Question: Does it shake on cold start and at hot idle, or only when hot? Does the motor have good power when you drive the car? Possibilities (no insults intended): A bad spark plug. Or a bad ignition wire; check each with an ohm meter. Two spark plug wires are switched, yes it could happen. A bad fuel injector or incorrect fuel system pressure. A vacuum leak as you mentioned, probably at the manifold to cylinder head joint. That can be found by CAREFULLY applying propane to all suspect sites (Standing well back so if it flares you don't get burned). The propane will get pulled into the motor and make it run well. Has the valve timing been checked? One bank may be off by a tooth. If none of these ideas resonate a compression test may be the next task to diagnose the problem. -- © 2008 T.G.Lambach. Publication in any form requires prior written permission.
