2008 4.0L belt/pulley squeaking, how to diagnose?

Per the title, my 2008 V6 has a pulley/serpentine belt squeak. My Tacoma has almost 90K miles, and I changed the belt when it was 10 years old (so the expensive OEM belt is ~3 years old).

I ran the engine while looking at the pulleys, and I didn't see anything that looked like it was wobbling like I've seen in the YouTube videos of a bad idler pulley bearing. Popped off the belt, and could only feel a small amount of play in those pulleys. I wobbled them, spun them, and none of them seemed out of the ordinary. The serpentine belt didn't show any signs of excessive or out of the ordinary wear. Other than some dust/dirt on it, it looked almost new. I reinstalled the belt, started the truck, and didn't hear any squeaking.

Took it for a test drive, and didn't hear the squeaking, but I don't think I solved the problem. Did I dislodge something from one of the pulley bearings when I spun and wobbled it?

If the squeak returns, how should I attack this problem? New belt? New idler pulleys? New tensioner? I hate throwing parts at a problem hoping I stumble across the solution.

Speaking of pulleys, I have three idler pulleys as depicted in the attached photo:



It's showing a "Idler No. 1," and two "Idler No. 2" pulleys which is what I have on my Tacoma. What are the part numbers of those?

I'm including a Toyota dealer's part's page:
Belts & Pulleys for 2008 Toyota Tacoma | OEM Genuine Parts

In the picture on the website, they show an "Accessory Drive Belt Idler Pulley" (16603-0P030), and an "Idler Pulley" (16604-0P011). From what I've seen, most of the OEM Toyota parts websites show a similar vague picture.

 

Spray bottle with water on each pulley while it's running to isolate it. If the squeaking goes away, problem is likely the belt, if it doesnt, likely the idler bearing

 

Which idler bearing?

 

I'd spray them all but one at a time and then replace accordingly. Not entirely sure what you're asking here, sorry

 

Those idler pulleys are a common item to fail. Just replace them all if one is bad.

 

OP, you might want to also check the AC pulley and alternator pulley bearing(s) also driven by the same belt.

 

as mentioned above when its squeaking spray the belt with water, if noise goes away its a belt, if it does not remove belt, spin each pulley by hand, if its bad you will feel/hear it.
I own a small automotive shop and do this for a living, water trick has never failed me.

 

is it one of those high performance engines where you have 2 or 3 belts and are trying to figure out which one it is

 

This may help for the part numbers

https://www.tacomaworld.com/threads/4-0-idler-pulley-replacement-oem-vs-gates-reference.516475/

From the link above

Part Numbers (OEM):
Tensioner - 16620-31013
Idler #1 - 16603-0P030
Idler #2 - 16604-0P011

 

Sometimes just cleaning the smooth pulleys will fix the noise. Scotch brite worked for me.

 

Thanks! That tells me the 4.0L has two 16604-0P011 pulleys, and one 16603-0P030 pulley.

Will wait until the squeaking returns to diagnose belt vs pulley. If I have a bad pulley, will replace all three and just be done with it. If it's the belt, will just replace it. If the squeak returns, all of the above will be replaced. If it returns again, the tensioner is next.

 

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If you do the tensioner, I’d just do the whole thing. I think you need to loosen the AC mount to get to it. But it’s probably worth it.

 

Great combined write up and parts to figure this one out. Replaced the pulleys and belts in the spring and haven’t had problems since. Big Thanks!!!

 

I just replaced the idler pulleys, tensioner pulley bearing and belt. It's not a super enjoyable job so while it's ripped apart, replace everything.
It was easier to tell any bearing noise was gone than it is to figure out exactly which bearing was bad.

 

I wouldn't replace the tensioner, it's spring only has 100K on it and it's unlikely tension is an issue since those typically last far longer than 90K, arguably the life of the truck. I'd suggest just replacing the double row bearing inside the tensioner pulley with a quality Japanese one like a Nachi. It takes about 10min once the belt is off compared to hours and probably a few skinned knuckles replacing the entire tensioner assy and it costs about 1/4th what the tensioner assy does.