3800 Learned something new about TB's today

[imported_Ron Vogel]

So, I'll try to keep this story short (since it spans about 8 months). This guy calls me asking about doing his front end work on his TB, since it was already bored out to 73mm. He was having high idle probs with it, so we tried to find a vac leak before I started on it and we thought it was ok. After the rest of the TB work from me was done, put it back on and, Viola; high idle again...A few months passed, and he had a new plate cut, but it still didn't work out. I recently agreed to try and fix it, but after hand making a new plate to custom fit, still idled high. So this morning I filled RTV around the blade and completely sealed it off. High idle gone! At the same time off the car the blade fit pretty snug, so it made no sense. It ended up being metal fatigue. The area around the throttle shaft on the linkage side was fatigued with the big bore and distorting the bore itself. I didn't see it until I polished the bore chrome. As the tb aged, the bore distorted. The blade won't fully shut on the car because it rides up the distortion and hangs up the opposite side of the throttle blade. I'm now a little concerned about boring them even to 72mm. In a couple of years the bore may do the same thing. I'm going to start reinforcing that area now to prevent it from happening. I've got about 15 hours into this TB already, I'm just glad to know finally what the problem was.

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[sweetness]

thank god there is someone around here who is able to fix stuff like this. Probably never would have occured to me to even check for something like that. Good job Ron.

BC

 

[rob]

So the TB aluminum body distorted/warped and prevented the butterfly from seating properly...I cant seem to swallow the idea that the bore predisposed the TB to warpage. I mean, your into these TB's. It would be something to look out for but I think is an isolated instance...Or are you seeing alot of this now?

There is a possibility that there was an abnormal amount of heat in that particular instance. Anyway, I guess, heat insulation is the proticol hear. A MAP TB spacer/isolater or your TB (in aluminum) relocation pipe/adapter that we spoke of could help with heat soak prevention..? Instead of renforcement? Just a suggestion Ron...

I like the RTV idea....

Happy tuning.

 

[imported_Ron Vogel]

Originally posted by rob@Apr 4 2004, 10:06 PM
So the TB aluminum body distorted/warped and prevented the butterfly from seating properly...I cant seem to swallow the idea that the bore predisposed the TB to warpage. I mean, your into these TB's. It would be something to look out for but I think is an isolated instance...Or are you seeing alot of this now?

There is a possibility that there was an abnormal amount of heat in that particular instance. Anyway, I guess, heat insulation is the proticol hear. A MAP TB spacer/isolater or your TB (in aluminum) relocation pipe/adapter that we spoke of could help with heat soak prevention..? Instead of renforcement? Just a suggestion Ron...

I like the RTV idea....

Happy tuning.

When you bore the TB out, the area by the throttle shaft gets pretty thin. There's a needle bearing about the size of a quarter that rests against the bore, and on a 72mm bore leaves less than 2mm of metal there. This particular core had been bored to 73mm. So over time as the thrttle was opened and closed during normal use it flexed the area the bearing sits in and buckled the bore ever so slightly. I didn't even see it when I made the first TB plate. After I ran a buffer wheel down it sttod out when it was shiny and I could see down it well. The heat couldn't have caused it; this is an intercooled car, the TB stays pretty cool. The TB was bored about 2 years ago, so this has had plenty of time to develop. This is the first time I've seen this, it's probably an isolated thing. But I thought it was noteworthy, because it actually can happen.

 

[rob]

Got it........How about epoxy....aka: J.B. Can you renforce it with that without interfering with the bearing?

 

[imported_Ron Vogel]

I haven't had too good of luck with the JB weld, but I have some other stuff that may work. I also have been playing with welding on another TB I have, and it works pretty well. I may just weld some webbing lines around that area, then apply some epoxy over it.