It's common to lose radiator heat transfer capacity with age and mileage. The degradation will be more rapid if "green" antifreeze is used as the silicates and other inorganic salts will tend to build up deposits in the aluminum radiator tubes and form a thermal barrier.
You may be close to having to replace the radiator. I doubt if the average 15 year old car that has received indifferent cooling system maintenance would make it through an entire Phoenix summer. A fifteen year old radiator probably has in the range of 10 to 50 percent loss of original heat transfer capacity depending on the quality of coolant and frequency of coolant changes.
The best coolants to use are MB brand, Zerex G-05 or Havoline Extended Life (Dexcool). Prestone also offers a licensed Dexcool.
The first problem may be vapor lock, even if the accumulator and pressure regulator are still in spec 110 ambient temps could cause some vapor lock as underhood temps could reach over 180 after five to ten minutes of hot soak in the sun.
Flooring the throttle might be the best response to lack of hot starting as this will call for maximum fuel flow and purge the vapor more quickly.
Last edited by Duke2.6; 05-29-2003 at 09:31 PM.