One of the questions I am most frequently asked is about idle speed control; idling is too fast, too slow or erratic. An engine might idle too fast because of an intake air leak, but it can be due to a fault with the idle speed stabiliser or auxiliary air regulator. Check for leaks first and do not omit the vacuum hoses.

The next step is to determine what sort of idle control your engine uses. Some do not have an idle speed control valve and idle though the throttle body, meaning that the required air must pass the throttle plate. After a time this passage becomes contaminated by the oil breather system, lessening the air supply resulting in cutting out and stalling.

Cleaning the throttle body normally does the trick, but do not forget the basics; faulty plugs, HT leads and distributor cap can all bring the same symptoms.

Cars with a plunger-type idle speed control valve may, with age, develop an idle that is too slow, causing stalling, or too fast. In the latter case this may worsen as the engine warms up.

The best bet here is to remove the throttle body and clean it and the control valve with carburettor cleaner and a toothbrush. Immerse the valve in carburettor cleaner for 10 minutes or so and give it an occasional shake.

Some control valves have a metal cylinder in which there is a window that moves in relation to an aperture in the valve body. As the cylinder rotates, the window opening varies changing air intake to regulate idle speed.

Because tolerances are tight, a bit of dirt can stop the cylinder rotating resulting in stalling and fast idling. Be extra careful when changing the air filter.

Idle speed may be affected by electrical loading. The engine might idle at, say, 800 revs, but drop to 500 when the headlights are switched on. This would be a classic example of a seized idle speed control and most times the engine would stall until warmed up.

In such a case, clean the throttle body. If this does not cure the trouble, clean the idle control valve.

To complicate matters further, some engines use a waxstat to open or close the auxiliary air valve. During cold starts this prevents mixture enrichment by giving extra air. It depends on coolant temperature.

As the coolant heats up, the wax in the element expands as it melts and shuts the air valve. The main snag with this is that the tiny tube through which the warming fluid flows can easily become blocked and the result is a fast idle for too long – in fact until the engine’s heat has become sufficient to melt the wax. The idle speed will then fall to normal.

You can try blowing a blocked pipe clear with an airline and flushing the cooling system. If this doesn’t work, you might have to strip the unit to unblock the pipe. Keeping the cooling system clean helps prevent such trouble.

I have merely scratched the surface here because there are many more causes of erratic idling. Check for leakage and then try cleaning the throttle body and/or control valve. A bit of effort can save a lot of cash. -Drive Times.