It is located in the throttle housing
What does a auxilary thremosat do in a 1994 BMW 325is. on 1994 BMW 325i
I don't see an auxiliary thermostat or axillary water pump in Alldata's information and they are the best out there. Auxiliary thermostats on other cars can be used to regulate cold idle speed. Auxiliary water pumps are often used for cars with automatic climate control. Bentley workshop manual may be the best source of information for you.
Throttle body thermostat valve is underneath air filter
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Its underneath the air filter n its hooked to to small hoses controls temp called the thermostat throttle body valve