sticking throttle - that was exciting

Otter74 · 10-03-2016, 09:55 AM

Yesterday was 'take the 911 for a drive' day and my GF and I drove to New Glarus, WI to see some nice scenery, drink some beer and stock up my pantry.

On the way back (much of the drive was on faster 2-lane country roads), I gave the car full throttle to illustrate a point I was making to her, and when my foot lifted, the throttle didn't. Gave it a wiggle, tried to reach around the back of the pedal with my toe (not so easy in a 911), then quickly signaled and braked to a stop at a turnout and shut the car off. That was a little bit exciting.

Pull the floor mat out, peel the carpet away, no footboard so no need to remove that, and take a look at the linkage. What was happening was that at full throttle, the end of the threaded rod with cup on the throttle linkage was catching on the edge of the clutch cable clevis.

What surprised me was that in ~14 months and 6000 miles this has never happened.
Obviously I do not normally drive in a manner that has me booting it all the time. I do occasionally give it plenty of throttle for acceleration when conditions allow for it because, well, it's fun. When events like this happen, I look for not just what is causing the event, but what has changed. So what changed?

Maybe I've never given it *quite* 100% throttle, and it could have happened at any time if I had.

I did replace the clutch cable earlier this summer. Previously, my clutch cable was crossed up with the throttle linkage - I could see the throttle pedal move slightly when I pushed the clutch in. I corrected this condition when I put the new cable in.

The pedal box has *seemed* OK, e.g. no obvious extreme wear in bushings, nothing bent, no elongated hole in the clutch arm or anything like this. It probably has not been rebuilt.

I popped the socket off the ball and wiggled the throttle lever on the pedal box, and could see some play in the bushings on the pivot for the throttle lever arm on the pedal box. So it looks like it's time to replace those.

With some twiddling, I was able to improve the apparent clutch/throttle relationship, enough so that it was not sticking every time I gave it full throttle with my hand while buried in the footwell. Drove home uneventfully. But obviously this is a safety issue. So I want to make sure I do not miss addressing anythiing.

-Obviously I think I should replace the bushing on the throttle shaft. I might as well just rebuild the whole pedal box as long as I have to take it out.

-I do not think that the clutch cable clevis and throttle rod end should be that close to begin with. Is a pedal box rebuild going to address this completely? Is there anything else I am missing? Anyone have a photo of a everything-known-right relationship between the two?

On a related note: my floorpan is missing the threaded stud that is used to hold down the bottom of a footboard, which is why I haven't been able to install one. If I am going to be working down there, I'd like to take care of this at the same time.

What are the thread specs of that stud? and what might you recommend for a solid way to mount it to the floorpan without welding? (I don't have a welder or the ability.) I do not want to drill a hole in the pan to mount it with a nut from the bottom. JB Weld with a sheetmetal bracket? Or whatever. Or just an alternate method to prevent clutch pedal overtravel. Right now I have something improvised against the back of the clutch pedal arm, but largely just do it by feel when I use the clutch.

KTL · 10-03-2016, 10:55 AM

I've had that same issue in a '78 SC I was storing for a friend. I crawled down in the floor area to relieve the binding but it occasionally returned later on. I never pursued it further because the car left before I could dive all the way into it. My situation was not always at full throttle. I could feel it hanging up in part throttle use and even when the throttle was completely closed, then starting to press down.

Yep you're smart for putting that stop on the back of the pedal. That footboard is important because it provides a stop point for the clutch pedal travel (and adjusting the clutch cable) and over time you can break the clutch by over-extending the spring fingers in the clutch housing.

The stud in the floor is a regular M6x1.0 thread size. But the size isn't super critical in that if you needed to install oversize for a typical M8x1.25, that'd work too. You'd just need to ensure the footboard is drilled out a bit larger to accept your bigger M8 stud.

tshebib · tshebib's Garage

Check the integrity of the weld at the throttle arm pin junction. They are known to weaken, bend and eventually just become floppy

chris_seven · 10-03-2016, 11:46 AM

The real issue is the missing pedal board.

The throttle stop is built into the pedal board. On early cars it was an adjustable screw and on the later cars is part of the pedal.

If you eliminate the board then the chances are that the pedal box, which is only 1mm thick , will bend even if you strengthen the reverse link.

Not only will the throttle jam but there is also a chance of damaging some of the linkage components.

I would strongly recommend that you refit the pedal board.

wayner · 10-03-2016, 12:26 PM

Holy crap, I'm running some accessory wiring and found a whole bunch of scariness behind that floor board just now!

Is that really a pair of fuel lies running back there?
I'll bet THEY could get damaged easily without that board!

Get a bolt quick !!!

