Is Your Car Idling Too Rough?

Decipher Car Idling Problems

car idle
Your idle quality can tell you lots about the health of your engine. Robert Llewellyn / Getty Images

The idle of your engine is kind of like a human's heartbeat ... with just a listen, you can detect all sorts of problems. Is your vehicle idling too tough or too slowly? If anything is going wrong under the hood, there's a good chance it will be affecting your car's idle speed and quality. Problems with idle speed -- things like slow idle, low idle, bad idle, lumpy idle, and fast idle -- are symptoms that should be investigated, diagnosed and repaired.

The following symptoms and related problems should act as a guide in helping you troubleshoot your idling issues.

Symptom 1: Rough Idle in the Cold

The engine will not idle smoothly, or it stalls during idle when the engine is cold. When the engine is cold and you take your foot off the gas pedal, the engine runs very rough and may even stall. When you run the engine at higher speeds, it seems to run fine, or at least it runs more smoothly.

Possible causes:

  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: Check engine control systems with a scan tool. 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. 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.

    Symptom 2: Rough Idle With Warm Engine

    The engine will not idle smoothly, or it stalls during idle when the engine is warm. When the engine is warm or hot and you take your foot off the gas pedal, the engine runs very rough and may even stall. When you run the engine at higher speeds, it seems to run fine.

    Possible causes:

    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. Replace fuel pressure regulator. (This is not really a do-it-yourself kind of job.)
    4. Idle speed 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: Check engine control systems with a scan tool. Test circuits and repair or replace components as required. (This is not really a do-it-yourself kind of job.)
    7. The EGR valve may be bad.
      The Fix: Replace EGR valve.
    1. The engine may have mechanical problems.
      The Fix: Check compression to determine engine condition.
    2. The fuel injectors may be dirty.
      The Fix: Clean or replace fuel injectors.

    Symptom 3: Fast Idling

    The engine idles too fast. After the engine has run long enough to become warm, the idle speed does not come down to normal. You really notice it when you come to a stop and must have to push hard on the brake pedal to keep the car from moving.

    Possible causes:

    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.)
    1. The ignition timing may be set wrong.
      The Fix: Adjust the ignition timing.
    2. There may be some type of ignition problem.
      The Fix: Check and replace the distributor cap, rotor, ignition wires and spark plugs.
    3. There may be a fault in the computerized engine control system.
      The Fix: Check the engine control systems with a scan tool. Test the circuits and repair or replace the components as required. 
    4. There may be a vacuum leak.
      The Fix: Check and replace vacuum lines as required.
    5. You have a bad idle speed control unit.
      The Fix: Replace the idle speed control unit.
    6. The alternator may not be working properly.
      The Fix: Replace the alternator.

    Symptom 4: Stalling Upon Stopping

    Car stalls when stopped quickly. You are driving along and everything is just fine ... until you let off the gas pedal and apply the brakes. The engine starts shaking and may even stall. Not a good thing to happen because you lose power steering when the engine dies and could risk an accident.

    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: Check the engine control systems with a scan tool. Test the circuits and repair or replace the components as required. (This is not really a do-it-yourself kind of job.)
    3. Broken linkage.
      The Fix: Repair or replace the linkage as required.

    Idling issues can be very 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 air conditioning system's demands on the engine.

    Format
    mla apa chicago
    Your Citation
    Wright, Matthew. "Is Your Car Idling Too Rough?" ThoughtCo, Oct. 11, 2017, thoughtco.com/is-your-car-idling-too-low-or-too-high-281322. Wright, Matthew. (2017, October 11). Is Your Car Idling Too Rough? Retrieved from https://www.thoughtco.com/is-your-car-idling-too-low-or-too-high-281322 Wright, Matthew. "Is Your Car Idling Too Rough?" ThoughtCo. https://www.thoughtco.com/is-your-car-idling-too-low-or-too-high-281322 (accessed November 20, 2017).