Stuck Throttle

[trentwat]

As I was going through the gears on a long on-ramp (shift hard), I felt something; and when I shifted to 5th and let off the throttle, the car simply maintained the same RPM. Ultimately, made it to a gas station. When in neutral, the motor stayed at 5,000 RPM.

This happened once before a few years ago. When it happened that time, the cable had come off the engine mounting bracket. It is now zip tied to it, so this didn’t happen. After playing with the throttle, and then moving the throttle linkage in the engine compartment, it cleared itself up.

Any suggestion on what might have caused this? Thank you in advance.

[silverlock]

LOTS of things. In rough order of likelihood:

- Pedal bushings (most common if you haven’t done the pedal box + gone brass)
- Throttle linkage + bushings in the tunnel: there are three of them. They disintegrate over time, go yellow, and can bind. < $10 to replace although a little fiddly in the tunnel as you need to remove the shifter, remove/move the handbrake and get to the one in front of the shift coupler.
- Throttle console behind the intake where the linkage comes up from the trans: bushings there. Usually rubber and decay -> bind.
- Bellcrank on the trans, which also has bushings that go bad over time.
- Return spring(s) failing

Given how safety critical this is: make a list and work on through it. The pedal box is the biggest PITA but it’s all cheap work.

[rsscotty]

See the picture of the clamp in the wrong position....

https://forums.pelicanparts.com/porsche-911-technical-forum/773057-3-2-carrera-throttle-could-get-stuck-open.html

[EC900]

Had similar situation at highway speed during downshift. Turned out to be the two old yellowed 45 yr old nylon bushings for accelerator bell crank in the pedal cluster disintegrated. It threw off the entire geometry of the throttle rod and hung up in the tunnel where three more bushing were also aged out and gone, t-rod was flopping around.
So that’s first to check. Easy to replace if it’s just the nylon bushings (and not pedal cluster rebuild) with driver seat out of the way. For the 3 tunnel bushes passengers seat removal makes getting and seeing these fitted.
Those Bell crank bushes can be replaced easily, nylon or brass but if they were worn down and gone a long time the rod could damage bush opening by enlarging them and will then will need to be replaced.

[Spetrow]

Similar, my 3.2 with Rennline after market gas pedal that i did not install a limit stud on back side, upon and an aggressive WOT run, the pedal stayed floored. Lucky 3.2 has a rev limiter with kicked in a few times. Turned key off and coasted to a stop. Root cause - Mashing throttle w/o a pedal stop caused the bell crank at pedal box to bend and bind on tunnel thus keeping WOT. A little scary but after doing leak down, compression test, adding pedal stop, all was fine.

[carreradpt]

Do you have cruise control ?

[Rodsrsr]

Originally Posted by Spetrow

Similar, my 3.2 with Rennline after market gas pedal that i did not install a limit stud on back side, upon and an aggressive WOT run, the pedal stayed floored. Lucky 3.2 has a rev limiter with kicked in a few times. Turned key off and coasted to a stop. Root cause - Mashing throttle w/o a pedal stop caused the bell crank at pedal box to bend and bind on tunnel thus keeping WOT. A little scary but after doing leak down, compression test, adding pedal stop, all was fine.

This is common. To the Op. You definitely need to install the pedal stop, and I also bent some of the metal back on the floor area just to give a little extra clearance. I also welded a small piece of flat stock to the bell crank essentially stiffing it up, but also check all the bushings as others have said.

[milnejk]

Had this problem and found that driving without the floor board installed caused the problem. The throttle stop relies on the floorboard for limiting the travel of the throttle stop/ throttle pedal.

[trentwat]

Originally Posted by carreradpt

Do you have cruise control ?

I do…don’t think I knocked into it. Is there a reason it might have been engaged?

[trentwat]

OK…plenty of places to check…I have Rennline pedals and associated updates…will need to start poking around and see if anything stands out.

[dkirk]

Had this happen in my '88 and it was due to the cruise control cable. A zip-tie and some aircraft safety wire fixed the problem.

[Crow911]

Cruise control cable…happened to me and I disconnected the cable. Too much risk to have something I never use.

[Crow911]

Cruise control cable…happened to me and I disconnected the cable. Too much risk to have something I never use.

[famoroso]

Originally Posted by trentwat

I do…don’t think I knocked into it. Is there a reason it might have been engaged?

Cruise control cable doesn't cause an issue as a result of engaging the cruise control, but rather, the application of WOT can dislodge a CC cable with failed tabs and this can cause the throttle to stay open. Easy fix is to apply two zip ties in a cross configuration.

This video shows the failure mode...

https://youtube.com/shorts/3TuLEUiUNeI?si=QgDs2f5LmOyUe6HQ

[PopsRacer]

I had a bell crank issue when i first got the car, so it got new bushings. It only happened when the car was really hot (hpde events. Kind of a code brown braking into a corner and the engine wants to go!)
Then this past spring the cruise control cable tried to kill me. The plastic bit on the end oxidized and broke.

Set up a tripod or get a buddy to video the engine bay and replicate the issue. If you dont have cruise then its either the pedal assem or the bell crank probably. Jm2c

[MrBonus]

I also had the cruise control cable fracture on my old '89. It happened on the highway and scared the living **** out of me.

[trentwat]

Originally Posted by dkirk

Had this happen in my '88 and it was due to the cruise control cable. A zip-tie and some aircraft safety wire fixed the problem.

How did you identify this issue?

[dkirk]

trentwat - Had the throttle stick at WOT when accelerating on an ON-ramp. Key off and coasted to a stop. Opened engine compartment lid and saw the cruise control cable was out of position on the mounting bracket as the plastic connector had failed, causing tension in the inner cable, thus holding the throttle open. The black zip-tie fixed the failure point and the safety wire is just insurance against some suspicious looking, old black plastic that will eventually fail.

[trentwat]

Originally Posted by dkirk

trentwat - Had the throttle stick at WOT when accelerating on an ON-ramp. Key off and coasted to a stop. Opened engine compartment lid and saw the cruise control cable was out of position on the mounting bracket as the plastic connector had failed, causing tension in the inner cable, thus holding the throttle open. The black zip-tie fixed the failure point and the safety wire is just insurance against some suspicious looking, old black plastic that will eventually fail.

And that is what happened to me previously…it wasn’t WOT…instead it was stuck at 5K RPM. Same this time at 5K RPM, but the cable is zip-tied to the mounting point.