Rough Idle When Starting Cold, Then Goes Away When Engine Is At Operating Temperature More Details Below

Posted in Ignition Systems | Asked on Aug 8, 2010

I have an older, low mileage vehicle with a V8 engine. When the engine is started cold, regardless of the outside temperature, the idle is rough and there is a noticable power loss. However, once the engine reaches FULL operating temperature the idle is smooth and power returns.

During the rough idle, I notice the tach needle quivering and that the engine is revving lower than normal. Also, there is an economy meter, fed off engine vacuum, which reads “lower than average” economy and its needle also shakes.

Some folks have told me to check the ignition system, others the vacuum system. Any thoughts would be very welcome!

There are 5 Answers for "Rough Idle When Starting Cold, Then Goes Away When Engine Is At Operating Temperature More Details Below"

  1. Totem T says:

    I would think that it is time for a full tune up, plugs wires filters and go from there

  2. kelly_f_1999 says:

    try a fuel filter and a bottle of fuel treatment

  3. Rudy H says:

    It’s going to be the engine temp sensor that feeds the computer, and also you may have a fualty or clogged injector. A shop can easily check this out if the car comes in very cold, like towed in, or you leave it overnight. It’s much harder to check once the engine is warmed. Don’t just start changing parts. Waste of money. It may additionally need plugs or wires, but these too can be checked first.

  4. Jon H says:

    is it fuel injected? many of the early fuel injected engines have cold start injectors that spray more fuel into the intake when the engine is cold to richen the mixture. Yours may have died and will not supply the needed fuel during warm up.

    The low vacuum is probably from the low rpm.

  5. Silverado says:

    You did not say that the vehicle is carbureted or fuel injected. If it has a carburetor, this might be the problem. If you have an automatic choke, check to see that it does not stay closed for too long. I had a simular problem with my old GMC pick-up.
    When the motor is cold, it will start right up, but will lump at idle and feel rough because the choke provides a too rich a mizture and overloads the engine to the point where it wants to run rough, almost to the point of stalling. Once the auto choke begins to come open it smooths out.
    Check the choke. I’m not saying this is the problem, but it sounds so much like the problem I was having with my pick-up.

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