The Toyota throttle body on the 2005 Elise (cable linkage, not the throttle by wire of later years), has a design flaw which can lead to the throttle not fully closing when released. I have had the problem on two throttle bodies, and isolated the problem as the being same cause in both cases. The first time the throttle body was replaced under warranty (although I diagnosed the root cause before bringing it to the dealer). The second time I made a positioning bracket to fix the problem (it was out of warranty, and I didn’t want to spend $900 or more on a new throttle body).
The problem is that the butterfly valve shaft lateral position (and hence the position of the flap inside the throttle body bore) is set by the flap touching the walls of the bore. When the flap is open, only the very small portion of the flap next to the shaft is touching the bore, and thus is all that is positioning the shaft laterally. If there was no lateral pressure on the shaft, this wouldn’t be a problem. However, the torsion springs that force the throttle to close put a slight outward (towards the cable quartile) force on the shaft. (See “spring” picture.) It wasn’t the design intent for the springs to do this, but the nature of how they are wound causes them to push out just a tiny bit (at least the ones on the two throttle bodies I have had).
Eventually, this pressure causes the flap’s inner portion (near the shaft) on the quartile side to wear, which allows the shaft to shift towards the quartile when the valve is open. When the valve closes, the now out-of-round flap outer portions (away from the shaft) rub against the bore as it is closing. As the wear becomes worse, the rubbing friction (counter-torque to the springs) becomes significant enough to make it stick open unless the throttle is snapped closed. (“Wear pattern” picture shows the flap wear on one side. The picture makes it look a little larger than it is due to the light blooming in the camera.)
A proper design would have used a retaining ring, or a shoulder on the shaft fitting into a groove, or other method, to set the lateral position of the shaft, and not rely on the flap itself to be the positioner. A good design would at least have made sure that neither the torsion spring nor the throttle cable (or anything else) could cause even a slight lateral force on the shaft.
My fix to this problem was to fashion a rigid steel right angle bracket with a set screw that positions the shaft (see bracket pictures). The bracket attaches to the full throttle stop and spring posts on the throttle body using a screw between the posts to clamp it to the throttle body. The right angle bracket reaches out and over the quartile, allowing a set screw to be centered on the end of the shaft. The bracket (after suitable cutting it to shape) does not interfere with the rotation of the quartile at all. After manually positioning the shaft where it should be (hold the quartile and push in towards the body while rotating open and closed), the set screw is simply lightly hand tight (snug – no play), enough to position the shaft, but puts no significant force on the shaft (if you put too much force on the shaft, you will cause stickiness and wear on the other side of the flap). The set screw is clamped in place with two nuts, one on each side of the bracket. (I could have tried to make threads in the bracket so that I would need only a single nut, but I wasn’t sure that I could drill the set screw hole accurately enough to center it, and was concerned I might have to oversize the hole a bit.)
After using this for 3 weeks, it is still working great. The throttle operates like it did when new. I even passed my California smog test with it last Friday (I wouldn’t have passed if it was sticking).
I did have to readjust the set screw after one week because I didn’t tighten it enough. Originally, I had backed off slightly from the screw touching the shaft when the shaft was properly positioned, but that turned out to be a mistake. I reset it, making sure it was actually touching the shaft, and wiggled the screw and the nut on the shaft side to make sure it was square to the bracket and there was no play in it, before clamping down the outer nut. I observed the screw closely to make sure that it didn’t turn as I was clamping the nuts torwards each other.
Material: (I bought all this at my local Home Depot – cost about $8)
- Right angle bracket: Zinc plated steel, 1/8 inch thick, 5 inch per side (got it from Home Depot). I cut it down and shaped it using a Dremel tool (using about 6 cutoff wheels ). I used a thick bracket because I wanted it to be very rigid. (I would have preferred aluminum, but couldn’t find anything big and thick enough.)
- Set screw: M4 (4mm diameter), 0.7mm thread spacing, 20mm long
- Set screw Nuts: two M4x0.7
- Clamp screw (clamps bracket to posts): M4x 0.7 x 20mm
- Clamp screw nut: M4x0.7 (in hindsight, I should have used a lock nut - Home Depot had them)
- Clamp plate on opposite size of posts from bracket: large thick fender washer, shaped with Dremel tool, and holes drilled for the clamp screw and for the spring ends.
- Small flat and lock washers for clamp screw.
In retrospect, I think I would want some extra clamping of the bracket to the posts (the one screw makes me nervous), but it seems to be staying secure (I check periodically). Also, if the bracket starts to rust, I will need to paint it with a rust-protection paint.