'78 S2 FI - Unintended Acceleration

Hi All,

I have recently purchased my first jaguar (1978 XJ6) and have been able to work through many maintenance issues with the help of this great forum.

One item I have not found any details on is a random unintended acceleration I have been experiencing.


  1. Engine suddenly accelerates without input or warning.
  2. The pedal does not respond to tapping or prying up with foot.
  3. Resolution has been to shift into neutral and turn off the engine.
  4. Car will restart at normal rpm and drive fine immediately after the event.


a. My speedometer does not work and has not worked since I have owned it (6 weeks).
b. This only happens when the vehicle is warm (after driving for a while)
c. The only other symptom I have is that the car will stumble off idle while warming up.
d. The car runs very well in any other way I have tested.

Checks and notes.

  1. Cruise control switch ( on stock) is turned off.
  2. I have unplugged the electrical connectors on the cruise control unit under the hood.
  3. The cruise control connector has been disconnected from the throttle body.
  4. The throttle body seems to work well and returns to a closed position easily when manually opened.

Any thoughts?

Kind regards,

To speed up, the engine needs both more air and fuel to mach, Jordan - my though is that without the cruise control; this is impossible…:slight_smile:

The first step would be to disconnect the vacuum line at the bellows - or indeed disconnecting the wire connection to the bellows. Though what you have done should really kill the cc anyway…

xj6 85 Sov Europe (UK/NZ)

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Yes I agree, it could be an intermittent short at the cables at the engine bay, energizing at will one of the solenoids.

Thank you both!

I will disconnect and plug the vacuum line at the bellows an report back.

As I think about this, I will also remove the lower dash panel and fully inspect the throttle linkage and cable inside the cabin for issues.


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Had some cruise control weirdness a few weeks back. Take a peek at the wires:



could be a air leak after the air flow meter

An air leak cannot cause such acceleration, Henry - and is unlikely to be intermittent…

xj6 85 Sov Europe (UK/NZ)

I think you are wrong hear is how it works
air leak so running weak oxygen sensor sees weak so increase the fuelling more air and fuel revs rise

Intermittent odd readings from the coolant temperature sensor, or its circuitry…wiring or connector. You can check the entire circuit by ohming the two harness wires at the ECU, and checking the readings against the chart. SD Faircloth

How severe is this condition? Akin to flooring the throttle pedal? Or just a slight increase?

Good idea on GP, as faulty/sticking throttle cables are fairly common on these cars…although a defect here would account for a stuck throttle, not unintended acceleration.


The unmetered air is available for the engine to consume, yes. But it is upstream of the throttle body. The amount of air actually entering the cylinders would be dictated by the throttle blade.

The typical result of a leak after the AFM is a very poorly running engine, as air is being consumed that the AFM doesn’t know about…and the fueling is thus bollixed-up.

I can’t see how it would cause an engine to self-accelerate.


You need an actual and significant throttle change (air and fuel, not just leaks) for actual acceleration. Must be the cruise control if it just happens so completely disconect it mechanically and the problem must be gone.
The hard part is figuring out where the fault is exactly, that for later.

I think that step has already been done

Jordan, is here any chance that the throttle is sticking and you are perceiving it as unintended acceleration?

They’re not quite the same thing, but close enough to make me ask.


How severe is this condition? Akin to flooring the throttle pedal? Or just a slight increase?

Not quite flooring the throttle as I can control the car with brakes (override the acceleration). When this happens, my strategy is to shift into neutral and turn off the car with the key. In the time it takes to move my hand from the shifter to the key, the engine will rev quickly and I believe it will hit redline although I have not allowed it to happen.

This happened again yesterday and one interesting development is that I heard an audible “Click” as the car shut down. While this could be any relay (or other) I add it to the list of observations for diagnosis.

Good idea on GP, as faulty/sticking throttle cables are fairly common on these cars…although a defect here would account for a stuck throttle, not unintended acceleration.

While I will do this work as a matter of completeness, I also do not anticipate solving the issue as it does not fit well with my understanding of the symptoms. In my mind, a stuck linkage would not consistently resolve itself by shutting down the car. If the linkage were jammed, I would expect the engine to race again upon restart???

Thanks again for everyone’s insight!

Kill the ignition first?

Click is probably the AFM door closing.

Might want to look at the air flow meter. There are instructions somewhere on this site on testing the output readings as it sweeps thru its range of motion. SD Faircloth

every one its a 1978 CAR IT MUSE BE CARBS

It woulf be but the last US S2 got FI and he‘ll know what he has. The AFM is working correctly.

Fuel Injection was installed on the Jaguar production line in some of the Series II XJ6s starting around May 1978.


Or a sudden release of a binding cable?