CAMARO6 - View Single Post

DIYguy · 11-08-2019, 08:38 PM

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.

11-08-2019, 08:38 PM	#10
DIYguy Drives: 2017 Camaro 1LT Join Date: Mar 2019 Location: California Posts: 241 View My Garage	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. Attached Images __________________ Gen6 Camaro LT (my daily driver), Cruze ECO (grocery getter), Chevy SS Pickup (wife's daily driver), Honda Shadow, Honda CBX Last edited by DIYguy; 11-09-2019 at 11:11 AM.