Inconsistent idle

2014 2.7L 4cyl MT, 50k miles

My Tacoma has an an unsteady idle. What I mean by that is, after it warms up a bit and idles down, it'll be at about 800rpm but then slowly drop down to about 550rpm, at which point I can feel a rumble in the cab. The shaking isn't anything drastic, but it just doesn't feel right - doesn't feel like a smooth idle to me. It'll stay at 550rpm for a few seconds and then shoot back up to about 800rpm, and then start the cycle over. It does this pretty regularly, any time I come to a stop. Happens a little less if the A/C is on, but it still happens. It's not a huge change in rpm's - just a couple hundred - but it feels like it knows something is wrong when it's idling down around 550rpm and then eventually corrects itself, only to slip again. This isn't normal, is it?

Since it's been doing this, I've done the following and none of these have had any affect on it:
- replaced MAF sensor
- replaced spark plugs
- replaced air filter
- new oil and oil filter
- cleaned the throttle body (and the dealership cleaned it once too)

I have plenty of ideas of things to check, but I'm not sure if any of them are more likely culprits than others. So if anyone has ideas of things to check first, please let me know.

Here are the things I'm thinking of checking, in no particular order:
- coil packs
- new throttle body / TPS
- injectors
- vacuum leak
- fuel pump / pressure
- compression test

 

Check your A/F sensor lambda readings.

 

Thanks for the tip. I just went out to get a reading and the truck idled better than usual (of course). It did fluctuate as I described above when the A/C was on, but that would make more sense than fluctuating with it off. Anyway, the reading I got was right around 1.0 (occasionally dipping down to .99). The next time it's acting up (as it usually does) I'll attach the scanner and get another reading. But so far, the lambda seems okay.

 

I checked the fuel trims while I was out there too. For what it's worth, both the short term and long term fuel trims were 1.6.

 

Try another MAF ... it got bad MAF ...once ..then went .OEM ..

 

Thanks for the suggestion. I'll keep that in mind. But I feel like it's a long shot to bet on the OEM replacement MAF that I installed as being bad from day 1. It had this issue before and right after replacing the MAF with an OEM one. I know it's possible to get a bad OEM part, but I think it's more likely something else.

 

ICV

 

As far as I can tell (from looking at other posts on this forum) this generation doesn't have an idle air control valve. If I'm wrong about that, please let me know where it is.

 

Yes that might have been on my 1st.gen

 

Subbed!
09 2.7 RWD. 70k miles, purchased used.
Idle always drops to the 500 range in gear, but in my case it stays there.

So far did idle relearn, cleaned MAF, replaced spark plugs, bad converters & o2 sensors, engine/trans mounts, throttle body.

Recently found and repaired damaged wiring in the AM1 circuit (half of the strands cut.)