My 2000 VR6 Jetta has an annoying problem of hard starting in warm weather. Last summer through fall when the weather was warmer, it would often crank but not catch. All winter it is fine. A jump will usually gets it after a while. Replaced the battery with limited success. I park in the shade whenever possible. It has the manf. alarm system, and, also, but not always, it is accompanied by key not turning off the alarm. Do not have remote. Replaced the Fuel Pump Relay. No success. Cannot predict, so I don’t want to have it sitting at the dealer for days on end. Whew. Any suggestions?
Don’t know. Ideas (brainstorming):
Get a remote. You want to remove the alarm system from consideration of problem. The alarm specifically will not allow the vehicle to start if an intruder. So, you want to make sure that is not armed. Plus, cars get HOT inside in warm weather. Electronic control units can be affected by heat. The alarm control unit may be acting up (intermittent) from heat; and not allowing the vehicle to start.
Or, fuel expands a tremendous amount. So, in warm weather - the fuel in your tank also expands. If your Fuel Evap Recovery system - is having some difficulties - this could cause difficulties in starting as well.
If you know about electronics and cars; a good Factory manual might be very helpful to you. (http://www.books4cars.com) Otherwise; a high quality shop. Independent or Dealer. Does not sound like an easy diagnosis. Maybe someone else here - has more info and better thoughts.
Alarm is your clue. Can it be disabled? I would start there.