Is Your Car Idling Too Rough?

Odometer
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The idle of a car engine is kind of like a human heartbeat: You can tell what shape it's in just by listening. Problems with idle speed—things like slow idle, fast idle, rough idle, or stalling after idling for a few seconds—are symptoms that something is wrong with your engine. These should be investigated, diagnosed, and repaired as soon as possible. Some of these repairs you can probably make yourself, while others will require a professional mechanic to sort out.

Rough Idle in the Cold

It's a cold day and you're stopped at a stoplight. You take your foot off the gas pedal to apply the brake, but your engine begins to idle very roughly, and then it stalls. Yet, when you restart the engine and run the car at higher speeds, it seems to run fine, or at least more smoothly. Here's what could be wrong:

  1. If you have a carburetor, you may have a bad accelerator pump or power circuit.
    The Fix: Replace accelerator pump or replace the carburetor.
  2. There may be a vacuum leak.
    The Fix: Check and replace the vacuum lines as required.
  3. There may be some type of ignition problem.
    The Fix: Check and replace the distributor cap, rotor, ignition wires, and spark plugs.
  4. The ignition timing may be set wrong.
    The Fix: Adjust the ignition timing.
  5. There may be a fault in the computerized engine control system.
    The Fix: Bring your car to your mechanic or dealership to check the engine control system with a scan tool. They'll test the circuits and repair or replace components as required. 
  1. The EGR valve may be bad.
    The Fix: Replace the EGR valve.
  2. The engine may have mechanical problems.
    The Fix: Check the compression to determine the engine's condition.
  3. The idle speed is set incorrectly.
    The Fix: Set idle speed to the car's original settings.
  4. The fuel injectors may be dirty.
    The Fix: Clean or replace the fuel injectors.

    Rough Idle With Warm Engine

    If you're having the same problem as you did in the cold, only your engine is warm, even hot, this could indicate the following problems:

    1. If you have a carburetor, you may have a bad accelerator pump or power circuit.
      The Fix: Replace accelerator pump or replace the carburetor.
    2. There may be a vacuum leak.
      The Fix: Check and replace vacuum lines as required.
    3. The fuel pressure regulator may be operating at too low a pressure.
      The Fix: Check the fuel pressure with a fuel pressure gauge. Have your fuel pressure regulator replaced. 
    4. Idle speed is set incorrectly.​
      The Fix: Set idle speed to specs.
    5. There may be some type of ignition problem.
      The Fix: Check and replace distributor cap, rotor, ignition wires, and spark plugs.
    6. There may be a fault in the computerized engine control system.
      The Fix: Bring your car to your mechanic or dealership to check the engine control system with a scan tool. They'll test the circuits and repair or replace components as required. 
    7. The EGR valve may be bad.
      The Fix: Replace EGR valve.
    8. The engine may have mechanical problems.
      The Fix: Check compression to determine engine condition.
    9. The fuel injectors may be dirty.
      The Fix: Clean or replace fuel injectors.

      Fast Idling

      Even after the engine has run long enough to become warm, the idle speed does not come down to normal but continues to idle at a fast pace. You really notice it when you come to a stop, after which you have to push hard on the brake pedal to keep the car from moving. Here's what could be happening:

      1. If you have a carburetor, you may have a bad accelerator pump or power circuit.
        The Fix: Replace the accelerator pump or replace the carburetor.
      2. The engine may be overheating.
        The Fix: Check and repair the cooling system.
      3. The fuel pressure regulator may be operating at too low a pressure.
        The Fix: Check fuel pressure with a fuel pressure gauge. Replace fuel pressure regulator. (This is not really a do-it-yourself kind of job, except for the most experienced of home mechanics.)
      4. The ignition timing may be set wrong.
        The Fix: Adjust the ignition timing.
      1. There may be some type of ignition problem.
        The Fix: Check and replace the distributor cap, rotor, ignition wires, and spark plugs.
      2. There may be a fault in the computerized engine control system.
        The Fix: Bring your car to your mechanic or dealership to check the engine control systems with a scan tool. They'll test the circuits and repair or replace components as required.  
      3. There may be a vacuum leak.
        The Fix: Check and replace vacuum lines as required.
      4. You have a bad idle speed control unit.
        The Fix: Replace the idle speed control unit.
      5. The alternator may not be working properly.
        The Fix: Replace the alternator.

      Stalling Upon Stopping

      You are driving along and everything is just fine—until you let off the gas pedal and apply the brakes. At that point, the engine starts shaking and may even stall. Here are some possible causes:

      1. There may be a serious vacuum leak.
        The Fix: Check and replace the vacuum lines as required.
      2. There may be a fault in the computerized engine control system.
        The Fix: Bring your car to your mechanic or dealership to check the engine control system with a scan tool. They'll test the circuits and repair or replace components as required.
      3. Broken linkage.
        The Fix: Repair or replace the linkage as required.

      Idling issues can be frustrating, but with some patient troubleshooting, you'll have a real chance at figuring it out. Remember to always check your engine idle with the air conditioning and defroster turned off, as both of these systems are designed to change the idle when they are on due to the demands they place on the engine.