I have heard many different opinions of these engines. What are y'alls opinions of these engines?
You're talking about a lot of different engines that underwent a lot of different generations and changes.
The 3100/3400 were one design that ended in 2005, and the 3500/3900 are different bore size and have VVT.
I had a Gen II 3.1L 'LH0' engine and it lasted a very long time.
Why do you ask?
There are several GM vehicles that have these engines that I am interested in. The Impala, Uplander, Equinox, Grand Am.
You probably want to skip the Grand Am. The newest one is 15 yrs old.
Yeah
the 3400 and 3100 are prone to lower intake manifold gasket failures.
The Grand Am is trash. Just check the warning sign of many automatic car washes to see how they highlight their refusal to cover "Grand Am mirrors" should any damage occur. And no, I'm not making that up. This is a thing. If GM couldn't build it well enough for the mirrors to stay put in a car wash, don't expect a high-quality build anywhere else on/in the car.