I replaced the exhaust manifold with a good used part on my 2.7 liter tacoma due to a slight crack in the manifold between the 2/3 cyl. The crack became much less of an issue when the motor got hot but was still noticeable. The used part was a deal since I got it on ebay for $45 bucks. That manifold even fits the T100 up to '99. Since replacing the manifold the engine has been running much smoother and getting better mileage. I find it hard to imagine it would make such a difference , it must have something to do with a variance in back pressure? Also I never replaced the heat sheild that covers the manifold and don't think I will, I've been running it for 8 months now and just went through a very hot summer. I really don't think the heat sheild is Necessary and may even lead to excessive heat and cracking. Does any care to elaborate on this?

The cracked 2.7 exhaust is a common thing. I went to header after the second time. With a cracked manifold you are losing back pressure and your 02 sensors are not getting proper air/fuel readings thus making changes to fuel and timing. You can run without a heat shield.

OBAT, Thanks for the information. I'm so happy with my truck these days. I'm sure there are many similar trucks out there with high miles running around with cracked manifolds, its been the one week point with toyota since my Toyota Celica and the 20R engine in 1978.