On High heat days(above 90degrees)my 98 Volvo S70 often hesitates and occasionally stalls. Any suggestions for cause and solution??
Your car has a duct that pulls heated air from a shroud around the exhaust manifold to the air filter box. There is a flapper valve in the air filter box that opens when it is cold to let in warmed air so the car atomizes fuel better and warms up quicker when it is cold outside. The thermostat for the flapper fails in the open position, letting in heated air all the time.
If this thermostat has failed, it will cut into your mileage and power all the time, but it may be that it only gets bad enough to cause the car to hesitate when it is hot outside. It is also really hard on your mass airflow sensor to process this very hot air all the time. A new thermostat for your car costs $25 on-line. I will be scolded for saying this - but after replacing several of these thermostats on my Volvos, I just disabled them. The smog check inspector will never notice. In my defense, I live where it is warm outside year-round, so pre-heated air has minimal benefit for my cars.
Interestingly…I had the trouble described above in June when we had a week of 90+ but with the latest two weeks of extreme heating including a few days of 100+, I have had no trouble???