Otter74 · 10-03-2016, 12:57 PM

I'll check that throttle arm pin junction, though I can see the pin moving a bit in its bore so I suspect that is most likely fine and it could just use new bushings.

I either didn't know or forgot that there was also a throttle stop on the pedal board - all the more reason to get one in there. I was a bit mystified recently (I posted a question about it) when I could not find a mounting stud at all - I knew where it should be, and could see it in photos of others' cars - but it just wasn't there on mine. I asked the PO, who owned the car for 15 years, and he didn't remember there ever being one. Odd, I must say.

That bolt doesn't see a whole lot of load - it's mostly just there to locate the bottom of the board - unless you slam it with your foot or something, so I am thinking of something along the lines of JB Welding a stud to the floor pan, or else using a small piece of steel sheet to attach to the pan instead and then attach the stud to that via a nut on the sheet. I really don't want to put a hole in the floor pan, unless perhaps it is covered by the front pan, and even then I'd rather not.

pmax · pmax's Garage

Quote:

Originally Posted by Otter74

I was a bit mystified recently (I posted a question about it) when I could not find a mounting stud at all - I knew where it should be, and could see it in photos of others' cars - but it just wasn't there on mine. I asked the PO, who owned the car for 15 years, and he didn't remember there ever being one. Odd, I must say.

How about posting a pic of the area in your car ?

Rodsrsr · Rodsrsr's Garage

As others have said, the pedal board is essential in these cars. Also, those lines look like brake fluid lines which go up to the reservoir and down to the master cylinder. For mounting the board, there is a small stud on the floorboard and the top part of the board has two cutouts which allows it to sit onto tabs which support the top.

Otter74 · 10-03-2016, 01:43 PM

Quote:

Originally Posted by pmax

How about posting a pic of the area in your car ?

Here's one:
Clutch cable clevis pin removal - argh!

avr924 · 10-03-2016, 02:01 PM

Had the same issue with my '80 Targa, only during an open track DE event, scary stuff! Pressing the clutch released it, I pulled the pedal cluster apart and did the bushings to fix it, I would highly recommend this cheap fix to anyone.

Dooodahman · Dooodahman's Garage

If that happened to me, my wife would never ride in the car again!!! (Glad everyone is safe.)

gomezoneill · gomezoneill's Garage

Cruise control cable hung up on the throttle linkage bracket next to the throttle body. Very common.

Here's a link one of many if you do a search.

Throttle stuck today...

Otter74 · 10-04-2016, 07:18 AM

Gomez,

Thanks, but I verified that it was caused by my throttle cable catching on the clutch cable clevis, and in any case the cruise control has been removed from my car.

Otter74 · 10-19-2016, 07:35 AM

I am going to pull the pedal box out and rebuild it on one of the next couple of weekends, and add the missing foot board, too. I've already got some of the parts I need, but it was only when I was ordering the rest that I realized that this may not have happened had I had a foot board, as the throttle stop would likely have limited pedal travel before this condition occurred. All the more reason to have everything right down there.

manbridge 74 · 10-19-2016, 07:53 AM

For those running w/o a board, make a pedal stop!!

manbridge 74 · 10-19-2016, 07:54 AM

And a clutch stop to prevent over travel issues..

brianlay · brianlay's Garage

I've had the same problem lately when other people drove the car..once at an autocross!
Mine hangs up the same way, catching on the clutch cable clevis. I do have a floorboard.
My theory is that others put more side load on the pedal than I ever have.

So far my fix has been bending the throttle arm back slightly to the left so the cable has more clearance. I can't see anything obviously wrong with the arm or bushings but maybe i'll just replace them anyway.

Brorag · Brorag's Garage

mine was binding at the lever point at the transmission under the car...

HarryD · 10:37 AM

Quote:

Originally Posted by Brorag

mine was binding at the lever point at the transmission under the car...

Mine did that too. Not a happy time. Happened as I was entering a low speed sweeper from a high speed straight at Autocross.

Be careful out there.

Pictures of the result.

The fix is a zip tie to move the cables for the e-brake etc. away from the bell crank.

This is the fix

wayner · 11-20-2016, 10:14 AM

Quote:

Originally Posted by brianlay

I've had the same problem lately when other people drove the car..once at an autocross!
Mine hangs up the same way, catching on the clutch cable clevis. I do have a floorboard.
My theory is that others put more side load on the pedal than I ever have.

So far my fix has been bending the throttle arm back slightly to the left so the cable has more clearance. I can't see anything obviously wrong with the arm or bushings but maybe i'll just replace them anyway.

After 40 years my pedal box got a rebuild. Had no idea that the pushing were so bad and other stuff was ready to snap! Don't take this area for granted