In Summary
  • Occasionally, poor engine idling can be caused by a dirty throttle valve housing or clogged idle air control valve.
  • The CTS can fail due to age, circuit failure or poor maintenance of engine coolant which corrodes its surface and makes it less sensitive to coolant temperature.

Hello Paul, I own an Alteeza Toyota 1998 model with beams vvti engine. In the morning my engine starts properly but when I hit the accelerator to reverse or drive the engine goes off. Then I have to start the engine again and when I hit the accelerator again to drive, the engine takes about six minutes to respond to the accelerator. I even press it till it is down and still the car will not really move or make that sound when the accelerator is hit hard. My my mechanic said I had faulty spark plugs which I changed. I also changed the engine coil but the problem still persists. What could be the problem? Allan.

Hello Allan, poor engine performance or stalling during a cold start can be caused by electrical engine management failure due to faulty coolant temperature or intake air temperature sensors. Occasionally breakdown of the throttle potentiometer’s idle air control valve can cause similar symptoms. The spark plugs or fuel pressure (filter and pump) will affect all round engine power regardless of engine temperature.
From your description, the erratic and poor engine performance of your Alteeza only happens when the engine is cold. When it heats up the car runs fine. The engine management computer (ECM) relies on signals from the coolant temperature sensor (CTS) or intake air flow sensor (AFS) to determine engine temperature by monitor engine coolant temperature and intake air temperature.

This information helps the ECM to activate or deactivate cold start engine warming phase which runs the engine at higher revolutions (1,000rpm). Warming up the engine is crucial for stable idling and efficient performance. As the engine temperature builds up, the ECM slowly reduces engine revolutions to normal idling (800rpm). Failure of these two sensors means the ECM is unable to determine the correct engine idling speed hence the erratic idling and engine stalling. The CTS can fail due to age, circuit failure or poor maintenance of engine coolant which corrodes its surface and makes it less sensitive to coolant temperature. The AFS on the other hand will fail due to age, broken circuit or poor maintenance of the air cleaner element.

Occasionally, poor engine idling can be caused by a dirty throttle valve housing or clogged idle air control valve. However, this would continue even when your engine warms up. Check the CTS and AFS. A computer diagnosis can help you pinpoint the cause of this problem quickly.