Check the harmonic damper pulley (mounted on the crankshaft). It has three parts: the inner section keyed to the shaft, the outer section on which the belt rides, and a rubber ring in between. This is supposed to dampen out minor imbalances, but if the rubber breaks up with age and wear, it can cause vibration. Another possibility is a cracked flywheel.
Check your vacuum hoses for leaks. A vacuum leak can cause all sorts of problems with the engine controls. A bad EGR valve can also cause rough running. (These may turn on the "Check Engine" light.)
A quick check to see if you are firing on all cylinders is to examine the exhaust. It should have a steady, uninterrupted rhythm and no gasoline, burned oil or "maple syrup" odors (just take a few quick sniffs - this is not good stuff to breath in!). Hold a narrow strip of tissue paper to the end of the exhaust pipe. If it gets pulled in once every eight beats, you have a bad exhaust valve (sucks exhaust in on the intake stroke, and leaks during the ignition stroke).
Exhaust odor diagnostic:
In most of these situations, a compression test may help to isolate the bad cylinder. You may also find it helpful to connect an OBD readout device to the engine diagnostic connector; it may show one or two cylinders operating with significantly different parameters than the others. (Hint: some auto parts stores will do this for little or nothing in hopes of selling you the parts to fix it.)
Check the erg valve, and for a vacuum leak
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