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Curing A Bad Idle

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On: Tue, Sep 6, 2011 at 10:18AM | By: Peter C Sessler

Curing A Bad Idle

Let’s start by saying that there’s little one can do to fix an engine with a bad idle these days because of all the electronics that govern idle speed. The way an engine management system works is really quite simple, very much like your home computer.

On a car, the Central Processing Unit (CPU) uses sensors to receive information, such as engine speed, temperature, throttle position, airflow, exhaust gas content, and so forth. From the information gathered, the CPU then instructs other electronics, called actuators, such as fuel injectors, the ignition module, EGR valve, idle speed motor, and others to perform accordingly. So, in my home computer, when I press the letter "p" on the keyboard (the sensor), the CPU thinks about it a bit and then tells the monitor to display (actuator) the letter “p”.

In the same way, if I step on the gas pedal (a really big "sensor"), the computer gathers all the information from the various other sensors, and then tells the fuel injectors to squirt more gas into the engine and thus the engine accelerates.

Problems crop up when there are sensors that tell the engine’s computer information that isn’t correct. The computer will try to compensate, but sometimes it can’t be done. In such cases, the engine won’t run or if it does, it runs very badly. But there are some things that can cause a bad idle that don’t have anything to do with a bad sensor.

Typically, a bad idle may be one that fluctuates way up and down or one that is just lumpy. Sometimes, the engine may not be able to idle at all when it’s cold but may idle when the engine is warm. Practically all engines today use an air box which contains the air filter. Sometimes, besides the large inlet and outlet rubber tubing/hoses there will also be other smaller hoses attached to the box. The outlet tube leads to the engine and is usually clamped onto the throttle body. On engines that use an airflow meter to measure intake airflow, you’ll find this meter located between the air boxes and throttle body.

So, what can one do to cure a bad idle? First, open the air box and check the air filter’s condition. If it is really dirty, replace it. I’ve found leaves, bugs, and other debris inside air boxes, so make sure you get rid of these too. Reinstall the air filter and make sure the lid is clamped on securely and that all hose connections are tight.

Check the condition of all the air tubing. On cars that use an airflow meter, ripped tubing from the airflow meter to the throttle body will cause a fluctuating idle. It won’t with the other type, the ones that use a manifold absolute pressure (MAP) sensor. Still though, make sure there are no rips or tears in the tubing.

Next, you have to gain access to the throttle bore to see what condition it’s in. To do that, disconnect (it’s usually clamped on) the large tubing that’s attached to the throttle body intake.

Once you remove it, you’ll be able to see the throttle bore and, depending on the throttle bore’s length, the throttle plate itself. All it really is is a round plate that lets air into the engine—you’ll also be able to see that the accelerator pedal cables are attached to the outside of it.

If there’s a lot of black residue in the throttle bore and plate, it’s likely that you’ve found the cause (or at least one of them) for a bad idle. Get some injector spray cleaner (it looks and smells like the old carb cleaner stuff) and spray the throttle bore and plate. You may need to use Q-tips or an old toothbrush to remove the residue on the plate, especially on the edges. You’ll probably also be able to see a small air bypass hole in the throttle bore—just spray that as well. If your engine uses an airflow sensor, never spray it with any type of cleaner, as this will ruin it.

If you feel up to it, the proper way to clean a throttle body is to remove it completely from the manifold. If you’ve gotten to the spray stage, you should be able to see that all that’s keeping it on is two or four nuts, the accelerator cable, and some electrical connectors. Once you carefully disconnect these, the throttle body comes out. Then you can spray the heck out of it until it’s clean. To reinstall it, you’ll need a new throttle bore gasket, which should be available at your local parts store or dealer.

Reconnect everything, and you’ll be in business, hopefully curing the bad idle at the same time. You’ve probably saved $50-100 in the process, as this is what most service stations charge for a throttle body cleaning.

If that doesn’t do the trick, then you will have to take your car to the service station where they’ll be able to diagnose what sensors aren’t working. But at least you’ve eliminated the possibility of some of the basic causes of a bad idle.


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