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I was pulling my car out of the garage yesterday my foot barely on the gas pedal when all of a sudden the car accelerated out of control in reverse and I did not get it to stop until it hit the wall of my garage damaging by driver's side door. The car just did the same thing in drive-again I was barely able to get it to stop while burning rubber. Does anyone know what I should look at for this? My car has been converted to a GM 700r transmission for reliability I have never had a problem like this before.

Sounds like a missing or broken throttle return spring.

Agree...or something laying across the linkage that should not be there. Ernie in Arizona

It does sound like the linkage return spring might have broken or become disconnected and is not returning the linkage to the idle position. The spring could be in one of two places or, in some cars, both places.

Originally a single spring was positioned under the floor board pulling on the pivot mechanism at the bottom end where the accelerator pedal pushes a rod thru to the pivot piece. Later another location was used in the engine compartment where the spring was mounted vertically, one end on a piece of linkage on the firewall at the rear of the engine and the other end thru a hole in the welded flange toward the bottom of the firewall where pieces of sheetmetal are joined. In some cases a spring was added so both places were used.

Another possibility is since you have the GM transmission, instead of a rod for throttle linkage the GM trans uses a flex cable connected to the accelerator linkage to work the throttle and kickdown mechanism inside the transmission. It is also possible something has happened to the cable or whatever piece was fabricated to attach its end to the accelerator linkage and cable is somehow binding and not letting the accelerator linkage return to the idle position. Depending on how the cable was attached, positioned, or clamped, make sure it did not kink or get damaged by being too close to a hot manifold.

Excess heat can cause a modern cable to stick.



Another possibility is an engine or transmission mount that has separated. In such a situation, the engine can shift and bind the throttle linkage, causing a situation like you experienced. Sometimes, a car with a separated engine mount will stick the throttle when power is applied, and release it if the engine is shut off, after which the engine may start and idle normally, and respond normally to the throttle until the car is put in gear and throttle applied, whereupon the linkage will bind up again. The left side motor mounts are more likely to separate on some cars, but any issue with motor or transmission mounts can cause the problem.

How did this go undetected while the car wasn't in gear???

Probably because he tapped the gas pedal once he was in gear 🤔

Thanks for all the replies and advice I admittedly scratching my head over what could have went wrong. Many times on the forum I have stated how pleased I was with the conversion (bendtsens) since it was done almost 10 years ago. it is a mystery to me. Has anyone else who installed a GM 700r in their Packard had this problem? Even if you had the conversion (and it proved to be trouble-free) I would like to hear from you.

Just to add my 3 cents worth (inflation:(
Was scheduled to take my 55 400 into the shop first thing in the AM and when I went to start it, pumped on the accelerator a few times with fuel pump running, ignition on. Third tap and the pedal stayed down near the firewall. elected NOT to try to start and came to this post to see suggestions. The firewall spring was in place so went under the car to see if there was another spring...no go.
Looked hard at the linkages and could not see from the drivers side any thing that was out of the ordinary...except their was a small stamped plate with slots, holes, tabs and very elaborate and it was hanging like a pendulum on one slotted screw fastening. It moved freely but could not determine how it affected the throttle spring linkage system. Pulled up a few pictures of Rochester 4GC carbs and saw the issue almost immediately. One of the two rod linkageages going upward had become disconnected from the "pendulum". I tried to dislodge it from where it was located and it was bound in tight.Finally worried it free and once I did, all of the linkages sprung back into place with accelerator pedal back in normal location. Looking around in the area, on the valley pan, there was a micro spring clip that was lying very close to the linkage connections. fished it out with a magnet, compressed the spring clip to make tighter and reinstalled. Pretty sure this does not answer all of the OP's questions but conceivably, it was in a bind, freed up and then bound up again. Now trying to figure away that is fool proof (I know...doesnt exist) but thinking a small drop of urethane adhesive on the spring clip might keep it from dropping off again. Still wondering if it fell off because of some misalignment in the articulated controls. A couple of pics...not great but...

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