I recently adjusted the valves on my two Toyotas: a 2002 Tacoma with the 2RZ engine and a 2000 4Runner with a 3RZ engine. I have done a lot of Internet searching to see what's out there on this subject. Not much.
There are lots of misconceptions and false statements about valve adjustment on these engines. I thought this might be a reasonable place to clarify things. The factory shop manuals, which I own for both vehicles, does state how to adjust valves. However, it doesn't always say so in the clearest fashion.
My Toyotas have 4-cylinder engines. Both are DOHC, shim-and-bucket adjusted. The 6-cylinder 5VZ-FE engine uses an identical system, though of course it's a V engine and drives its camshafts via belts rather than a chain.
A recent question on this thread concerns where the shim is placed. In the 3 engines named above, the shim is over the bucket. It fits between the bucket and the cam lobe. This means the shim can be replaced without removing the camshafts. A special tool is needed to do this.
Regarding shim under bucket: This arrangement will be found on high RPM performance engines. Many motorcycles use it. Also, I think, certain exotic Italian cars. The major reason for putting the shim under the bucket is that the shim cannot be spit out at high RPM. It has little to do with being more impervious to wear. (Furthermore, shim-under-bucket aids high RPM by being slightly lighter due to smaller shims than shim-over bucket.) Replacing shims on these engines requires removing the camshafts. No special tools needed, other than standard mechanics tools.
Your 5500 RPM Toyota truck engine does just fine with the shim on top of the bucket.
What's the advantage of shim and bucket anyway? What's wrong with rocker arms? Plenty of DOHC engines use rocker arms.
In the shim-and-bucket system, the camshaft pushes straight down on the valve. Side thrust on the valves, which causes the valve guides to wear, is thus kept to the minimum possible, very near zero. Shim-and-bucket is the kindest system to valve guides.
In any engine with rocker arms, there is some degree of side thrust on the valve, thus causing valve guide wear. The end of a rocker arm always moves in an arc. Think of a see-saw. When you ride one you're not moving in a straight line, but instead are moving in an arc.
Now, on with valve adjustment. In my Tacoma, which I bought new, I first checked the valves at 38k. Perfect. No adjustment needed. Again recently at 90k. Again, perfect.
In my 4Runner, which I bought in '03 with 65k on it, I don't think the valves had ever been adjusted. I checked them at 90k - this may have been the engine's first adjustment - and 15 of the 16 valves were perfect. One exhaust valve was .009 inches (minimum is .010, maximum is .014), so I replaced just one shim.
That 4Runner now has 143k on it. In a recent check, 6 of the 8 exhaust valves needed adjustment. 2 had shrunk to .009, 3 had shrunk to .006, and one had shrunk to .005.
All intake valves were still within spec. This is understandable. Exhaust valves have a much tougher life.
Sadly, one of the valves that needed adjusting was the one closest to the firewall. Access is extremely difficult to that valve with the special tool used to compress the bucket to permit shim access. Primarily, the heater hoses are in the way. I worked at it for a long time, and even got under the vehicle, unbolted the motor mount at the transmission and jacked the engine/transmission assembly up an inch or so to rotate the engine away from the firewall a little bit. Still not enough room. (The 2RZ in my '02 Tacoma would have the same problem.)
I don't think I'd encounter this problem on a V6 engine. I should state right here that I have never adjusted the valves on a 5VZ engine.
Anyway, I have the knowledge, tools, and time to remove the exhaust camshaft. So that's what I did, primarily to ease access to that hindmost valve. It turns out that removing the camshaft made changing the shims on the other 5 valves easier as well.
My rule in the future will be this: if more than 2 valves need adjusting, or if the hindmost valve needs adjustment, forget about using the special tool; just remove the camshaft.
Here's how you have to use the special too. First, rotate the engine to TDC on the #1 cylinder's compression stroke. Check all 4 valves on #1, the intake valves on #2, and the exhaust valves on #3. For each valve needing adjustment, remove the shim, measure it, make a note, and put it back in. NEVER TURN THE ENGINE WITH A SHIM OUT.
Okay, next rotate the engine (clockwise only) 360 degrees (the cam rotates 180 degrees). The engine is now at TDC on the #4 cylinder's compression stroke. Check all 4 valves for #4, the exhaust valves for #2, and the intake valves for #3. Remove, measure, note. (Reinstallation isn't necessary if you're now going to replace these shims with correct thickness shims before turning the engine back to TDC on #1 to adjust the first set.)
Now buy the shims, and repeat the process of removing and reinstalling.
Consider the process if you remove the cams to replace shims. It's still a 2-stage process to check all 16 valves. However, you never remove - and reinstall - any shims to measure them. Just note the clearances. Now remove the cams as explained in the shop manual. (No special tools needed for the 2RZ and 3RZ engines to do this.)
All 16 shims are readily accessible. Measure, note and replace. Easy.
However, if I had just a few accessible valves needing shimming, I'd use the special tool method.
A word about noise: there are many out there on the Internet who advise only inspecting and adjusting your valves if they are noisy. I don't understand that. Certainly a valve that's too loose will be noisy, but a valve at .002 on its way to zero clearance is not noisier. Though it might get noisier once there's negative clearance and the valve is being held open during engine operation. But clearance below spec makes no extra noise.
And frankly, diminished clearance is likely what you'll get on shim-and-bucket engines. The cam and the shim itself - the rubbing surfaces - are nearly impervious to wear given proper oil changes. The diminished clearance is due to the very gradual but inevitable eroding of the valve's contact face and the valve seat. The valves - and in particular the exhaust valves - are very slowly eating their way into the cylinder head.
The word out there on the Internet is that the exhaust valves on the 2RZ and 3RZ engines need regular checking when you've got over 100k. Lots of reports of 3RZs with 120+k and tight exhaust valves. As I discovered, my 3RZ tighened not at all in the first 90k, and then considerably in the next 50k.
If a valve runs with near zero or negative clearance it's only a matter of time before it burns and the engine will need the head removed and a valve job. Big bucks.
Proper clearance gives the exhaust valve the most time to sit on its seat and dissipate heat. That's key to longest life.
I think it behooves any owner with one of these 3 engines (2RZ, 3RZ, and 5VZ) to have the valves checked when the engine hits 90k and then every 30k thereafter.
Even non-mechanics can learn to check valve clearances and then leave adjusting - shim replacement - to someone else. A few wrenches and a set of feeler gauges suffice to check clearances.