Hi, I have noticed a strange issue with our 1984 L4 2.5L Fiero. On a cold start, the throttle sometimes doesn't want to open from its resting position at idle. It takes a considerable amount of force just to get the pedal to start moving. I thought it was a throttle cable issue, so I tested it at the throttle body with the cable off, and it is still hard to move the throttle buttery fly with the engine running. When the engine is off, the throttle moves smoothly at both the pedal, and at the throttle body. The only time the throttle seems to move smoothly with the engine running, is when the engine warms up and the idle speed drops back down to ~750 rpm. Could the manifold pressure somehow be to blame?
just to clarify, this issue only happens with the engine running, and with it cold.
You may just have some carbon build up. Try cleaning the Throttle plate with some choke cleaner from the auto store. Make sure it says "sensor safe". If that doesn't do the trick, remove the IAC "idle air control valve" on the Throttle body and check for buildup in the air passage.
Just How You rebuild a TBI matters. Examples: If you pull the throttle plate out of the shaft then very likely you reinstalled it wrong. Most plates are made to install one way to top. If you adjust idle stop screw or screw/level is worn/bent then very likely allows the shaft to close tight. Screw should never let the shaft and plate to close completely. see idle stop in my cave.
You never lube the shaft. For a start Many are wrong types and cause worse problems over time. Most quickly wash away by fuel. Some Graphite lubes can cause big problem used w/ aluminum parts.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
I didn't mess with the idle speed screw or take the throttle butterfly out. (the idle screw still has the welsh plug covering it) But I don't know if the previous owner(s) ever had the butterfly out.
There is a picture of the throttle butter fly in the configuration as when I got the vehicle. I checked the idle screw where it meets with the rotating mechanism, It felt like it came to a flat end but I don't know if that's worn or not.
Edit: used Patrick's advice for the photo
[This message has been edited by Owiko (edited 07-11-2018).]
That HUGE image effectively destroys this thread. If you can't resize it to something that works in a forum environment (such as a width of 1024 pixels), then use the following code...
quote
Originally posted by theogre:
Use [img thumb] not [img] to post big pictures...
CLICK FOR FULL SIZE
[This message has been edited by Patrick (edited 07-11-2018).]
So, I know most of you wont like this, but I adjusted the idle stop screw using the tach on the dash (I don't have a separate tach, and the last time I called around to see if any stores or shops did automotive tool rentals, they told me to call Rona...) I did disconnect the IAC, and I blocked the IAC hole on the top with something that fit snugly. I got it around 750-800 rpm before I decided it was close enough. Visibly, the throttle butterfly is no longer in contact with the walls of the throttle body so hopefully that will fix the issue.
The only valid reason IMO to turn that stop screw in would be because the location where the end of the screw sits had worn down.
That's probably what was wrong with this one, the screw must have worn enough that the throttle plate closed completely. when I first started the procedure it asks for you to start the engine with the iac disconnected and plugged, but in its current screw position, it wouldn't even start. i had to turn the screw in a bit to even get it to run.
I finally found someone else on another car forum who had the same issue, and the fix was turning in the idle screw because the throttle butter fly was being held closed by the manifold vacuum.
[This message has been edited by Owiko (edited 07-13-2018).]
That's probably what was wrong with this one, the screw must have worn enough that the throttle plate closed completely
I doubt the screw itself would've worn. It's made of rather hard steel. Much more likely whatever it rests against might've worn away.
When I've had to adjust that screw (after a previous owner messed with it), I adjusted it just enough so that the butterfly wouldn't bind in the TB bore while in the closed position. I presume that's what you've done. Should be fine.
IMO, manifold vacuum has nothing to do with holding the butterfly valve closed. The throttle shaft is in the middle of the butterfly which pivots... therefore manifold vacuum is exerting equal force trying to both close and open the valve (on either side of the throttle shaft).
[This message has been edited by Patrick (edited 07-15-2018).]
Yes the screw isn't the problem most times... The "arm" is worn bent or both. (I've seen screw tips digging thru the "arm" on some old cars...) Someone messing w/ the screw.
Correct adjustment for all engines is in my Cave, Idle Stop 300 TBI need something to block IAC passage. Many things can work but make sure you can remove even part of the plug after.
IAC can give some adjustment to bad air mix but isn't make for big problems for idle screw etc.
I had a bent arm on the linkage that was allowing my throttle plate to fully close. I bent the arm back, and the problem went away. I have spare throttle bodies to compare to, so I know what it was.
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