Valve clearance - how much out of spec is actually bad?

[celeron55]

I have this emissions issue on my Previa #2 (*) and after checking and fixing many small issues, the issue remains.

(*) 1992, 410 000km, bought last summer for cheap; http://www.toyotavantech.com/forum/s...via-adventures

Specifically, it emits slightly too much HC at idle, and slightly too much CO at 2000rpm. You can hear a very slight inconsistency in the idle, which was also pointed out to me by a mechanic. The mechanic said the issue is probably either in ignition or in the valves or head, so I am now taking a closer look at both.

So, now I measured the valve clearances. Here are the results: (1. intake, 2. exhaust; in mm)

1. 0.20-0.25 0.20-0.25 0.20-0.25 0.25-0.30 0.15-0.20 0.15-0.20 0.20-0.25 0.20-0.25
2. 0.20-0.25 0.20-0.25 0.15-0.20 0.20-0.25 0.25-0.30 0.25-0.30 0.25-0.30 0.25-0.30

The specification for these is 0.15-0.25mm for intake and 0.25-0.35mm for exhaust.

From the intakes, 7 are to spec, and 1 is 0.25-0.30. These are fine.

From the exhausts, 4 are to spec, but 3 are 0.20-0.25 and 1 is 0.15-0.20.

So, QUESTION: Is 0.15-0.20 bad enough to send a little bit of unburned fuel into the exhaust in these engines? Or how likely is it indication that something else is wrong with the valve?

[pdgizwiz]

I'm not going to pass a judgement on what is "fine" if it's out of spec. Loose valves make noise. Tight valves bring you closer to a condition where the valves don't close, which means they will leak and burn. If you want to experiment with tight clearances, the risks are your own to judge.

Valves too tight were either assembled that way by a careless mechanic, or the result of erosion at the valve/seat interface.
I suppose your measurements were made cold. I don't know how they change when the engine reaches operating temperature. But the factory specs take that into account.

[celeron55]

I measured the shim on the tightest exhaust valve and ordered one that is 0.10mm thinner to get it into spec (was 2.85mm, ordered 2.75mm).

This is a cheap experiment and I will at least avoid burning that valve. The others will still be rather fine for a good while. Let's see if this makes any immediate difference. It might, or might not.

Yes, I did my measurements quite cold, which indeed is what the FSM specs are for.

I guess I should be asking this question on some Supra forum or something as they have more experience messing around with valve clearances. 8)

[Burntboot]

Tight valves are way worse than loose valves as already noted by PD but if way loose, they will start to affect lift and duration.
Emissions will be affected by either condition.

The factory gives a specific spec for damn good reason, trying to fudge the clearances is a recipe for further trouble.
As you've been chasing this ghost for some time, does it really make sense putting things together, knowing their out of spec?? and what if it still fails emissions after all this work, do you pull it apart again to correct the clearances you KNOW are wrong, hoping it makes a difference? or is it better to suck it up now, while you're right there, and make sure that everything is within spec before going further??

I have always done clearance in Thousandths, probably because that is the gauge I have.
I will stack gauges to get an exact measurement.
Maybe I am brain dead this morning but I see a big difference between .20 and .25mm, does that mean the 20 won't fit and the 25 does?

If I was working to a min spec of .25 and had .24 I might take the chance, doubtful but possible.
If I was looking at a min spec of .25 I would never in a million years think that .20 would be close enough, just sayin'

In my world, I have learned that when a range is specified, I always shoot for the middle of the range, to give a margin of error to either side.
So if I am adjusting valves that are spec'd at .008-.010in, I would use the .009in gauge.

[celeron55]

Maybe I am brain dead this morning but I see a big difference between .20 and .25mm, does that mean the 20 won't fit and the 25 does?

Yes, exactly.

So, what I take from your answer is that in this case 0.20mm is pretty bad.

-- does it really make sense putting things together, knowing their out of spec??

Yes. the information itself is valuable to me.

if it still fails emissions after all this work, do you pull it apart again to correct the clearances you KNOW are wrong, hoping it makes a difference?

If adjusting this single valve gives indication that correcting the others is likely to fix the emissions, yes. If not, then no. I'm mostly doing it for the experience at this point though. I still have other things to dig.

Pretty much everything on this van is out of spec. It would be insane to try to fix everything to be as good as new. I am not that comfortable with the base condition.

Correcting that absolutely worst valve is going to give me indication about whether the valve clearances are causing some of the known symptoms.

It's not such a big deal to open the valve cover, I can do it any day. And it's not like I'm going to stop fixing things after it barely passes emissions. But that's the base level I want to be at before being comfortable with actual investments.

Don't worry so much I know this is pretty reckless, but you have to understand that this van has been already driven by the previous owners with all the same conditions for like 100 000km. It has zero known service history. Me digging things as a hobby using 0.05mm gauges is only going to make things better.

