I think the problem is the HGF. I opened the reservoir now the night and saw a small smoke with exhaust smell.
B34R said about the sniff test, but I got it wrong because of translation into Portuguese.
I will buy a transparent hose and will change the output of the intake manifold. So I´ll see which cylinder head is leaking compression gas. If confirmed, this weekend I´ll remove both cylinder heads to change de head gaskets.
Just to warn you. Doing the KV6 head gaskets with the engine in the car is a really difficult job. The front head isn't to bad but the rear one is a swine. These limited access to the exhaust manifold nuts. The head is awkward to refit as you can't see where it locates. Last KV6 heads I did, I removed the engine to make life easier.
Good luck