To assist them in diagnosis, and not dilute with extraneous info?
Specifically driveability problems. For ex: a concise description of the problem, when it occurs, etc. Such things as if prob is intermittent or steady, occurs when engine is cold, hot, or both. If prob is worse during cold or hot ambient temps, or damp weather.
If it was related to any previous repairs or changes, when it started to occur.
What else should be added to this list?