I would suspect the manifold itself is leaking around the cooling passages. It will need to be removed and the gasket replaced.
Check to see if there is coolant on the engine block. It is common for the Monte Carlos, along with other GM cars equipped with the 3400 V6 engine to experience leaking intake manifold gaskets. A leaking intake manifold will cause oil to leak into the coolant system and even worse, for coolant to leak into the oil system.
Look where the thermostat is attached to the intake manifold
the intake gaskets are prone to leaking. Get a hanes book and good luck Actualy, the intake manifold has 2 different versions. The first version is prone to leaks. The second is much better. Also the replacement throttle-body gaskets(GM# 12580919, $40.00) fit BOTH versions and have been redesigned to help with this. The coolant passageway is located at the very bottom of the throttle-body. Your engine sometimes idles rough because it is also leaking into the intake manifold. I won't go into details here, but it should take you only an hour the first time using common hand tools. I'll warn you that the throttle-body is fragile and you do not need to remove any sensors(doing so will sometimes strip the threads or break the screws off!).
I just had my intake gasket changed on my 99 Monte Carlo for the same thing. My cousin is a mechanic and has changed the same gasket on several cars with the 3.1 engine. The gasket is about $60 - $75 plus labor.
In the intake manifold near the thermostat.
i suggest you get a repair book.it is not easy
The temperature sensor is located on the driver's side under the radiator hose, on top of the intake manifold.
The Manifold Absolute Pressure (MAP) sensor is located on the firewall side of the upper intake manifold plenum
the coolant sensor is in the radiator on the passanger side the oxygen sensor is in the exhaust system on the manifold and the pipes and sometimes both
im having the same problem with my 99. i know the V6 engines have problems with the intake manifold gaskets. only problem is my leak is coming from somewhere by the reservoir as well. check the gaskets and see if it is leaking around them. if not...hop on the troubleshoot train that I've gotten onto. good luck bro, they are great cars. i love mine.
can the gasket go bad behind the water pump houseing
You need to replace your intake manifold gasket. Chevy had a bad problem with these gaskets.