Stumble/Miss/Hesitation

[Jackpine]

Originally Posted by BigSur

Wow...what is that exactly? What happens when a truck goes into fail safe mode?

"Failsafe" is actually designed to get you home if your truck engine overheats. (It's also known as "limp home mode"). Basically, the PCM shuts down spark and fuel to half of the cylinders so that the engine can run cooler.

As I said, it's supposed to be triggered by overheating, but it seems that the PCM can apply this strategy any old time - possibly caused by a false sensor signal. If you're really paying attention, you'll see the temperature gauge peg on full hot as it goes into failsafe mode. The PCM is actually moving the temp gauge there, since it is not actually telling you what the coolant temperature is (there's no sensors for that value - it's inferred by the PCM from the Cylinder Head Temperature).

The fact that an engine shutdown and restart often clears this condition is almost a sure sign (to me) that some sensor is malfunctioning.

- Jack