I cannot take it anymore! Boost/radiator?

[The Hardway]

OK my 2003 1.8T slightly modded (chip, injectors, fmic, cat-back) is giving me headache as I can't fix the problem that causes :

-The radiator fan to start as soon as I start the car, even if the car's cold!
-My boost to drop from 18 psi to 5 psi in a second (happens when the fan finally turns off, after 1/2 mile)
-A sweet check engine light that I scanned : Unknown Code - Intermittent. Missfires.

WTF??

The boost can come back for a couple of days and everything is normal, even the CEL was once gone when I had boost, but I always lose it again! And it happens in a second, you really feel like the drop of power as if I the asphalt would suddenly turn into glue!

I need your help, as my local shop don't even know where to start!

Thanks a lot

Simon

[dirtybrd]

Sounds like you have a bad fan control module, it's located on the rad. fan shroud in frt. of the intake manifold. It might be throwing the ECU into limp mode. This just a guess.

[The Hardway]

Well you were right about the limp mode, but I figured out that the temperature sensor was guilty, once changed everything went back to normal!!

And now my headaches are gone! YAY

Thanks!

[dirtybrd]

No problem. Was it the one on the back of the head on for the fan in the lower hose, for future reference.