How to Diagnose a Cracked Exhaust Manifold

Place the vehicle in "park" for an automatic and "neutral" for a manual transmission. Firmly set the emergency brake. You can perform an exhaust manifold check while the vehicle sits in a garage, but leave the garage door open for optimum ventilation. Start the engine and raise the hood.

Lean over the fender panel nearest to the exhaust manifold if you have a small 4- or 6-cylinder in-line model engine. For a V-6 or V-8, choose one side of the vehicle. Listen carefully for any noise that resembles a click, popping or plapping sound that comes with a regular firing cycle of the engine. A crack in one of the lead pipes attached to an exhaust port will make this sound with regularity -- every time that cylinder fires. A crack further down inside the manifold collecting chamber will have a more muffled exhaust leak noise. Check both sides in the case of a V-6 or V-8.

Lean over different points of the exhaust manifold, from the front of the engine to the rear. Smell for any odor that resembles raw gas, or a sickly sweet odor, which is a sign of raw carbon emissions. A cracked exhaust manifold will emit a very strong smell of unburned gas, since it has not passed through the catalytic converter and muffler. Look for telltale signs of black smoke coming from the manifold -- evidence of rich, unburned fuel. Have an assistant rev the engine a few times and look for black or dark gray gaseous plumes.

Don a stethoscope and place the probe over the valve covers of the engine. Listen intently for any mechanical clicking or clacking as you move the probe over the entire length of the valve cover. For V-6 and V-8 engines, check both valve covers. If you hear such a noise originating from a certain spot on the valve cover, you can safely rule out an exhaust leak. If you hear no such noises, run the probe over different parts of the exhaust manifold. A leak in the exhaust manifold will set up tiny vibrations, and you will detect it in the stethoscope.

Use a floor jack to lift the vehicle high enough to place two jack stands under the rear frame and two jack stands under the front frame. Hook up a portable smoke machine to your exhaust pipe, according to the directions in the kit. Place the pipe cone adaptor over the exhaust outlet hole and turn the smoke machine on. Let it pressurize the entire exhaust system. For a dual exhaust system, use the kit adaptor to plug the other exhaust pipe. The engine should not be running for this test.

Look for the white smoke from the smoke machine, which will leak from any hole or crack in the exhaust system. Start at the tailpipe and work toward the front of the engine. You will discover an exhaust manifold leak immediately by the white color of smoke exiting the manifold crack. If the smoke machine kit comes with a halogen or UV light, use the light to better see the smoke emissions. Use an angled inspection mirror to look under the manifold for hidden leaks.