Random small dip in revs at idle

[Danneman]

Hello all. 99 Yukon with 124k miles, engine and transmission are almost new - they were replaced (with stock) at around 116k or so when I threw a rod. Handful of other non-regular maintenance replacements over the last 10-15k miles, kind of as opportunities presented themselves (for instance, leaking intake manifold, decided to replace fuel injectors while they were in there).

Anyway, thing I've been noticing for a while now is a periodic dip in revs when I'm sitting at idle. It doesn't happen all the time (meaning not every "driving session"). It's just momentary, around one second or less - but when it's happening (that driving session), it will happen every 10-30 seconds, at the lights. I don't notice any obvious hesitation once I'm underway again. The tach will drop around 100 or so, then right back up. (Voltmeter as well.)

Next time I drive, it might happen or it might not.

Does that sound like anything obvious to anyone?

 

[Sgtduckyboy]

mine started something similar. happens when its warmed up from driving. it wasnt so bad when i upped the RPM's 100 with my tuner. but i want to fix it, so i put it back to stock RPM.

 

[bigfootchiro]

Hello all. 99 Yukon with 124k miles, engine and transmission are almost new - they were replaced (with stock) at around 116k or so when I threw a rod. Handful of other non-regular maintenance replacements over the last 10-15k miles, kind of as opportunities presented themselves (for instance, leaking intake manifold, decided to replace fuel injectors while they were in there).

Anyway, thing I've been noticing for a while now is a periodic dip in revs when I'm sitting at idle. It doesn't happen all the time (meaning not every "driving session"). It's just momentary, around one second or less - but when it's happening (that driving session), it will happen every 10-30 seconds, at the lights. I don't notice any obvious hesitation once I'm underway again. The tach will drop around 100 or so, then right back up. (Voltmeter as well.)

Next time I drive, it might happen or it might not.

Does that sound like anything obvious to anyone?

Have the battery and alternator checked. There are ways to check at home, but best way for alternator is to have it bench tested.

 

[Danneman]

Thanks, I appreciate the suggestion - to Autozone I go. Will report back.

 

[Danneman]

Update: battery, alternator, and starter check out. Sgtduckyboy's comment about idle speed reminded me that i only see this rev dip happen the engine is warm and idling low - and that it has always seemed like the idle is a little lower than it should be. 500 rpm when warm. So, cleaned the throttle body (wasn't terrible, but could definitely use the cleaning) hoping that was it - but no change in issue. So, suspected and replaced idle air control valve...thought that did it at first, but nope. Any other thoughts? Going to try to post a video that shows my gauges when this is happening (haven't done that before, hopefully the video works)

.

 

[Danneman]

UPDATE - RESOLVED - I still had the original alternator after swapping it out for new, confident that it was the source of an annoying noise I was hearing, but that turned out to be the power steering pump. Put that old alternator back in and have had no further rev dip problems since - going on four or five months now. Of course, the new alternator was about a month past its warranty.