RomRaider

I'm chasing an issue where my ECU seems to be going in to limp mode when ever I hit boost, but no CELs are being thrown. In the spirit of discussion, I'd like to keep most of the specifics of my issue out of this thread and concentrate on the question "What causes the ECU to go in to limp mode?" (although I am more than willing to post my details in the tuning discussion or where ever would me most appropriate). In this case, I have a 16-bit ECU (03 JDM STi). I would guess that the circumstances would be similar to the 32-bit DBW ECUs also?

The main, obvious item would be out of control knock. Other items? Front O2 sensor going bad? MAF sensor is wonky? Other bad sensors that can cause an issue? I don't have access to the assembly code, so I can't do what I normally would and just start pouring through it.

Knock. That is all. Limp mode is defined as going into the high DET fuel mapping and WG boost enabled. The only trigger is a low IAM.

Any other sensor failing will not cause the BCS to be disabled or the rich fuel map to be enabled.

topic1840.html

I had read the knock control thread, which was really the impetus for the post. The thread states "Limp-home mode not active (IAM would already be 0 in this case)" so I was just trying to get some clarification. It seems to imply that limp-home mode could be activated in some other way.

So, that's good to know. Thank you gabedude.

I think the limp-home mode is possibly disabled in USDM models (that is, it never comes into play). Haven't really checked out non-USDM models to see if this is the case. I don't know for sure if some ECUs have a universal limp-home mode or not, but they all have some response due to sensor failures or certain CELs. Outside of a CEL being set, the only known "limp-home mode" would be what gabedude describes.

Aren't there limp modes that kick in when sensors go bad? Like if the MAF sensor goes way out of range, for example.

Or is "limp mode" the wrong term for that?

I would expect these modes to come with CELs, but if there's no other good theory then it couldn't hurt to try logging all of the different sensors to see if any are reading bizarre values. O2, EGT, MAP, MAF, etc, etc.

AFAIK, if no CELs displayed it's failsafe mode rather than limp mode.

I think of limp-home mode as a universal mode that impacts fuel, timing, boost, etc. Sensor failures are usually specific and only impact a narrow functionality like maybe forcing open loop, impacting knock control, calculating MAF via MAP, etc.

That suggests that the OP's car is falling into a failsafe mode then, doesn't it?

It couldn't hurt to view the data from all of the sensors and see if any of them are way outside the normal range.

Anytime your check engine light is flashing, some refer to this (including dealers) as limp home. The thing is you can still hit max boost. If your MAF breaks, your car will not start. The only other limp mode I know of is unplugging the MAF and the ECU will revert to running MAP. AFIK, the only way the car is ever truly "limped" is due to low IAM enabling WG boost and the high DET fuel mapping.

There are hundreds of CELs and each of them has a set of failsafes. It's hard to say what "limp mode" really means when someone says "limp mode" on an internet forum with no further explanation of their issue or a list of CEL codes.

Ignition advance can be disabled, fine ignition learning can be disabled, boost control can be disabled, long term fuel trim can be disabled, AVCS can be disabled, DBW can be disabled,etc.