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My 2001 Ford Focus judders when idle, and is most seen under load. The rev counter goes from roughly 500rpm to 1500rpm during this juddering cycle, as if it is "hunting" for the correct place to be.

When the lights are on, they noticeably dim during this time - which lasts for about 2 seconds, every 15 to 20 seconds.

The alternator is the likely culprit, however this was replaced two years ago and the battery is still being charged.

This behavior is most noticeable under electrical load, but also happens when the lights are off. This behavior is intermittent, and does not occur when the car is in motion - only when idling.

So, is the alternator the culprit, or is there something else which may be causing this?

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It is most likely NOT your alternator. Everything is going to dim in the car when the idle drops down to 500rpm because the alternator cannot put out enough juice to keep things up to snuff. To check it, have someone keep the engine up over 1000 rpm while you put a multi-meter on the battery terminals. Voltage should be around 13.5-14.1 vdc.

When you say "juddering" ... Is this just your way of saying "shuddering" or rough idle? Assuming this is what you are saying, there could be a plethora of things causing this. Is the car over a 100k miles? If so, have you done a tune up on it (ie: plugs and O2s))? Have you tried cleaning the MAF and throttle body? Does it need a clean air filter? Do you have any codes? The Air Idle Control (AIC) valve might be your problem. Any/all of these and more could be causing your rough idle and bouncing rpms.

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Yes, shuddering, and the car is > 100k miles. I'll hook up an ODBII scanner to it soon and see if any codes are thrown - nothing to cause the MIL to flash just yet. I was thinking of the throttle body, alright. The plugs are a bit old as well (about a year and a half to two years), however the O2 sensor was replaced about 3 years ago - most times I've gotten 5 years out of an O2 sensor. Time to go cleaning, and checking! – Nick Jan 4 '14 at 23:11
Cleaning the idle air control valve might solve it. A decent cleaning can be performed with seafoam (spray version) without removing it, spraying into it from the throttle body next to the throttle plate. – R.. Jan 5 '14 at 20:24

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