One thing I noticed and was a bit surprised about was that judging by the chain tensioner's position the timing chain has been replaced at some point in this van's history. That and the fact that the windshield doesn't leak and the A/C works are things that keep me motivated!

[Burntboot]

I hear ya, but you have 4 tight exhaust valves, correcting one will help but why not just correct the 4 and be done with it?
Then you'll know for sure that at least that part of the equation is dialled in
BB

[originalkwyjibo]

Fyi Burntboot, .001" = .025mm so his exhaust specs of .25-.35mm = .0098-.0138" This means his measurement of .20-.25mm puts him within .002" and .15-.20mm within .004" of the low end of spec. I would agree both of these would warrant correcting. Celeron, one of the tricks of shim and bucket lifters is to reuse the old shims where possible. These means for example, if you had six valves out of spec you would remove and mark them for their original location and then measure their thickness writing this down as well(I usually clean off the oil and write it directly on the shim with a sharpie. In your case I would write #3 2.85). You would then use an adjustment chart printed in many manuals and widely available online to determine the sizes needed. Once this is done you look through your out of spec shims and use the ones that meet your needs and replace the ones that don't. What I'm getting at is while you're there, you could pull your next worse out of spec shim and mic it. You may be able to use the #3 valve shim to bring one of the other valves within spec. This keeps with your as little expense as possible concept. You could even pull the other out of spec shims and possibly swap some of them around as well. I have had valve jobs where 10 of 16 were out of spec and only ended up buying 2 or 3 shims. Good luck.

[Burntboot]

And doesn't he still have the cracked head from #1 with another 16 possible shim choices.

We always used to clean up used shims with a sheet of glass and wet #4000 micron paper, basically it just removes any carbon build-up, with hardened shims you don't have to worry about removing metal or altering the profile.

[celeron55]

Got the shim today and put it in. As a result, fuel trim went from the +11...+20% range down to the +4...+10% range.

So it really does seem that fuel and air was ending up flowing directly from the intake to the exhaust, and as I expected, correcting only the worst exhaust valve got rid of most of it. This would also indicate the valve itself is OK, so far only the timing was wrong.

I'll go and take new measurements on monday to confirm these findings.

[pdgizwiz]

Wow. That's a terrific thing to know about. I guess this is part of what defines a "finely tuned machine".

[celeron55]

Emissions test taken, and it barely passed now.

The values I was previously stuck with were roughly these:
CO%: idle 0.40, 2000rpm 0.70 (limit 0.50, 0.30)
HCppm: idle 135, 2000rpm 65 (limit 100, 100)

This is what I have after adjusting the worst exhaust valve:
CO%: idle 0.33, 2000rpm 0.28
HCppm: idle 88, 2000rpm 9

Getting those 2000rpm values required a fair amount of revving to get stuff into proper temperatures but I guess that's not indicative of much when the van was sitting at -26°C prior to the test...

I guess adjusting the rest of the exhaust valves might get these values further down, but that will be much later because the next priority is clutch and brake work.

[Jonny]

This is just for anyone reading this old thread wondering whether they should check their valve clearances.
As this thread show, having the correct clearances has a significant effect on the running of the vehicle.
Both vehicles I have owned with shim and bucket designs have had clearances which reduce over time, with the exhaust clearances reducing more rapidly than the intake.
As another data point for Previas, I finally did my first valve clearance inspection on my '91 at a little over 200,000 miles (I know, I know). All the intakes were still in spec. However, the majority of the exhausts were under spec. If I recall, when I extrapolated for an assumed even wear rate, I think I calculated that the exhausts started going out of spec somewhere between 120K - 150K.

[man_btc]

I'm appending this onto an old thread as it might be useful for more valve-adjustment info in one place. While doing something else, I just ran across a Previa valve adjustment spreadsheet I had once saved from the old Yahoo Previa Group - attached below between two relevant posts. As I always remind folks, this is copy-and-pasting of years' -old stuff from a long-gone message board, not authored by me (I have yet to deal with valve shim adjustments beyond maybe a quick check during a previous head gasket job).
Please excuse any gunked-up text, it's a quick conversion and some hand-editing of Yahoo's "JSON" source file minus their code that formatted it for viewing.

The Valve-Measurement Post with Spreadsheet (converted to pdf to attach)

authorName": "kfbryski" "msgId": 17387
"subject": "Valve Measurement"
"postDate": "1101647430", (no idea how to translate that)

Further to my original post (17088 - Rough Idle - Poor Compression
on One Cylinder) I submit my most recent findings. First, the
transmission fluid I added to motor oil did not help. So I proceeded
to do a engine flush/clean using the Gunk Motor Flush product (pour
a quart into a cold engine and run for five minutes, change oil and
filter). The engined did seem to run a bit smoother, although, I was
not satisfied that it had solved the problem, which I thought may
have been a sticky valve.

