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Old 10-11-2019, 11:02 PM   #1
DIYguy
 
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Drives: 2017 Camaro 1LT
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Smile Sticky Throttle Plate

Slight update, see text-

I searched this forum (and others too) repeatedly for a solution to a sticky throttle problem but found nothing helpful.
I don't know if sharing this will help anyone else, but hopefully it might.

Over the course of the year plus that I have owned my '17 the throttle had been getting progressively "stickier" for lack of a better term.
While there is no physical connection from the pedal to the throttle plate, the way it responded mimicked a classic sticking cable or throttle plate perfectly.
Push the pedal slowly, nothing happens, and as you push a little more all of the sudden the engine snaps up to a power level.
I had suspected, checked, cleaned, reset, re-taught and otherwise tweaked a number of things and nothing helped.
So after finally discovering witness marks in the throttle bore (shiny spots, scratches and a very tiny ridge) using a 10x jeweler's eye loupe, I did the following-

Using 220 grit paper I carefully sanded out the marks and ridge.
This was followed by 800 grit, 1500 grit, then buffing compound.
Then I gently rounded off the knife-sharp edges of the disk where it is closer to the bore as the throttle is being closed and polished some manufacturing burrs.
I did not round the edges farther from the bore as it closes, if that makes sense.
In this process I did not re-shape the bore or disk noticeably, perhaps 0.001"-0.002" at most anywhere.
I then carefully cleaned any grit out using carb cleaner and re-installed it.

It was much better immediately, but the problem was not completely gone.
So I repeated the process which finally worked.
After many hundreds of commuter miles and probably 50-70 start/warmup/drive cycles it is still holding up.
No codes, no odd behavior and no pseudo-stickiness either.

I do not have "before" photos of the bore and the witness markings, sorry.
But I have added a couple of "after" photos because I happened to have it off the car today for another reason anyway (23OCT19).
The witness marks were scratches along the axis of the bore toward the top and bottom of the bore as shown in the photos.
They are totally gone now with no re-occurrences.

If you feel that you are having a problem like this, before taking it to the dealer or buying a new throttle body why not give this a try?
If you see any odd marks, scratches, scrapes or wear ridges in your throttle bore, even if it takes a jeweler's loupe to see them, the plate is contacting it for some reason or another.
Try taking small corrective steps until the problem is gone.

There was another slight benefit that is worth mentioning.
The car feels a bit more responsive in general.
Nothing that would likely register on a dyno or anything of that sort, but it is definitely more fun to drive now.
Perhaps this is just because it was sticking before and was limping a little, I don't know.
But polishing the throttle bore and plate may be something worth considering doing even if your throttle plate is not sticking.

I do hope this helps someone else.
Thanks for reading.
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Last edited by DIYguy; 10-24-2019 at 12:37 PM. Reason: Adding photos
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Old 10-26-2019, 09:34 PM   #2
17rsvert
 
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For the benefit of those of us who never took one apart, can you tell us what's involved and how much time it takes to take it off and re install it?
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Old 10-26-2019, 11:09 PM   #3
DIYguy
 
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Smile Throttle body R&R

Sure, glad to.

My car has an LGX V6, but it would be a pretty similar process for a V8 or an earlier V6 such as the LFX.

The removal and installation time is less than 15 minutes total.
It took me about 45 minutes to do the polishing the first time and about 20 minutes the second time.
So less than an hour total either time.

Removal-
  • Remove the engine cover. In the case of the LGX you remove the oil filler cap and then remove the Torx bolt at the front-left (T30 bit). Lift the front of the cover and pull it toward the front of the car.
  • Remove the intake tube, which in the case of the LGX involves loosening three large hose clamps, un-clipping the sound tube by squeezing the sides where it connects to the tube (assuming that you still have a sound tube), and pressing the white clip on the left side of the small vent tube and pulling it gently off the intake tube.
  • Then remove the throttle body from the intake manifold. This is done by removing the four bolts using a 10mm socket.
  • Remove the electrical cable from the throttle body using a small screwdriver to depress the lock tab. I find it easier to remove the electrical cable from the throttle body after removing the throttle body from the manifold.

