Idle Speed Too High

Troubleshooting a Toyota Corolla with a High Idle Speed

Question: Corolla Idle Speed Too High

Good day Vincent, I have a 1993 Toyota Corolla Gli, 1.6 liters, manual transmission with 138,000 kilometers, EFI, Power Steering and A/C. Recently, I noticed that the fuel consumption went up. I figured that it must be because of the high idling at 1,400 RPM, with the A/C turned off. When the A/C clutch engages, the RPM drops to 1,000, but this is still way too high since nominal is 800 +/- 50 rpm.

I have already checked the vacuum hoses for leaks and clogs, the air filter is clean, the throttle cable was not too tight, and I reset the ECU. But idling was still high even with a warmed up engine. Incidentally, I also noticed that the A/C is intermittently not engaging even if it is supposed to. What could be causing this?

My mechanic friend says it may be the MAF sensor. I tried swapping it out with another MAF sensor, with a different part number, though, since I couldn't find an exact match. But still, idling was high. I'd appreciate your help on this.


Answer: What to Check When the Idle Speed Is Too High

The first thing to do is to see if there are any diagnostic trouble codes stored in the PCM. If there are this will give us a good starting point for troubleshooting. Many parts chain store will read your codes for free, all you have to do is ask. When you find these codes, you will be able then to follow them to possible causes, or consult with a mechanic for further interpretation.

Lacking any codes, I would have to start with the Idle Air Control Valve/Bypass Air Control (IACV/BAC). You can try cleaning it and see if that improves your idle speed. A throttle body cleaning is likely to cure the high idle speed as well.

Possible Causes of High Idle Speed

There are many possibilities when your engine is idling too fast.

Here are some common ones to help guide you towards the root of the problem. More: Car Idling Speed Problems

  • In vehicles that have carburetors, a bad accelerator pump or power circuit.
  • The engine is overheating, and the cooling system needs repair.
  • Fuel pressure regulator may be operating at too low of a pressure and would need to be replaced.
  • Ignition timing needs adjusting.
  • Ignition problems stemming from the distributor cap, rotor, ignition wires or spark plugs.
  • The computerized engine control system is faulty.
  • Vacuum leak
  • Bad idle speed control unit
  • Bad alternator

For the do-it-yourself mechanic, this is a problem you may be able to trace to its source with some patient troubleshooting. Some of the solutions are best performed by a service shop.

One tip is to check the engine idle only with the air conditioning and defroster in the OFF setting. You won't get a true idle speed if they are at work. They trigger changes in the idle speed that can mask the real problem.