New Timing Belt Changes Fuel Trim

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Re: 99 Toyota V6 with 135k miles and 9 yr old TB with 50k miles.

Splain this one. With old belt LTFT was consistently +3-4% on Bank 2 and -5-6% on Bank 1. With new belt, LTFT is close to zero on both Banks. In fact, I checked after running errands this morning and LTFT was 0.0% on both Banks. Old belt looked perfect except it had stretched a little. Why did new belt improve/balance LTFT's? Thanks.
 
Originally Posted By: artbuc
Re: 99 Toyota V6 with 135k miles and 9 yr old TB with 50k miles.

Splain this one. With old belt LTFT was consistently +3-4% on Bank 2 and -5-6% on Bank 1. With new belt, LTFT is close to zero on both Banks. In fact, I checked after running errands this morning and LTFT was 0.0% on both Banks. Old belt looked perfect except it had stretched a little. Why did new belt improve/balance LTFT's? Thanks.


The battery was disconnected and it's learning again?
 
Belt stretch over time.

In fact, if you look at the routing of the belt, I suspect you'll find that, given the direction of belt travel, bank 2 was closer to the crank, and bank 1 was farther, so the greater stretch was realized at bank 1, with commensurately greater cam timing error. Cam timing error required a fuel trim correction for proper operation.
 
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Originally Posted By: javacontour
Originally Posted By: artbuc
Re: 99 Toyota V6 with 135k miles and 9 yr old TB with 50k miles.

Splain this one. With old belt LTFT was consistently +3-4% on Bank 2 and -5-6% on Bank 1. With new belt, LTFT is close to zero on both Banks. In fact, I checked after running errands this morning and LTFT was 0.0% on both Banks. Old belt looked perfect except it had stretched a little. Why did new belt improve/balance LTFT's? Thanks.


The battery was disconnected and it's learning again?


Too obvious. Hopefully the OP took this into account.
 
Originally Posted By: Astro14
Belt stretch over time.

They do not.

OP's fuel-trim changes are not due to any "stretch" of the belt. Tooth-wear, maybe, but not "stretch".
 
Originally Posted By: dlundblad
Was the old belt off by a tooth somewhere?


I am quite sure it was right because I carefully marked before removing and counted teeth.
 
Originally Posted By: Tegger
Originally Posted By: Astro14
Belt stretch over time.

They do not.

OP's fuel-trim changes are not due to any "stretch" of the belt. Tooth-wear, maybe, but not "stretch".


Old belt was definitely stretched a little. It was obvious in a side-by-side comparison. If the teeth were worn, it was so little I could not see it with my naked eye.
 
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Originally Posted By: javacontour
The battery was disconnected and it's learning again?


No, I don't go to the trouble of disconnecting battery even though it is recommended, especially since I use the starter bump to loosen crank bolt.
 
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Originally Posted By: zzyzzx
Originally Posted By: Astro14
Belt stretch over time.


Do timing belts really stretch?


When I take them off after 90,000 miles, the tensioner has usually taken up some slack. So, tooth wear? Stretch? Does it matter? The slack means that the cam timing gets off by a few degrees...
 
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Originally Posted By: zzyzzx
Originally Posted By: Astro14
Belt stretch over time.

Do timing belts really stretch?

No. They are specifically designed to NOT "stretch".

Timing belts (and all other Gilmer-type belts) have a fabric carcass that is dimensionally-stable. If that carcass begins to change dimensionally, it's because the cords are beginning to separate, and failure is minutes way.

Most timing belt tensioners BOLT in place. They do NOT adjust to take up wear, but are locked in a fixed position on installation.

(I say "most" because there may well be some oddball design somewhere out there that does have an adjusting-on-the-fly design.)
 
Originally Posted By: zzyzzx
Originally Posted By: Astro14
Belt stretch over time.


Do timing belts really stretch?


I had a Dodge 2.5 with a bolt on tensioner, the belt stretched to the point where it started slapping on the cover.
 
Originally Posted By: Lubener
I had a Dodge 2.5 with a bolt on tensioner, the belt stretched to the point where it started slapping on the cover.

It did not "stretch". No cogged belt "stretches".

Either it was too loose to begin with, the tensioner was fastened with insufficient torque, or the belt's rubber surface wore down. But there was no "stretch".

Timing belts do not stretch.
 
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Originally Posted By: Tegger
Originally Posted By: Lubener
I had a Dodge 2.5 with a bolt on tensioner, the belt stretched to the point where it started slapping on the cover.

It did not "stretch". No cogged belt "stretches".

Either it was too loose to begin with, the tensioner was fastened with insufficient torque, or the belt's rubber surface wore down. But there was no "stretch".

Timing belts do not stretch.


Nope. I seen it first hand.It was once properly tensioned when I took delivery of the vehicle and the tensioner was tight. Show everyone with some documentation that the belts NEVER stretch.
 
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Originally Posted By: Tegger
Originally Posted By: zzyzzx
Originally Posted By: Astro14
Belt stretch over time.

Do timing belts really stretch?

No. They are specifically designed to NOT "stretch".

Timing belts (and all other Gilmer-type belts) have a fabric carcass that is dimensionally-stable. If that carcass begins to change dimensionally, it's because the cords are beginning to separate, and failure is minutes way.

Most timing belt tensioners BOLT in place. They do NOT adjust to take up wear, but are locked in a fixed position on installation.

(I say "most" because there may well be some oddball design somewhere out there that does have an adjusting-on-the-fly design.)


From my own experiences I have to agree with this. I just looked at one I did on a VW golf with a mechanical tensioner and pointer 40K ago, the pointer is still where it was when I put it on.
When you get one too tight that it makes noise the belt doesn't stretch over time and it goes quiet, it stays tight until it either wears the teeth out or damages itself or tensioner/WP bearings. The teeth do wear and make it appear it stretched though that's true.

https://books.google.com/books?id=4pcXBg...tch&f=false
 
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