Polishing-
  • I did not remove the throttle plate from the throttle shaft and don't recommend doing so. Services that specialize in throttle body work probably would, but to me it is not worth the risk of getting it misaligned plus the issue of securely fastening it again.
  • I used small strips of sandpaper (about 1" wide) for the scratch removal and coarse polishing work and did it by hand (fingertips). This technique works well to get in close to the throttle shaft.
  • As mentioned, I did not remove a lot of metal, I just polished out the scratches and high spots until there was a uniform gap all the way around when the plate was held completely closed.
  • I wiped out debris frequently during the process to check progress.
  • The entry throat had the roughest finish by far. I spent a lot of time on the entry throat using 220 grit paper but a Cratex rubber abrasive tip and a Dremel tool would be a lot faster. Just don't use the Cratex on the precision portions of the bore because you could cause problems.
  • To polish everything to a mirror finish, use a small (3/4" diameter or less) cloth polishing wheel or felt pad on a Dremel tool (or similar) and abrasive compound such as MetalGlo or Harbor Freight 96775 (brown). Don't dwell too long in any one spot.
  • Hold the throttle plate wide open and you can get into all but the tightest spots quite easily.
  • Clean up the bore, plate and shaft very carefully using spray carb cleaner and clean paper towels. Then spray it down at least a couple of times until it looks like the photos and wipe dry.
  • I would not suggest using an air hose on it because it might drive dirt into the body casting along the shaft.

Installation-
  • Re-attachment to the car is the reverse of the removal process pretty much.
  • I like to smear a thin film of white grease or wheel bearing grease onto the rear mounting surface and o-ring of the manifold.
  • Install the four bolts in a cross pattern tightening a little at a time. You really should use a torque wrench for the final pass (80 lb-in of torque), but otherwise just make sure they are all four tight.
  • Put a little dielectric grease on the connector plug face and click it back into position on the throttle body.
  • Re-install the intake tube (and the sound tube if you really must) in the reverse order you took it off, then the cover.

That's all there is.
I hope that answers your question.
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Last edited by DIYguy; 10-27-2019 at 12:31 AM. Reason: Formatting
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Old 10-27-2019, 06:01 AM   #4
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Thanks!
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Old 10-29-2019, 11:33 AM   #5
crosive

 
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so, dumb but honest question, are you positive it's sticking physically via the plate? I have a guy local that had an ss that had a *terrible* lash problem with the gearing inside his tb, which was replaced via warranty. His had almost the same problem that you're describing. Basically his tb motor was turning properly, but the gearing connected to the plate itself had too much play, and it would just "snap" open after he gave it some juice.

once he had the tb replaced, the problem went away completely.
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Old 10-29-2019, 01:34 PM   #6
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Smile

Actually, that is not a dumb or unreasonable question at all.

I was initially very doubtful myself because after all it is a round brass disk in a presumably round aluminum bore and the problem was worse when the car was completely warmed up.
The thermal coefficient of expansion for the brass disk should be a bit smaller than for the aluminum bore, so if it had clearance cold it should only have had more clearance when hot.
I did think of gear lash early on and there didn't seem to be much at all when gently pushing the plate back and forth at its non-powered position, which is slightly open.
Yes, there is some, but it is very tiny.
The throttle-by-wire plate on our Cruze has more than that and it doesn't stick.
For those reasons it took me a long time to even consider looking for what I eventually found.

I also thought that there could be a dead spot in the motor itself (it is just a DC brush motor after all) and considered removing it and bench testing it.

Nonetheless, when I saw the scratches and shiny spots on the bore there could no longer be any doubt that the plate was (for whatever reason) contacting the surface of the bore at some point in its travels.
These are referred to as "witness marks" in the machinist's trade (I am an engineer BTW).
Proof positive that two things are contacting one another.

If I had to guess at a probable cause for all of this I would also blame the gear train and some burrs on the edge of the disk that I polished away.
There is a hard machined stop for the travel under the plastic cover (a PITA to access because of the spot welded motor connection tabs).
I tried shimming that months ago to prevent the plate from contacting the tiny wear ridge I found, and it did help temporarily.
But it threw a code even though I had re-taught the throttle position using my scan tool.
So it was back to square one.

If taking into account the build ups of expansion coefficients and tolerances for the entire gear train, perhaps they just all add up wrong under certain conditions.
Or perhaps the slightly greater expansion of the aluminum body opens up some radial play on the shaft when it is really hot allowing the plate to move sideways, particularly because it is spring-loaded.
I had something like that happen on a TBI-equipped truck I used to have, and I installed Oilite bronze bushings which cured it (replacement throttle bodies for it were nosebleed expensive).

As I sort of implied in the original post, I was on the verge of buying another throttle body but thought I'd try this first.
Perhaps if I had done that this post might have never happened.

If nothing else it was fun to experiment with polishing and mildly porting the bore just to see what would happen.
So far I am quite pleased with the result and would do it again even to a brand new one.

That said, I totally take your point and greatly appreciate that input.
Something to consider.
And better yet now there is a discussion to search for if someone has this issue in the future.
Before there was nothing.

Thanks!
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Old 10-29-2019, 01:40 PM   #7
crosive

 
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Quote:
Originally Posted by DIYguy View Post
Actually, that is not a dumb or unreasonable question at all.

