[Biturbos4] Detecting "limp" mode?

Chris Newbold chris at newbold.org
Mon Mar 10 09:50:02 PDT 2008

For the last several months, I've noticed that under many circumstances I'm
only getting about 6psi of boost (according to the AWE center-vent gage).
Other times I'll get upwards of 8-9psi, which is what I'd thought was the
stock limit. My impression is that I used to get 8-9psi far more regularly
than I am now. I've pretty much ruled out intake leaks, as this apparent
change in behavior has persisted despite replacing all of the rubber in the
intake system with Samco bits. I also have Bailey diverter valves, and so am
less inclined to suspect them (as compared to stock valves).

Some how I'd gotten the idea that 6psi was the "safe" limit used when the
ECU dumped into "limp" mode due to some detected failure or error condition.

So, three basic questions:

1. Should WOT always result in 8-9psi of boost while in the 2500-5000RPM
range? Or are there other factors, such as ambient temperature, transmission
gear and road speed?

2. Is "limp" mode always accompanied by a check-engine light? Is there a
definitive way to determine if I'm in "limp" mode via VAG-COM?

3. Can the ECU toggle in and out of "limp" mode "frequently"? For example,
during my usual 40-minute commute I've seen both 6psi-limited behavior as
well as 8-9psi limited behavior.

2001.5 S4 Avant 6spd

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