Ok here??s your problem.
Your car has been feed a steady diet of 93 octane gas which combined with short trips has lead to carbon build up. This explains your bad idle and your knocking noise after your long trip.
What you experiencing is a ??carbon ring? at the top of one (or more) of your cylinders. It was formed after your long drive at high speed. When you shut your engine down the carbon broke loose and got trapped in the top of the cylinder at restart. When the piston hits the ring it makes a sound like a bad rod bearing. At higher RPMs the sound is masked so you don??t here it as much.
How to fix it.
1) Use the correct octane (for a standard compression contour that??s 87).
2) Have the engine de carbonized at a shop with the correct equipment.
Remember this simple rule high octane gas is for high compression engines. You are getting lees performance and spending more money using so-called ??high test?. The ??high test? gas will not burn at a high enough temperature in a standard engine to burn completely, leaving behind the carbon that is causing your problems.
jeff
