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High Idle speed - '03 Corolla

When I start my '03 Corolla its idle speed is around 1200 and later after driving for a few miles it settles down to around 700. Other than that it starts well and runs smoothly. I tried to back off the idle adjustment screw and found out that the screw was barely touching the stop. In older models I used to be able to fix or repair the cold idle in engines with a carburetor, but am totally clueless regarding fuel injectors. This condition also happens when I start the car during the middle of Texas summer (100+ deg).

The car has 123,000 miles. I have not ever changed the plugs, but as i mentioned the engine starts and runs smoothly. As expected, the idle speed responds well whenever the air conditioner is turned on. The engine temperature comes to the mid gauge as always after driving a mile or so. No engine check lights come on. The car always passes the annual smog inspection.

Any guidance here will be greatly appreciated.

Your 03 corolla does not have an “idle adjustment screw”. It has a throttle stop screw and you should not be screwing around with it. Its function is to protect the throttle body from being damaged by a rapidly closing throttle plate.

The idle speed is controlled by the computer. It sends signals to a stepper motor called an IAC (idle air control). But to send these signals, it has to first know that your foot is off the gas pedal. It uses the TPS (throttle position sensor) to determine this. If you screw the throttle stop screw in too far, that will get interpreted by the computer as you still having your foot on the gas and it won’t attempt to control the idle speed. If you unscrew it, the throttle body can/will get damaged.

1200 rpm is a normal cold idle speed. Nothing is wrong. Your engine uses Iridium tipped spark plugs with a recommended change interval of 120k miles so your due for a change. I recommend the NGK iridium plugs, they are already plated with an anti-seize so you don’t need to add an anti-seize compound to the threads.

Try replacing the thermostat so it will warm up faster.

As mentioned above, the idle speed is supposed to be higher at first, when the engine coolant temp is low. It may be higher now than it used to be, I’m assuming that is what you mean. And the idle speed usually doesn’t pop up much when the outdoor temp is really hot, like you mention, the 100+ Texas summers. So there may indeed be something askew. The ECM decides how much gas to give the engine and how to adjust the IAC based on the air temp and the engine coolant temp. So the the engine coolant sensor or the air temp sensor could be on the fritz. That’d be my first guess

I concur with @keith, it is unwise to monkey around with that adjustment screw as a means to address this problem. If that screw gets set beyond its normal range, it confuses the ECM, prevents the ECM from setting the correct fuel/air mixture, and the result can be various surging symptoms which can be very difficult and expensive to diagnose and fix. I’d recommend the first thing to do at this point is return this screw to its nominal setting. Hopefully you’ve marked it with witness marks before moving it. If not, consult the shop manual for how to reset it to the nominal setting. Then have your mechanic test the air and coolant sensors.

BTW, having all the routine service suggested by the owner’s manual would be a common sense thing to schedule. At 120K, you are risking unscheduled and possibly expensive repairs.

Thanks Keith, circuit & George.
I realize that the idle rpm should be higher when the engine is cold. My comment for high idle speed was based on it being higher now at start up then it has been these past years. Sorry - I mis-typed the idle rpm and wrote that it is 1200 when cold. I meant to say that it is 2000 instead of 1200. As for the adjusting the throttle stop screw. When I noticed that the screw was barely touching the throttle body and screwing it out wouldn’t do anything I put it back to its original setting.

I will definitely change the plugs and see if it makes any difference, otherwise I suppose the cause can be attributed to the air and/or coolant temp sensors

Thanks for your feed back.

A cold idle speed of 2000 is a bit high, but not unheard of. I don’t think the plugs have anything to do with it, but they are due for replacement according to your owners manual. I know this because my daughter has an 03 Corolla, but with a few more miles on it than your has.

This higher idle does indicate that something is not quite right, but not bad enough to be an issue yet. For some reason the computer is thinking that it needs this to keep the engine alive or to keep emissions down, probably the latter. Eventually you will get a CEL (check engine light) and repairing that will probably cause the cold idle to drop down. But for now, I would not worry about it. Just keep up the PM (preventative maintenance) and your car will serve you well for many more years.

Post back when you get that CEL and we will help you get the right repairs done for it. You can get the codes pulled for free at most autoparts stores, but be sure to get the actual code, i.e. P0420 and not some explanation like, “the computer says your cat is bad”. The computer never says that and many codes have multiple possibilities, some of which are a lot cheaper than the code interpreted by the person at the parts counter.