I have a 2004 Volvo S40. For the past few summers, when it gets hot out, especially if the car is sitting in direct sun, when I get back into the car to start it up it stutters and often won’t move when I press the gas pedal. I have taken it to several mechanics and they never fix it. When it’s really hot, sometimes it will stutter when I am stopped at a light and threaten to shut off. So, any ideas of what is going on?!?
In days of old cars did this happened frequently as gas vaporized in fuel lines. Fuel injection pretty much eliminated these problems so modern cars rarely see it. Heat can also affect car electronics negatively.
A fuel problem would be a weak fuel pump. Low pressure in the fuel lines increases the vaporization possibilities in the fuel lines or fuel rails leading to the fuel injectors. If the fuel is vaporized before it gets to the injector(s) the correct amount of fuel isn’t getting the cylinder(s) affected. Rough running would result. Fuel line pressure is relatively easy to check. A new fuel pump for my '98 Volvo was $900 for the part and Volvo was the only source. Check the fuel pressure and go from there.
The heat effect on an electrical component is even harder to pin down. A different ECM (the cars main computer) is worth a try if you can figure out how to do it without a major expense as a “test”. An new ECM for a Volvo is pricey, like maybe $1000.
If this only happens when it is very hot, you might just try living with it. A fix from the old days was to raise the hood and allow hot air to escape. In a few minutes the cooler air allowed the motor to cool enough to run properly. I used to carry a gallon of water to pour on the fuel pump and fuel lines of an old Mercury to get it out of the intersection when this happened. Pouring water on a modern motor today is too risky. Water and electronic stuff don’t do well together, so don’t do the pour water trick. When you turn off a motor it actually gets hotter over the next few minutes since coolant isn’t circulating anymore. Therefore raising the hood at an interstate “pit stop” could make a lot of sense and smooth your journey.
As UncleTurbo suggests it does seem that a vaporizing fuel problem is a good possibility. This would likely be because the fuel pump isn’t maintaining pressure.
Try this - the next time the conditions are right for this to happen, rather than just starting the car right up, cycle the key from off to the run position about 10 times, pausing in run for a few seconds each time. (The run position is the one just before it cranks when all of the dash lights come on). This would keep cycling the fuel pump on and off. First it will build the pressure. Then it should be cycling any vapor back into the tank through the return line. After 10 times or so you should be left with fully pressurized liquid fuel.
If that doesn’t help at all then its probably something else - like overheated ignition components or something.