A trick I learned to check if you have micro cracks or a leak in the gasket for the exhaust or intake manifold is to spray carb cleaner or starting fluid (ether) in the area around the exhaust or intake manifold. It is highly advisable to do this when the engine is cold and just started. When it gets warm it works too, but you need to be careful not to use too much because you can cause a fire. Carb cleaner and ether are not as flammable as alcohol and will plug the crack for a second or two. This gives you enough time to see if it starts running smooth again.

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White 1998 4 door KIA Sportage 4x4 w/Eibach 2.5 spring lift and TJ rear coils, 4crawler 1.5 body lift, Monroe Sensa.trac shocks, Bosch 4+ plugs, Eaton LSD, 31x10.5x15 BFG KM tires, 2.5 cat back exhaust w/SpinTech muffler