Sticky Throttle Body - Hanging Idle

[Captain Obvious]

Sometimes when I come to a stop, my idle hangs up and will not return to the correct RPM. Just started like two weeks ago. I know it's got nothing to do with the linkage or the dashpot, because I removed both of them and it was still sticky. I also know that it's not sticky goop inside the throttle body. It's clean.

It's not sticky all the time, but when it gets that way, I can feel that the throttle shaft is a little more difficult to turn than "normal". Due to the fact that this is a recent development (now that it has gotten colder), my first thought was that maybe my throttle body was icing up. But heres the thing... I can shut the engine off while the throttle is stuck open a little, and as the engine cranks to a halt, the throttle plate closes just like it should, and moves freely. In other words, the instant the engine stops, it will move free and easy and "clank" hard against it's screw stop just like it should.

Two seconds later I can start the motor, and it's fine for a minute or two, and then it gets sticky again. Doesn't seem like ice to me...

It's not sticky goop.

It's not grit in the bearings.

It's not the linkage.

It's not the dashpot

Anyone have any ideas?

[rcb280z]

Have you tried unplugging the BCDD to see if it stops?

[Captain Obvious]

Have you tried unplugging the BCDD to see if it stops?

Thanks for the suggestion, and no, I have not disconnected the BCDD. I know the BCDD is often the cause of what people call a hanging throttle, but in my case, I can't see how it could be related.

My problem isn't the BCDD path around the throttle plate... It's the throttle plate itself is not returning to it's stop. There's no mechanical connection between the active parts of the BCDD and the throttle butterfly.

[240260280z]

Some thing in one of the passages of ported vacuum that pop out of the throat around the edge of the plate?

[Captain Obvious]

Some thing in one of the passages of ported vacuum that pop out of the throat around the edge of the plate?

Well you know the history of this car with the blasting grit and all. My first thought was a return of something related to that. The confusing detail is that the throttle shaft always feels perfectly smooth and free moving when the engine is off. It's only sticky when the engine is running? What causes that??:paranoid:

I guess I'm gonna have to pull the throttle body and look at it on the bench.

[sblake01]

If your car was originally a California car, a bad altitude switch could cause what you're experiencing.

[Mikes Z car]

I am the last guy who should say anything about carbs but what if you unhooked the springs and then turned the throttle shaft by hand, pushing in different directions that would simulate the forces that would be put on the butterfly in normal operation? Maybe push towards the engine, down a bit and also in line with the springs.

[Wade Nelson]

Check the pedal itself, pull out the pivot pin, etc see if carpet is jamming it, etc.

Yes, actually take it out or apart, not just the flashlight routine.

[Zed Head]

If everything is clean and nothing is rubbing, and the problem goes away when air flow stops, then it seems like the only think left is a pressure imbalance between the two sides of the blade, the trailing edge and the leading edge.

I had a similar problem and started thinking about this but then found that the problem was varnish/gunk/residue in the throttle body. I cleaned up with liberal amounts of carb cleaner and some Scothbrite so don't really know if there was crud in the ported vacuum holes or not.

Here's one off-beat thought though - how about the AC idle-up mechanism, the thing with the vacuum pot and the hook that pulls the linkage when you turn the compressor on? Maybe it's activated through a shorted or leaky solenoid. You might not notice it if you're staring at the throttle body.

[Captain Obvious]

and then turned the throttle shaft by hand, pushing in different directions that would simulate the forces that would be put on the butterfly in normal operation?

Mike, Your suggestion about simulating the forces of operation is what I had planned to do when I pull the throttle body. I believe you and I are thinking the same thing... That maybe the throttle shaft bushings are worn in some weird way such that the vacuum pulling the throttle plate and shaft towards the engine or the return spring is causing something to bind at times?

If everything is clean and nothing is rubbing, and the problem goes away when air flow stops, then it seems like the only think left is a pressure imbalance between the two sides of the blade, the trailing edge and the leading edge.

I had a similar problem and started thinking about this but then found that the problem was varnish/gunk/residue in the throttle body.

Here's one off-beat thought though - how about the AC idle-up mechanism

Zed Head, About that pressure imbalance on the edges of the throttle plate... I was thinking of that too, but I don't know enough to be sure - Does that really happen? If so, it seems like it would plague just about every butterfly valve device out there.

I'll double check for crud when I pull it too. I did check for cleanliness while on the car, but you can obviously do a more thorough job off the car.

As for the A/C idle up stuff, I had the linkage disconnected right at the throttle body. There was no linkage connected to the throttle shaft at all, and it was still sticky. I can state with 110% certainty that it has nothing to do with any part of the linkage. Guaranteed.

So anyone coming up with any theories that could account for the seeming temperature dependence?

Thanks for the help guys!!

[Captain Obvious]

Since it's not going to be Z driving weather anytime soon, I spent a little time on the hunt for this gremlin.

I pulled the throttle body off and verified that everything moves smooth and easy. Took it all apart again and didn't find anything astounding. No wear marks on the butterfly. No wear marks on the inside bore. No gunk. A small wear spot where the throttle shaft spins in the throttle body bushings, but not enough that looked like a significant problem.

Then I pulled a vacuum on the back side with my shop vac, and it was more difficult to move with the vacuum behind it. Has anyone else ever seen anything like this?

On a potentially related note... How many rotations should the return spring receive before it hooks onto the post. It's easy to get a half turn. It's difficult, but possible to get one-and-a-half turns. I don't think I could possibly get more than that. It would certainly be less prone to hanging up if there was more return force.

So between those two options 1/2 and 1 1/2, which is the proper return force?

[Captain Obvious]

How many rotations should the return spring receive before it hooks onto the post. It's easy to get a half turn. It's difficult, but possible to get one-and-a-half turns. I don't think I could possibly get more than that. It would certainly be less prone to hanging up if there was more return force.

So between those two options 1/2 and 1 1/2, which is the proper return force?

Any thoughts on this?