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Diagnosing An Idle Issue Without An Engine Light or Error Code, Pt 1

Vacuum gauge

Most idle problems, such as rough idle, the engine cutting out at idle, or idling too fast, are problems with the fuel:air mixture. Finding the problem causing that mixture issue can be difficult, especially on modern engines with sophisticated fuel injection systems that are made to trip sensors and give codes when a fault occurs. When no codes are set, it can get confusing.

The basic idle circuit begins with the idle air control valve (IAC), which opens and closes a bypass for the throttle plate based on input from the engine's computer (ECM). The engine's sensors (O2/AR, coolant temperature, RPM, ambient air temp, etc) tell the ECM the information it needs to know to adjust the IAC. Most problems will trip an error code from one of the sensors involved. It's those problems that do not that are at issue here.

Since most idle problems are either too much fuel or too much air (or not enough of one or the other), we'll focus on that.

Air Intake Issues

Your first culprit will be that IAC valve, which may not be opening or closing properly. It could be sticking, its actuator may not be doing its job, or there could be something causing air to get around a closed IAC. Begin by cleaning the IAC thoroughly with an old toothbrush and some electrical parts cleaner or brake cleaner. You might get lucky. Most likely this won't change anything. But it's worth a try.

Next, start looking at your positive crankcase ventilation system (PCV). The valve works harder and harder the older the engine gets. This is because the "blow-by" inside the valves and chambers increases as the engine ages and tolerances waver. Cleaning the PCV valve is likely the most sure way to resolve a rich idle (too much fuel) as you are probably getting enough air, but too much other stuff from the PCV for reburn as well.

Cleaning the throttle body is probably your next best bet. To do so, it's recommended that you leave it intact and on the engine. Do not unplug or remove it. Instead, turn the key to the "on" position (don't start the engine) and place something on the throttle (gas) pedal to open the valve completely. Then clean with the proper cleaner and a light brush.

Finally, the most common engine idle issue is a vacuum leak. In modern engines, this will usually result in some kind of error code somewhere, but rarely will that code lead you to the problem. It just takes you to the first sensor to notice something is amiss. Test the entire engine for vacuum leaks. They can be elusive, but once found, repair is easy and often results in instant improvements to idle and running.

In our next part, we'll look at fuel injectors and more.