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Re: Overboost Protection


I am not familiar with the intimate details on the Motronic
code used in the 91 200TQ, (at least not yet, but soon?)
but in the earlier versions of the Audi I5 turbo (1986-90)
the ECU code (Mac11/Mac14) is setup to allow "overboost"
above the preset boost pressure limit that shuts off the
fuel pump. This overboost is allowed for a certain length
of time depending on air temperature. In a modified MAC11
and MAC14 ECU, with a boost map set to provide 1.8 bar,
this is why the boost is allowed to overshoot to slightly
over 2.0-2.1 bar for a brief time, even though the boost
limit cutout may be set to the upper limit of the stock
pressure transducer i.e. 1.95 bar.

On your friends '91 200TQ 20V, if the pressure transducer
has not been upgraded to a 2.5 bar unit and has not been
reprogrammed to use this new sensor, your friends boost
level is probably momentarily exceeding the cutout limit
and shutting off the fuel pump. He should connect a good
boost gauge and see where his boost is running. He may want
to verify the hose coming from the intake manifold to the
ECU is in good shape with no leaks. The WG frequency valve
should be checked for correct operation as well, a few have
reported failures of this valve. He may want to ask TAP
what the boost cutout is set at with the modified chip(s).

Scott M.