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I have a 91 s10 blazer with the 4.3.
Occasionally it had white smoke out of the exhaust, but it was mainly noticeable in park or when stopped at a red light.
The coolant levels are consistent and haven't needed to be topped off.
It has a slight misfire during idle in park, but runs rough in reverse and drive.
There also seems to be a consistent clank sound coming from the CAT.

I was told the white smoke can either be from a bad head gasket, or and overfill of trans fluid.
Since the coolant levels are unchanged and the smoke doesn't smell sweet, i'm ruling the bad head gasket out.

Could a bad CAT cause it to run rough?
What do you think it could be?
 

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I would advise that you do a complete compression test , remove all spark plugs , open the throttle all the way open , and put a battery charger on the battery for consistent readings . This will give the most accurate readings on all 6 cylinders . This can greatly help in diagnosis of the problem , and where it's coming from .
After performing this test there's another that will definitively let you know if there's a head gasket out or cracking in the head . You'll have to go to Auto Zone , or Advance Auto , perhaps a different store that rents tools , but it'll check for engine combustion gasses in the radiator ; the tool is inserted into the radiator opening , filled with the fluid that comes with it , then a squeeze bulb is put on the top of the tool & squeezed , with the engine at operating temperature , any combustion gasses are drawn up through the liquid , and it will change color , if it doesn't change color , there are no combustion gasses getting into the antifreeze , so no bad head gasket , no crack in the head 馃槑
 

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I'd go with Franks idea,then you know for sure ...word of warning tho- none of the advance or az's in my area have the combustion gas checker ,only the pressure tester, so maybe call ahead. BTW,are you sure it isnt blue smoke ? Valve seals do go bad on those and can crop dust pretty good, esp if you have clogged return ports.
 

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Well if that's the case , you can use the pressure tester , with the engine at operating temperature , install it in place of the radiator cap putting it to the pressure of the cap , if the pressure rises in 20 min. , then chances are there is a combustion leak into the cooling passages , yet the combustion leak tester is more accurate in my opinion 馃槑
 

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Blue smoke is dark, and not usually as thick as white smoke. You most likely have a bad lower intake manifold gasket. Do a coolant pressure test.
 
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