As far as the motor there are a few things you can check. You can do a compression/leakdown test. If you go to any Advance Auto or Auto Zone they will loan you the tool to test the compression. The 3.3L has a minimum compression pressure of 100PSI and the 3.5L is the same. The maximum pressure difference between the highest and lowest pressure cylinders should be 25%. It should take a little while, at least 10 minutes or so for the compression to leak off. The longer it holds pressure the better. The main thing is that the pressure does not leak off immediately.
You can also look at the engine oil in the sunlight or a bright light and look for specs of metal. If you see little silver pieces of metal it is going to need a rebuild in a while. If you see Brass or Copper colored pieces in the oiol it needs a rebuild now. The metal will settle out in the bottom of the oil.
The same goes for the transmission except you will just be looking for silver specs of metal. The larger the piece of metal the worse the shape of the tranny. Also the fluid should not look dark or smell burnt. That means it needs a rebuild.
If you can, you can seal off the coolant ports on the engine and get a coolant system pressure tester to check the system. However doing a compression test is at least a way to tell if the head gasket is leaking in or out of the combusiton chamber which would affect performance if anti-freeze is getting into the combustion area.

Answered by DrCranknWrench (expert)
3,292 answers provided