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We had the timing belt and water pump replaced about 2 weeks ago for routine mtce. The other night the car started running really rough, the engine light came on and when the hood was opened the exhaust manifold was red. Next morning you could see a very small crack in the ex manifold. I was told it was likely the cat clogging so I replaced it and the car ran fine on a 5- 10 mile trip. My son jumped in the car after it had been off for a couple of hours and in about 1/2 mile the same problem happened. I haven't looked at it to see if it cracked again, waiting for first light. Could it be a bad O2 sensor? showing

sortsa77

 
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wont be a bad o2 sensor, could be clogged exhaust from the cat going bad, if the cat went bad.

 
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We just looked at it. No visible signs of a crack. Disconnected the upper O2 sensor and it runs fine, so far.

 
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He said that he replaced his cat and came back with the same problem. I was just saying as a side note his O2 sensors more then likely burned out after words as well. The only thing that I could think of is that the O2 sensor went bad which could have cause a lean condition (I know it's suppose to go in to Limp home mode and run 14.7 a/r ratio) Purphase a lean condition caused higher exhaste temps which turned the manifold red?? These are all just quick thinking so hopefully you figure it out.