One problem related to manifold/header/muffler/tail pipe has been reported for the 2002 Toyota Highlander. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2002 Toyota Highlander based on all problems reported for the 2002 Highlander.
I noticed coolant leaking from underneath the plastic into intake manifold in the rear of the engine. I brought it in for repair and the mechanic said he noticed a large piece of foam rubber between the intake manifold and the engine block and head. The leak was behind the foam piece. The only option to discover the source of this leak was to remove the plastic intake manifold, which the mechanic did. After this was removed, it became obvious that coolant had been leaking a minor amount for quite some time due to build up between the cylinder head and block. The only option left is to remove the head, which requires an extensive disassembly (r and r cylinder head). After loosening the bolts in sequence, he noticed three of the head bolts in the back of the engine were loose. After he removed the head sure enough, one bolt came out with aluminum in the thread thus indicating a stripped bolt. And the other three were loose. He said the only option was to replace the engine block, which will cost about $6000 including shipping/labor. I can not afford this and so had the Highlander towed back to my home.