what sensor or combination of sensors cause it? and what does it do? is there a default table you can see it goes to?


i remember years ago when my air pump went out the fans would kick on? is that a sign of it?

 

Yes, fans that are running all the time generally indicate the limp-home mode.

If you lose a key sensor like the engine coolant temp sensor then it will go into limp mode.

Not sure what tables or fueling strategy it uses in limp mode.

 

from what i have read this is generally not true, when the car goes into limp mode it attemps to run without any data from any sensors, and it is very rare for anyone to actually encounter it

the fans will run any time the SES light is on

edit:

 

The only time I experienced it was when I had a half-toasted PCM. The fans ran all the time, the SES light was on, and the PCM would not respond via the ALDL connector. So it must have had some sort of internal strategy for running. Interesting phenomenon.

 

Limp mode is basically the PCM running blind. Its not what happens when the PCM is missing a single sensor input, and can default to another method to approximate the input data. If you lose the AIR pump fuse, the fans run all the time, but that is definitely not "limp" mode.

 

That's what causes "fuel backup mode" - PCM problems.


I have wondered if the 94+ even has such a mode, because when we fry our EEPROMS, the car won't run at all.

In the earlier ECMs, the limp home code is in the CAL-PAK.

The Corvette Fuel Injection book seems to indicate that the later LT1/4 also has a fuel backup mode, but it's vague and that book isn't 100% trustworthy.

Can't find anything about such a mode in my 94 FSM.

 

This was on my 1994 Impala that this happened. In my vast experience tinkering with PCMs to develop the GT Tuner, I learned that if one "side" of the PCM was corrupt, the car would still run in limp mode, but if the other was corrupt it wouldn't run at all. It's been awhile so I don't remember which was which.

 

thanks for the info