I was initially very doubtful myself because after all it is a round brass disk in a presumably round aluminum bore and the problem was worse when the car was completely warmed up.
The thermal coefficient of expansion for the brass disk should be a bit smaller than for the aluminum bore, so if it had clearance cold it should only have had more clearance when hot.
I did think of gear lash early on and there didn't seem to be much at all when gently pushing the plate back and forth at its non-powered position, which is slightly open.
Yes, there is some, but it is very tiny.
The throttle-by-wire plate on our Cruze has more than that and it doesn't stick.
For those reasons it took me a long time to even consider looking for what I eventually found.

I also thought that there could be a dead spot in the motor itself (it is just a DC brush motor after all) and considered removing it and bench testing it.

Nonetheless, when I saw the scratches and shiny spots on the bore there could no longer be any doubt that the plate was (for whatever reason) contacting the surface of the bore at some point in its travels.
These are referred to as "witness marks" in the machinist's trade (I am an engineer BTW).
Proof positive that two things are contacting one another.

If I had to guess at a probable cause for all of this I would also blame the gear train and some burrs on the edge of the disk that I polished away.
There is a hard machined stop for the travel under the plastic cover (a PITA to access because of the spot welded motor connection tabs).
I tried shimming that months ago to prevent the plate from contacting the tiny wear ridge I found, and it did help temporarily.
But it threw a code even though I had re-taught the throttle position using my scan tool.
So it was back to square one.

If taking into account the build ups of expansion coefficients and tolerances for the entire gear train, perhaps they just all add up wrong under certain conditions.
Or perhaps the slightly greater expansion of the aluminum body opens up some radial play on the shaft when it is really hot allowing the plate to move sideways, particularly because it is spring-loaded.
I had something like that happen on a TBI-equipped truck I used to have, and I installed Oilite bronze bushings which cured it (replacement throttle bodies for it were nosebleed expensive).

As I sort of implied in the original post, I was on the verge of buying another throttle body but thought I'd try this first.
Perhaps if I had done that this post might have never happened.

If nothing else it was fun to experiment with polishing and mildly porting the bore just to see what would happen.
So far I am quite pleased with the result and would do it again even to a brand new one.

That said, I totally take your point and greatly appreciate that input.
Something to consider.
And better yet now there is a discussion to search for if someone has this issue in the future.
Before there was nothing.

Thanks!
I'm glad you actually have at least the train of thought to look at all avenues, and the knowledge to go through with it. I, too, have an engineering background, so I appreciate the steps you took to solve the issue.

I didn't realize (or didn't read) that you saw marks on the inside of the tb itself.
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Old 10-29-2019, 02:00 PM   #8
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Good find, thinking outta the box sometimes nets rewards.
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Old 10-29-2019, 03:19 PM   #9
DIYguy
 
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Smile

Quote:
I didn't realize (or didn't read) that you saw marks on the inside of the tb itself.
Yes, that was the "tell-tale" for the issue.
I feel a bit embarassed that I did not look closer when I saw the tiny ridge earlier (which had prompted the earlier shimming of the hard close stop).
Regardless, both are gone now.

If you look at the complex shape of the bore, it is obvious that GM (or whoever produced it) went to a lot of trouble to design and manufacture it.
So I am pretty sure it is only a matter of cost that they did not finish out the bore better, particularly in the entry throat area.
Diminishing returns and that sort of thing.
All the more for me to play with.

The other thing I discovered in this process is that there are two little perforated air dams just inside the opening to the manifold on my car, top and bottom.
I am curious if anyone might know why they are there.
I am reluctant to trim them out until I know why they are present, though I was tempted to do so.
My guess would be something to do with off-idle airflow balance when the AFM function cuts out the middle cylinders.
Any thoughts?
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Old 11-08-2019, 08:38 PM   #10
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Smile They're back........

OK, so I choose to treat this as an education moment.
A minor novella follows for the amusement of engineering nerds like me who thrive on details.
Feel free to skip to the conclusion at the end.

Since my original posting, slowly the hesitation has returned.
To be fair, it was not even remotely as bad as it was before, but it was there.
So off the throttle body came again.

The sharper edge of the plate on the lower side that I hadn't polished off still had some tiny burrs that had formed fresh scratches on the throttle bore.
But the upper side remains polished and squeaky clean, so whatever is happening is happening on the bottom side only now.
And no my OEM air intake is not ingesting crud nor am I failing to clean things up scrupulously during assembly.

This time I took a few photos for future reference and hope that they may help someone else recognize these signs, should they have a similar problem.

The scratches shown on the bore were totally removed top and bottom by the prior polishing work I did.
These scratches are fresh, and are deep enough that you can feel them with your fingernail.
The mystery then is how, with visible, generous and uniform clearances as the plate is held in full closed position, could this happen?

I believe that I have discovered the roots of the problem (more than one).

