Unintended Acceleration

[Frank D.]

I have a '65 with a freshly-rebuilt powerglide and carburetor. Today while driving home from my mechanic's shop, I rolled up to a stop light. As I sat there idling, the engine started to stall so I moved the shifter to neutral to up the rpms. The engine suddenly revved up past 5,000 rpm on its own without me touching the accelerator. I moved the shifter into "drive" again to slow down the revs and of course, off I went into the intersection, with my brakes on and tires smoking like I was piloting a C-5 in front of the grandstands at Carlisle. I had to turn off the ignition to stop the car from racing.

Before I started the car again, I checked under the hood for any signs of binding at either the throttle or the accelerator linkage. I found nothing irregular and in driving around the neighborhood tonight, I could not get the car to repeat this harrowing behavior.

Has this ever happened to anyone else? With a powerglide OR a manual? The car has also been shifting well since the transmission rebuild. Any ideas?

Thank you,
Frank O'Donnell
Charlottesville, VA

[Paul H.]

Re: Unintended Acceleration

Did you check the throttle return spring for the carb? Sometimes they get sloppy and weak and allow alot of movement both ways.

[Joseph K.]

Re: Unintended Acceleration

I have had this happen on other cars not on a bow tie. It is usually the pedal, linkage, rug on the pedal, linkage binding under motor torque, foot on both the brake on the gas, and the like. In order for the engine to race the throttle plate has to be open.

[Donald T.]

Re: Unintended Acceleration

Are you sure it isn't a Toyota?

[Robert G.]

Re: Unintended Acceleration

5000 rpms is practically wide open throttle. In addition to the above, check for bad motor mounts-maybe the fan blade has scraped against the shroud, the air cleaner bumping against the hood. Things like that. Also, I wouldn't move the shifter into drive under those circumstances. You could break something in your drive train and possibly hit someone. Shut off the engine first.

[Terry B.]

Re: Unintended Acceleration

It happened to me once as a result of the floor mat being too thick and pushed up near the accelerator pedal not allowing it to reset as I took my foot off the pedal. They were not the stock floor mats. I never want to experience that again. I'm glad you didn't get hurt or hurt anyone. I hope you can figure out the cause before it happens again.

[Clem Z.]

Re: Unintended Acceleration

at least you are more intelligent than some toyota owners when this happened. . if i remember correctly PG linkage is a solid rod attached to the carb for downshifting and could this have caused the problem if it stuck ???

[Stuart F.]

Re: Unintended Acceleration

This is a new one on me too. I've never, under the conditions you describe, experienced or heard of this happening. The most common cause in an old car is a mechanical linkage problem of some sort (no electronics to speak of). I had a tri-power progressive linkage go past zero and jam all three carbs open during a drag race. I took foot off pedal and it stayed at WOT. I killed the ignition switch asap in gear (manual) and the engine popped all the way to stop. I swear I saw well over 7000 rpm's (in the heat of the moment). I fixed the linkage on the road and the engine started right up (thought I might have blown it). The thing ran better than ever after that, specially after I got rid of the tri-power and went to a highly modified 4-barrel instead.

Stu Fox

[Richard D.]

Re: Unintended Acceleration

5000 rpms is practically wide open throttle. In addition to the above, check for bad motor mounts-maybe the fan blade has scraped against the shroud, the air cleaner bumping against the hood. Things like that. Also, I wouldn't move the shifter into drive under those circumstances. You could break something in your drive train and possibly hit someone. Shut off the engine first.

Happened to me once in a 65 Pontiac Catalina, and it was a broken motor mount that allowed the engine to torque over and bind the throttle linkage.

[Edward J.]

Re: Unintended Acceleration

Frank, Rich is more than likely right, I have seen many motor mount failures and if the rpm,s are raised enough the engine will pick up off the drivers side mount and cause unintended acceleration.around 68/69 I remember a gm recall for cars with this problem, they were installing cables to tie down the engine if this happens.

[Ray G.]

Re: Unintended Acceleration

Hello Frank;
Clem diagnosed this it in my opinion.
In the 60s there a campaign adding a shield over the Throttle Valve linkage to stop foreign material from jamming same.
This shield had a rubber extension. If that rubs on the TV linkage it can cause a temporary problem sticking the accelerator.
Ray

[Roger O.]

Re: Unintended Acceleration

As I sat there idling, the engine started to stall so I moved the shifter to neutral to up the rpms. The engine suddenly revved up past 5,000 rpm on its own without me touching the accelerator.

You were sitting at idle , the engine started to stall , this is most likely when something was starting to happen. For the carb to go to above 5000 the throttle plates would have to open( a huge vacuum leak wouldn't have done it ). If during the carb rebuild the mechanic didn't put the screws back in the throttle plates and 1 fell through it would happen but would not recover and run OK.
Is that an AFB type carb, could the secondary's have flopped open , I believe they are the weighted type. If this happened the mech screwed up on the carb.
The trans downshift linkage is connected to the carb , could a move of the shifter have pushed the downshift linkage opening the carb ? If this was the case the mech screwed the shift linkage up.
If I were you with the engine "off" try to go back and retrace your exact steps as it happened and see if you can see the carb linkage move. Until you find the exact cause I would not put that car on the street. You were very lucky the first time chances won't be in your favor the second time.

[Frank D.]

Re: Unintended Acceleration--UPDATE

Checked both motor mounts, they are fine. Checked both linkages (powerglide) and could find nothing that was binding at any point. I did discover that the speedometer cable was coming in contact with the long connecting rod of the linkage where the two cross at the front of the transmission tunnel. I moved the cable up and out of the way and the hanger for the speedometer cable popped off.

I will re-attach the hanger in a location so that it keeps the cable away from the linkage. I have driven the car a few more times (carefully) and can't get it to repeat the unintended acceleration.

Will keep you posted as I continue to search.

Frank

[Robert S.]

Re: Unintended Acceleration

I am starting to get unintended acceleration on almost all of my vehicles. I have found that moving my right foot about three inches left cures the problem.

[Frank D.]

Re: Unintended Acceleration

Please remember before you reply to a post, that this is the most helpful discussion board for Corvettes on the internet.

Frank