Don’t stall on fixing sticky idle air control valve

Dear Car Talk:

I inherited my elderly dad’s 1998 Lexus ES300 a few years ago. It has only about 65,000 miles on it and I’ve never had any major engine problems. But starting about a year ago, it occasionally stalls right after I start it, before I put it in gear. When this happens, it restarts easily and runs smoothly as long as I give it a little gas, but as soon as I take my foot off the accelerator, the tachometer drops to zero and it stalls. I can drive it, if I keep the gas pedal slightly depressed all the time, even when I’m braking or stopping. After a short time, the problem corrects itself, and it might not happen again for months. I am assuming that something like a governor is sticking. Do you know what it is? — Bruce

It is more likely a dirty idle air control valve than a governor, Bruce.

In the prehistoric days, when car engines had something called “carburetors,” they also had devices called “chokes.” Chokes were mechanical flaps that choked off the air supply when the engine was cold, so that the fuel-air mixture contained extra fuel. That kept the engine from stalling until it warmed up.

Now, everything is fuel-injected, computerized and controlled by electronic sensors. And the choke has been replaced by something called an “idle air control valve.”

When the engine is cold, the IAC sends air past the air-flow sensor, fooling the computer into thinking your foot is on the gas. So, the computer sends in more fuel. But if the IAC is dirty, sticking or broken, the cold engine won’t get the extra fuel it needs and the car will stall ... until it warms up. Once it’s warmed up, the IAC is no longer needed, so the car runs fine.

It won’t happen when you’ve been driving because the IAC is relevant only when the engine is cold and idling. Once the engine is warmed up, it won’t happen even if you do stop and idle again.

Ask your mechanic to use a scan tool to check if an electronic code was stored, which could confirm that the IAC is at fault. But even if there is no code, have the mechanic start by cleaning the IAC. That may cost you $150 or so. If that doesn’t fix it, a new IAC could be a few hundred bucks more.

You shouldn’t wait another year to fix it. If you pull out from the curb and the car stalls when you’re halfway into the street in front of an oncoming trash hauler, the consequences could be serious.

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(c) 2018 by Ray Magliozzi and Doug Berman

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