Idle randomly dipping

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Apr 13, 2013
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This is in my Pilot.

90% of the time, the car idles at a solid 750 RPM at a traffic light with all accessories off. But the other 10%, the idle dips to like 600-650. When this happens, the engine makes a rumbling noise and you can feel the center console shake a little. BUT...turn on the headlights or A/C, the idle rises and everything smooths out again. It's strange that this happens intermittently. Car drives great otherwise.

Any ideas what could be going on?
 
Maybe the 5 year old battery is starting to fail and the alternator is having to engage more to keep the system at 13 or so volts.
Many cars the AC system still exercises to keep seals good even with the switches off.
 
Battery was replaced earlier this year; connections are all clean & tight.

In addition:

- All fluids are clean & full.
- I mainly use BJ's gas but fill up at Top Tier stations every 3rd tank or so to keep things clean.
- Inspected the air filter. It's 25k miles old but light still shines through; manual says to change every 30k.

I'm thinking about cleaning the MAF sensor & TB. I don't expect them to be dirty at only 45k miles but maybe it'll make a difference.
 
My Civic will idle rough (haven't heckled RPM) after being fully warmed up and at higher ambient temps. This seems to coincide with the ELD sensing low electrical load and lowering supply voltage to 12.6 or so.

Turning even just the running lights on (all but headlights on) will raise the voltage to normal. The vibration doesn't go away completely (likely something else is the root cause of that) but the the vibrations do lessen.
 
Go get the battery, alternator, voltage reg, etc. checked anyways. Sometimes you get a bad battery. Autozone with do it for free.

I've had bad or fouling plugs give me a rough idle, as well. Might be good idea to check those out and test the coil packs.
 
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