There is a small amount of radial play on the throttle shaft even at room temperature as previously mentioned.
Not enough to close up the gap, but it is unquestionably there.
When the case of the throttle body gets hot, that play most likely increases.
And the return spring and forces on the driven gear may be combining to drive the plate downwards.

Also when actually running and at idle (or after a deceleration when the stickiness happens the most) there is around 70 pounds of force pressing on the throttle plate due to the pressure difference across it.
This is based upon the 72mm disk diameter and about 80% of standard atmospheric pressure.
I cannot apply remotely that amount of force by hand while also shining a light from behind to see how much the gap changes.
So it is rather difficult to recreate the same conditions on a work bench.

But the fresh scratches prove beyond a doubt that the gap is closing up.
Moreover they correspond precisely to the remaining burrs and dings on the lower plate edge (see photos).

As a test I took 1500 grit paper to the scratches quite generously until they were totally gone again and also to the lower edge of the plate until it was squeaky clean.
A quick 10 mile test drive and the hesitation was gone again (or at least far, far better).
So far so good.
But not so fast-

Based upon the posting from @crosive (thanks!) I decided to disassemble the gear train as far as I could short of disturbing the throttle plate.
The metal motor gear is just fine as could be expected, but the three plastic pinion gears (two are combined on the center cluster) are all worn significantly.
The photos I took do not really show the extent of the wear very well.
The teeth that appear light colored are not due to lighting effects, they are actually deeply worn and are that color.
The teeth around the idle position on the output shaft and the corresponding small pinion teeth on the center cluster are worn nearly triangular.
On top of all that, there is significant wear on the center bore of the middle gear against its support shaft.
They were not in any danger of skipping teeth, but I discovered that there was about three tooth's worth of gear lash at the motor shaft before all the play was taken up.

The gear ratios from motor to output shaft are: 12:60:14:60 for a total reduction ratio of 21.4:1
So to move the output shaft through 90 degrees requires 5.36 turns of the motor shaft, or 64.3 "teeth" of motion.
But with a free play of 3 teeth that is a 4.6% motion loss which is quite sloppy in a "servo" system like this.
Looking at it another way, to achieve that 64.3 teeth of motion it actually has to move 67.3 teeth, so the lash is even a higher percentage in actual practice.

But wait, it gets worse-
When the problem is happening it is transitioning from the motor trying to close the plate against the return spring to the motor trying to open the plate.
So the effective "play" is far higher as a percentage over that smaller range.

Add this to the chronic contact between the plate and the bore and it is little wonder that the teeth wore out and the throttle feels "sticky".
All of this in 46,000 miles since new........

Conclusion-
So I ordered a brand new throttle body (OEM- 12671014) which appears to actually be a Bosch part.
It also appears to be a replacement for the original part because it supercedes the older part number.
Hmmmmm, I wonder why?
When I receive it I will probably polish it too because I liked the definite improvement in performance and feel so much.
But the original one is on its way out.

So if you see marks in the bore like in the photo, I would still suggest trying to polish them out and to smooth the edges of the plate, but beware that buying a new throttle body is likely to be in your future.

And there is another takeaway from all of this IMHO-
If you are considering purchasing a ported and polished throttle body made from an exchange core or having yours ported and polished, you may want to go for a brand new one instead.
Unless the service is doing something with the gear train (at least checking them, but preferably replacing them) you may be spending a lot of money for a unit that has worn gears that will only cause issues later.
Because there do not appear to be any sources for these gears, at least not that I have found, a brand new unit would be the safest bet.

For your amusement......

Thanks for reading this.
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Last edited by DIYguy; 11-09-2019 at 11:11 AM.
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Old 10-01-2021, 01:00 PM   #11
davef
 
Drives: 2017 SS 1LE Garnet Red
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I know that this thread is old but I want to thank DIYGuy for the detailed explanation. My 2017 1LE 1SS (18,500 miles) is doing this same thing and getting progressively worse. It has some mods including a Tony Mamo ported/polished TB that was installed very early in the car's mileage. I still have my original TB in the basement, I'll swap them this weekend and report back on what I see on the Mamo and how it drives with the stocker.
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Old 12-06-2021, 05:10 PM   #12
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Smile

You are quite welcome.
(I literally just now saw your posting, BTW).

I hope it helped you.
Mine is holding up very nicely more than two years and thousands of miles later.
I happened to look into the throat of the throttle body a month or so ago while doing something else to the car (hey, why not?).
There are no new scratches on the polished bore, so there is no binding like what used to be happening.
And no hesitation or "snap-on" behavior like there was before.
There is absolutely no question in my mind that the new throttle body fixed the problem.

Whether polishing the bore had any performance effect is certainly debatable, but it looks nice.

Oh, and the throttle plate and bore are still squeaky clean.
No oily film, no dirt, nothing.
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