I then decided to measure the valve clearances. I followed the
advice of Neil's excellent Valve Adjustment post (3981 - Previa
Valve Adjustment -

I did not
use a special SST or a spring compressor, rather, I removed the cam
shafts, which is a very easy way to get at the shims (and buckets).
In hindsight, this is probably the easiest (less frustrating) and
less costly (do not require spring compressor $USD 75 or the SST -
even more). You do require a feeler gauge to measure the gap between
the cam lobes and shims, micrometer to measure the shims, torque
wrench, 18 mm wrench for the cam bolt, a new gasket for the valve
cover, sealant for the half moons (need to remove to remove the
timing chain cam sprocket) and a small bolt to hold together the
main and sub gear on the exhaust cam. It would also be wise to
prepare a parts bath to clean the cams and valve cover.

You must start with a cold engine. I followed the procedure in the
FSM to remove the cams, which is excellent. I also read Carl
Valderama's informative website on valve adjustment for a 4A-GE
engine:

Note that the
Previa exhaust valve clearance specs/tolerances (.01"-.014 " are
different than what his engine requires. The intake side has the
same tolerance (.006"-.01 ". I modified Carl's Excel worksheet to
fit the Previa. I have posted my results on the files portion of
this site. Download it and clear the values that I recorded, should
you have to do a valve adjustment. [***Download attached below ****]

I took me 9 hrs to do the job. This included removing the cam shaft
twice as I crossed the bucket and shim on number one, which resulted
in an even tighter clearance (not good). (that was the first pair I
removed and after that I only removed one bucket/shim at a time!)
Most of the time I spent was reading and re-reading (FSM, internet,
posts) as it was the first time I had removed cam shafts and timing
chain from a live engine. This is a job not for the faint of heart!
I also spent lots of time cleaning parts. All in all, it was a very
rewarding experience, which gave me great confidence to procede to
phase II.

I found 5 of 8 of the exhaust valves where out of tolerence (too
tight) with the other three marginally in tolerance (@ the lower
limit of .01 ". I found only one intake valve out and four
marginally in tolerance (@ the lower limit of .006 ".

I also found a fair amount of sludge, so I recommend considering
cleaning your engines if they have over 100K kms. I will also be
removing the oil pan to clean it next. There was no sludge on the
walls of the buckets, they were very clean and still pristine. I
also noticed that the shims were not worn. The loss of tolerance is
caused by the valves seating deeper in the cylinder head through
constant wear.

Rewardingly, the engine ran after I reassembled it. I noticed a
slight bit more power but still a rough idle.

Phase II is the valve adjustment - replacing shims. I will move some
of the old intake valve shims to the exhaust side (they are thinner)
and buy (or obtain from a donor vehicle) some new shims.

Kevin
91 LE with 265K kms

*************************************************
91 previa valve measurement.pdf
*************************************************

And here's the message referenced by the poster above:

Previa Valve Adjustment

The Previa's "shim & bucket" valve adjustment system does need checking and
adjusting. This type of system is more difficult to adjust but holds its
adjustment much longer than the screw and locknut type systems.

My two local Toyota dealers also told me it doesn't need adjustment or,
after listening to it, that its fine. That's BS! The only way to know is to
remove the valve cover and measure the valve clearance.

Typically, proper clearance is reduced as the engine gains miles or hours
of operation. Each time a valve is closed by the valve spring, wear (as in
microscopic loss of metal) of the valve's sealing surface and the valve
seat in the cylinder head occurs. This process is slow, but steady. It
causes the valve to move farther into the cylinder head eventually reducing
the specified clearance to zero.
This loss of clearance will alter valve timing, as pointed out in an
previous posting. It can also cause compression loss if the valve does not
close completely. However, the greatest danger of this is when valve
clearance is smaller than it should be (tight) and the valve does not fully
close. This leads to burned valves, usually exhaust, and compression loss
caused by overheated valves. Valve cooling depends on proper contact of the
valve's face or sealing surface with the valve seat in the cylinder head.

Again, the shim and bucket system must be checked and adjusted. It is
difficult to adjust, but holds its setting for a long time. There are
special tools for the job, but it can be done without them. It's not easy,
but its possible.

The special tools allow you to compress the valve springs to remove and
replace the shims. Without the special tools, you can change shims if you
remove the cams. I recently did it this way. It was not easy, but it
worked. If you try this make sure you have a FSM, a torque wrench, an
understanding of the four stroke cycle & top dead center, and lots of
patience.

By the way, my exhaust valves were tight. I'm glad I discovered that and
corrected it. Now, on to 300,000!
Neil Tolhurst
91 5 speed