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Most of the engine on my 2007 V6 Tacoma is in a really good shape but both exhaust manifolds coming out of the engine block are really rusty.

Q1: Is that a reason to worry or replace them, provided that they don't actually malfunction?

Q3: Is it an indicator/manifestation of a more serious engine problem?

Q2: Do exhaust manifolds have a propensity to rust faster than other parts and why?

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    Very good question, this happens especially when you live near a costal area.. :-)
    – Shobin P
    Commented Jun 10, 2015 at 5:41

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Q1: Is that a reason to worry or replace them, provided that they don't actually malfunction?

The only reason you'd want to replace stock manifolds is if they are cracked (or broken) or if you want to do a performance upgrade.

Q2: Do exhaust manifolds have a propensity to rust faster than other parts and why?

Yes. The reason is because of the heat. The extreme heat of the exhaust causes any type of protection which would normally be on them to burn off, which allows the cast iron to show more rust.

Q3: Is it an indicator/manifestation of a more serious engine problem?

No. It's just a fact of life. Cast iron rusts. Cast iron under extreme heat rusts faster. In most cases (there are exceptions), the cast iron exhaust header will probably outlast the usefulness of the car.

EDIT: I was going to mention, there are products on the market which can withstand the heat of the exhaust. Some of these will bond to the cast iron and inhibit the rust. This POR 15 product is a great example.

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  • thanks man, your answers always exceed expectations. also, why are they made of cast iron vs. steel like the engine block?
    – amphibient
    Commented Jun 9, 2015 at 22:14
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    @amphibient - Engine blocks are made of cast iron or aluminum, not steel. And thank you for the kind words :-) I appreciate you asking questions I can answer, lol! Commented Jun 9, 2015 at 22:14
  • so how come the engine block doesn't rust as much as the exhaust manifold ?
    – amphibient
    Commented Jun 9, 2015 at 22:19
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    @amphibient - The engine block heats to about 240° when up to temperature. The exhaust manifolds heat to around 1100-1200°. There is coolant in the heads which keeps the heat down in them, but the exhaust manifolds don't have that luxury and frankly don't need it. Cast iron exhaust has been around for many, many years and has been doing the job wonderfully. Commented Jun 9, 2015 at 22:26

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