Valve lifter replacement based on clearance check

Question

Since I have the valve covers off on my 2006 Tacoma V6, I decided to do a valve clearance check. The manufacturer recommended ranges are:

TYPE	MIN	MAX
IN	0.006	0.010
EX	0.011	0.015

^{meaning the intake valve clearance should be between 0.006 and 0.010 etc"}

Here is the data. The "off" column is populated if the valve is outside the range (other than 0) or if it barely makes the range by being on one of its limits (zero). As you can see, five of the 24 are just barely outside their range (none more than 0.001", a single thousandth of an inch only) and another seven are borderline (at one of the limits of the range).

CYL	TYPE	POSITION	CLEARANCE	OFF
1	IN	1	0.005	-0.001
1	IN	2	0.006	0
3	EX	1	0.014
3	EX	2	0.015	0
2	EX	1	0.013
2	EX	2	0.015	0
6	IN	1	0.009
6	IN	2	0.008
2	IN	1	0.006	0
2	IN	2	0.005	-0.001
4	EX	1	0.014
4	EX	2	0.014
3	IN	1	0.008
3	IN	1	0.01	0
5	EX	1	0.014
5	EX	2	0.014
1	EX	1	0.015	0
1	EX	2	0.016	+0.001
4	IN	1	0.005	-0.001
4	IN	2	0.005	-0.001
5	IN	1	0.008
5	IN	2	0.008
6	EX	1	0.015	0
6	EX	2	0.014

QUESTION 1: Since my range violations are very small, none more than a single measurable unit, does this test result imply that I should replace the violating valve lifters to get them in the range or do you think I am fine like this since the violations are all very small?

QUESTION 2: How do valve clearances that are outside the range affect the engine function and perfomance?

Answer 1

1. First of all you don't replace the lifters to adjust the clearance on this engine. It's a "bucket under shim" design where there is a small metal disc called a "shim" sitting on top of the lifter (or bucket) that is between the cam and the lifter. You adjust the clearance by removing and replacing the shim with another one of different thickness. There is a tool that is used to press on the edge of the lifter and push it down which then allows you to remove or replace the shim. Whether you should do it is entirely up to you, but in my view you're already 80% of the way there so why not?

2. Yes, this does affect engine function and performance. When the clearances are out of spec that affects the valve timing and can cause some performance problems. The biggest concern is that the clearances tend to tighten up over time due to the valve seats wearing and this causes the valves to contact the cylinder head less firmly than designed. This can cause the valve to run hotter than they should since that's how heat is dissipated from the valves. This can result in burned valves.

If you check your Toyota Service Manual you will find the full procedure for checking, removing, and replacing the shims. I also found any number of YouTube videos on people doing this so that may be of assistance to you as well.