Below is a list of common issues that can cause a rough running idle:


  • Bad Motor Mounts or Mechanic Failures: Broken, weak, cracked, or ruptured motor mount don’t actually cause the engine to idle at an erratic rpm but when they fail, it can feel like your car has a rough idle. The accessory items that are driven by the engine, such as the alternator, power steering pump, and AC Compressor, can have internal failures that cause them to bind up or be much harder than normal to rotate. Such failure cause excessive drag on the motor, especially small engines that result in a poor running idle.
  • Vacuum Leak: your engine needs to generate a certain amount of vacuum to power some accessories and to regulate proper air to fuel ratio for proper combustion. All cars, whether carbureted or fuel injected, must meter a measurable amount of air to be mixed with a correct amount of fuel. Any loss of engine vacuum or vacuum leaks beyond the metering point can result in poor air to fuel mixture for proper combustion and cause a rough idle.
  • Damaged, worn, or incorrectly installed Spark Plugs (engine misfire): Spark plugs are necessary for creating the spark that ignites the fuel-air mixture in the combustion chamber of gasoline engines. If the spark plugs are worn out, or damaged due to debris, or incorrectly installed, they will not be able to create enough spark for proper combustion. The car’s idle will be affected. The engine will vibrate or run really rough. The engine note will can sound noticeably different. Often times this can be referred to as engine misfire.
  • Poor Airflow or Bad Airflow Sensor: A severely clogged air filter or obstruction in the engine air intake system, such as sludge buildup or carbon fouled intake ports, can literally choke the engine and cause it to idle or run rough. A bad airflow sensor that fails to meter air properly will have the same effect.
  • Clogged Fuel filter or Low fuel Pressure: The Fuel filter will eventually get clogged with debris and starve the engine of fuel. A weak or failing fuel pump can do the same thing. Again, the engine will run rough and erratically, due to a lack of proper mixture of fuel to air.
  • Sensor failures and Oxygen Sensors: Your modern computer controlled engine uses many sensors to help it calculate and fine tune the fuel to air mixture and ignition spark for proper running at idle. Some of these sensors are the engine coolant temperature sensor, throttle position sensor, camshaft sensor, crankshaft sensor, oxygen sensors, intake air temperature sensor, fuel pressure sensor, and fuel injector or fuel injectors.
  • Incorrect Idle Speed and Engine Spark Timing: Carburated engines  and early fuel injected motors had idle adjustment screws, and throttle plate adjustments that could get out of whack and need calibration. Modern cars use devices such as idle control valves to regulate airflow that can start to fail. The most recent cars from BMW, Audi, Lexus, Porsche, and Mercedes have eliminated idle control valves and other components in favor of controlling camshaft timing and valve lift for maintaining idle. The latest valvetronic and variable camshaft timing systems are very complex and expensive to repair.
  • Internal Engine parts failures: Internal engine parts failures resulting in a Blown Head Gasket, Cracked cylinder heads, Bad Piston Rings, or Damaged Valve train components can result in a rough running idle that can be anywhere from minor roughness to severe rough, bouncy, and erratic idling.


What do you need to expect to inspect and resolve your car’s rough running idle?


As you can imagine, there are numerous issues that can result in a rough running. A top-level technician should inspect and diagnose your car. If you own a European car such as an Audi, BMW, Volkswagen, Mercedes-Benz, Porsche, Ferrari, and Maserati, we suggest that you contact Sprint Motorsports in San Jose. Sprint also service Lexus, Toyota, Datsun and Nissan Z cars, and Subaru. Rough idling issues can be quite complex and your money is well spent at a shop with expert level staff that have dedicated dealership level equipment for your car.


How Important is it to service or repair a car that has a rough running idle?


Often time when we answer the phones, the customer will state that my car has rough running idle or my car idle is running rough, what could it be and can you help? Of ten times, after discussing the issue with the customer, we find that they have noticed the issue for a while, but put off getting the issue inspected and diagnosed. Unfortunately, the delayed service and attention to their car has resulted in compounding failures and issues.  The issues could have been resolved with less money and parts if they had the rough running issue taken care of early.  When an engine is running rough, it is not burning fuel correctly. Unburned fuel will prematurely cause the catalytic converter and oxygen sensors to fail.  The emissions produced due to a rough running condition are extremely harmful to the environment and will also cause you to fail the smog test.


Sprint Motorsports provides expert car service, car diagnostics, and vehicle repair for European and Japanese Luxury and Sports car vehicles. We take pride in caring for our customers, and are invigorated by the opportunity to provide competent and comprehensive automotive repair services. We are the leading repair, tuning, and race facility in San Jose.  We  provide you personalized, detailed, and friendly service. Our relationships with our customers are built on trust, communication, integrity, and commitment to our purpose to solve and competent repair performance issues on your car.

To schedule an appointment for your Your European car (Audi, BMW, Volkswagen, Porsche, Mercedes Benz) or Japanese Luxury Car ( Lexus, Toyota, Subaru, Nissan Z, Datsun Z) that has a rough running idle, please contact service@sprint-motorsports.com or 408-831-3730. We welcome you to stop by our shop at 2099 S 10th St Unit 40, San Jose, CA 95112. If you have any questions about the issues that your car may be experiencing or a check engine light, please feel free to ask our friendly staff for assistance.