It could be an issue with air/fuel ratio (running lean), or the replacement manifolds could be inferior to the Toyota manifold.
I would ask Toyota if there was an issue with this particular year regarding cracked manifolds.
I would have the PCM scanned to see if there is an indication of a lean engine (PCM trying to add fuel).
In 1991 Toyota had a problem with cracked manifolds caused by the heat shield (the factory shield had no vents). They replaced the shield with one that had several vents (slits in the steel) that helped cool the manifold and it solved the problem.
Monday, November 26th, 2007 AT 6:09 AM