'78 S2 FI - Unintended Acceleration

I missed where the poster indicated the AFM had been tested as ok. ? SD

Simply because the AFM doesn’t cause unintended acceleration. There’s no AFM failure mode that suddenly opens the throttle and holds it open.

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Update!

As suggested, I pulled off the lower dash panel and inspected the throttle assembly.

Discovered a possible culprit. There was a bracket hanging loose on the throttle cable which could easily have jammed the cable if it bounced into a compromising position.

The bracket is approx 1.5 inches long and the throttle cable was routed through a hole on one end. My guess is that this small bracket originally guided the cable through the pedal box and was probably attached to the pedal box. It appears to have broken off of the pedal box and was dangling on the cable at the point where the cable was attached to the pedal arm.

The bracket has been removed from the cable. I will drive for a while and update with any developments.

Thanks again for all of the help.
Jordan

PS Yes, it’s an FI car. I was surprised to see FI on a car with points ignition! Other than some interesting surprises, I am loving the car.

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Right ! No matter what’s going on with the AFM the throttle blade dictates how much air is admitted to the cylinders.

And I can think of nothing, knowing that the cruise control is out of the picture, that would cause a throttle blade to open on its own; that is, accelerate.

This is a weird one!

Cheers
DD

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Sounds like a proper action, Jordan…

…but while the bracket may have jammed pedal release it cannot increase throttle - which seems to be the case? The same applies to the throttle butterfly opening; it cannot open more than the throttle pedal input(?) - though opening the butterfly is the real cause of engine reving, as Doug says. The pedal input just opens the butterfly - and somehow pulling the pedal up should close the throttle…

As a retrospective aside; the cruise control bellows directly act on the pedal, of course, but a series of electric safeguards removes vacuum and releases the pedal. Electric disconnections may just nullify the safeguards - but as long as there is bellows vacuum it continues to pull on the pedal as long as the engine is running.

Frank
xj6 85 Sov Europe (UK/NZ)

I think I had my V12 hat on. Was thinking about the TPS on the V12 sometimes going wonky and feeding wrong data to the ECU. SD

…but while the bracket may have jammed pedal release it cannot increase throttle - which seems to be the case? The same applies to the throttle butterfly opening; it cannot open more than the throttle pedal input(?)

With this new evidence, I am willing to entertain that my initial perception of events was incorrect.

At this time, I am thinking that the bracket jammed the throttle pedal during a transient acceleration event (climbing a hill, accelerating from a stop, etc), and my incorrect impression was that the pedal was pulled down.

I will use Occam’s Razor on this one and wait for additional symptoms before any more intervention.

Back to my regular scheduled programming of bringing the rest of the systems up to daily service spec.

Thanks again for everyone’s thoughts and assistance. It has been a great introduction to the forum!

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