For the V8 126s:

High idle can be caused by a dirty idle control valve. This can often be fixed with a long soak in brake cleaner. If that does not work it should be replaced.

Rough idle is often caused by vacuum or air leaks. Check for broken or disconnected vacuum lines in the engine bay. The injector seals should also be checked.

Know when the timing chain and guides were last changed. If you don’t know and the car has over 100,000 miles, replace the chain and guides.

300 SE/L:

Stalling is often due to a failed fuel pump relay (MAS controller in 90 – 91 cars).

Hard starting cold and hot with gas smell under the hood can be a bad EHA.

M103s must have a good cap and rotor to run correctly. Check for cracking or burned points. The distributor oil seal can also leak and foul the components.

Keep a quart of good oil in the trunk and check the oil at every fill up. It is normal for the engine to use oil, especially at highway speeds. Also at highway speeds it is normal for the engine to run near 4000 rpm.