Exiting the freeway I noticed that the engine was accelerating without being depressed. The engine continued to run at a faster and faster rate and would not turn off with ignition key - had to shut off manually with engine STOP lever. Is this a linkage issue? The accelerator pedal does spring back when I push it down. The levers which operate the accelerator adjacent to the STOP lever appear to be intact. Where to start???

If the exit from the freeway had a noticable downslope it probably is a failed vaccum pump letting oil into the intake system. Check the lines for an indication of oil inside. When the engine is tilted nose down a little more oil gets to go out the vaccum pump if it is defective.

When you stopped the car on level ground and the key would not shut it down enough vacuum may have been missing. Or the engine had not yet used up all the available base oil. The manual shutdown worked because the oil feed from the vacuum pump had reduced the amount of base oil available on a flatter surface by the time you got to the shutoff.

This senario is just a possibility. If that exit ramp was downhill much more of a possibility. The car may have been accelerating on its base oil. Check carefully as you do not want to land up with a runaway engine.

Having the means on hand to stop a runaway would not hurt. I've heard of discharging a CO2 fire extinguisher (not any other kind) into the intake. You can also starve it by blocking the inlet- not at the mouth of the air cleaner, but the manifold inlet. Your hand doesn't seal as well as you would think- maybe a thick, flat piece of sheet metal would work better. Whatever it is, make sure it's something the engine won't be able to ingest.

If the engine was stopped with the stop lever on the linkage then it doesn't sound liek it was running on its own oil. If it was running on its own oil, you have to smuther it or mechanically slow the engine(braking). I had a 1.5 VW diesel that had run away issues and the only way to stop it was leaving it in gear and stalling it with the brakes. It was a stick shift.

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.