Engine hesitates and stalls when warm

Question

My car is a Toyota Vitz 2007 and it has 80k KM on it. I usually drive short trips (about 5km) most of the time in a low traffic road. Car drives mostly fine in these short trips. But, if I drive for about 2 hours in a congested road with frequent stops, engine starts to hesitate when idle and even stall sometimes.

Hesitation is more apparent if I apply breaks suddenly and stopped. Also, when I step on the accelerator when engine hesitates, it doesn't respond correctly, no matter how much I press (it slowly accelerates).

If I enter a highway when the problem occurred, I can't feel any difference when I drive at higher speeds. Hesitation only occurs when driving in traffic.

Also, even in short trips, car sometimes idles very roughly (sudden heavy vibrations). But it doesn't hesitate or stall.

Also, in my morning drive to office (short trip), car idles smoothly. But in evening drive to home, idle is much rougher.

What I have done so far:

• Cleaned throttle body
• Replaced spark plugs
• Replaced ignition coils
• Replaced fuel filter

Can someone tell me what is wrong with my car?

Answer 1

The stalling issues point toward problems with an engine control system (air, fuel, or spark), but again short of replacing the fuel pump, you have already replaced the parts most likely to cause those issues. The idea that the car will act normally on short trips is also strange.

The problem that stands out to me is that the car has trouble accelerating, but accelerates normally at highway speeds. This could indicate that there is something wrong one of the solenoids involved in shifting the transmission, or with the torque converter. If the torque converter is not disengaging properly, or the transmission is not properly lubricated, this could also cause your intermittent stall issues. You might not experience the problems on short trips because you never reach high enough speeds for the obstructed solenoid to be activated.

I would first make sure the transmission fluid is at the correct level, and regardless, if the fluid has not been replaced I would do so. Assuming the OBD-II system is not throwing any codes (indicated by the Check Engine Light), the solenoids are probably functioning properly, but the dirty transmission fluid could be limiting their function. If changing the transmission fluid does not solve the problem please add that to your question and see if you get another answer, but you may have to look into replacing the torque converter.