Random low idle issue

I have an 2008 4.0L V6 with about 174k. I've been having an issue which happens intermittently where when the truck is stopped the engine drops as low as 300 even 200 rpm and the voltage drops as low as 11.7volts and the truck begins to shake vigorously. While stopped (truck is shifted into drive), it tends to have a back and forth cycle where the rpm drops down only for about 5-10 seconds then jumps up back to normal idle rpm and voltage and so on. Most days it doesn't happen at all, but some days it does it constantly. Acceleration and driving seem to be normal.
Battery, alternator and serpentine belt are all new.

Any idea what could cause it?

 

Clean the maf and throttle body

 

Were you having any of these issues before you replaced the things you mentioned?

 

I'll give cleaning the MAF and throttle body a shot, thanks Anthony.

Now that you mention it, I'm almost positive I didn't have the issues until after the replacements. I had Firestone replace the alternator since i was in a pinch. They damaged the belt in the process and half of it tore off and got caught up in my engine fan on my way home. Bought it back and they fixed it. Ever since, I've been having issues with the low voltage every now and then.

 

It's getting worse, it's been happening all day and the rpm drops to below 200. My truck finally stalled out at a stop just now.

I haven't had a tune up in a really long time, maybe it's the plugs or fuel filter?

 

If this started after the alternator/belt problem, maybe it's possible that when the belt let go it damaged something else with it? Maybe a pully is seizing up? Would cause the belt to slip and cause issues like that.

 

That's a good point. My A/C compressor seems to be making more noise lately. It's rusty as hell and the bearing sounds like it might be shot.

 

The bearing might be okay actually, but the clutch has a good amount of rust on it. There's some between the pulley and the clutch too.
I just tried turning my heat on and off a few times and it seemed to idle fine when I have the heat on, but once I turn it off the idle dropped down to 200 again. Did it a few times with the same effect.

10 min later I tried doing it again but couldn't replicate the results, it's running fine now.. what do you guys think?

 

Sounds like it may be an AC issue, I had that happen with another car (audi), it was stalling out and at the end it was stalling out the engine. Depending what type of AC you got you may try just turning the compresor to see if it's binding. Mine had an internal clutch so it froze up and had the compresor pulley spin all the internals all the time.

 

What vent were you using? Defrost?

 

I've had the same issue whenever I unhook the battery. It resets the computer and it has to re-learn stuff before it smooths out.

 

Yeah I didn't think about the fact that I'm sure they disconnected the battery to replace the alternator. Might be worth taking the ground off, waiting 15 min or so, connect it back up, and try again.

 

Have you checked to make sure that all of your electrical connections are good? Including both terminals on the battery, and the 10 mm nut on top of the alternator.

 

The a/c compressor clutch is external, right in front of the pulley.

I was using windshield/floor setting on hot.

Right! It's exactly how the truck behaves whenever I disconnect the battery, going through that process. Really low rpm and shaking.. probably a good time to mention I have a half-ass spliced battery cable running from my positive battery terminal to my fuse box. No idea why it was done but thats the way I bought it. I try to stay away from more involved electrical work, but it sounds like I finally have to address the issue..
Only I can't find the proper cable, and instructions on replacement. I have some mechanical experience but I'm no expert..
One of the wires on my alternator pigtail connector also corroded away and broke off in the past, for the charging system sensor relay circuit I believe. It was triggering my red battery dashlight indicator. I managed to soder it back on and the light went away. I bought a replacement connector but the knuckle busting tight space and my fat fingers convinced me to procrastinate..

 

Yes, if you have it on windshield it's going to kick on the AC compressor. That's why I was wondering.

 

I see, so do you think the A/C compressor is a potential culprit? I'm not trying to go through all that time and money unless I'm sure.

I bought MAF sensor cleaner and new spark plugs and will do those in the morning, since it's probably overdue as it is.

 

Well, after you just went on about all the dodgy wires.... I would start with that now lol.

 

I hear you.. not gonna lie, a number of memorable unrelated past experiences have kept me away from electrical work.. last thing I want to touch lol.
I'll see how it goes and let you guys know. Thanks a lot for all the help. This site is a lifesaver.

 

It shouldn't be that bad. The cable from the battery to the fuse box is just held in with 2 nuts. If the cable is totally toast, and you don't want to solder up a new one, I would bet a car audio shop would have a proper sized wire and ends, they would be capable of soldering and making one up.

http://tacomaface.epizy.com/FSM/data/ileaf/06toyrm/06toypdf/06rmsrc/rm2006ta/02400.pdf

 

They more than likely used a POS alternator from Auto Zone or some other places that sells cheap Chinese crap.

In my area - they are a shop to be avoided.