In a effort to make my '88 740 dependable enough for my daughter to use (she lives about 200 miles away), I've replaced a number of components. However, I've been unable to eliminate a vibration while the car is at idle, auto trans in drive, and foot on the brake. The vibration shakes the whole car, but disappears when the transmission is shifted to park or neutral or when you increase the engine speed by slightly depressing the accelerator. When you put the car in gear, the engine speed drops about 200 rpm's (the tach doesn't read accurately so I can't tell exactly what the engine speed is, only that it drops). I've replaced the cap & rotor, spark plugs (gapped at .028), intake manifold gasget, amm to throttle body bellows, vacuum hoses and cleaned the IAC and flame trap, checked the timing (12 degrees BTDC). Plug wires were new within the past 2 years. Any help to diagnose and cure this vibration would be appreciated.
Marlin