What would cause it to crack again so quickly. The part was new purchased on eBay.

2007 Hummer H3.

 

You should be asking your "shop" that question.

If my 07 would have had a cracked manifold, I would not have replaced that with another OEM Manifold. I would have went aftermarket to avoid a repeat.

 

The reason is simple. Your engine is overheating. Temperature gauge below average?

 

Oh Boy.................. and ......................... uh...... No.

Exhaust Manifolds crack because of repeated heat cycles (expansion and contraction), not high heat of a "engine overheating". High heat alone does not do it or every exhaust manifold out there would be cracked. There can be other factors, like improper exhaust fasteners, improper fastener torque, improper mounting, poor gaskets, defective casting, poor materials etc...

An overheating engine might get to 300F or even 350F, before complete failure. The exhaust manifold and pipes close to it including Pup Cats regularly heat to 500F+ and under load over time, some get to over 900-1000F +.

 

Exhaust system blockage, improper timing, or unburned fuel entering the exhaust manifold, will cause the exhaust manifold to overheat/crack.

 

in 2009 they began to make a collector with an additional mount. a manifold crack is almost all H3 owners. I removed the overheating and for 5 years now I have been moving on a certified manifold. There is another one with a crack in the garage. many problems with H3 with I5 are precisely because of the high temperature of the engine. very bad engine. and the mounting of the collector is also unsuccessful. everyone has a crack in the same place.

 

@Disel where does the crack develop?

 

 

My '07 manifold is still OEM, installed on the assembly line, back in late 2006, and I still don't have a crack!

 

have you checked to see if the cat is clogging?