Don't Stall on Fixing Sticky Valve

Dear Car Talk

Dear Car Talk | May 03, 2018

Dear Car Talk:

My 1998 Lexus ES300 has only about 65,000 miles on it. I inherited it from my elderly dad a few years ago. It has 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, as long as 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? Is this likely to happen when I am already driving (it never has yet)? Is it going to be difficult to diagnose, since it is very intermittent? And how expensive is it going to be to fix? -- Bruce

I don't think it's a governor, Bruce. It might be the lieutenant governor. More likely, it's a lazy or dirty idle air control valve.

Back 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 And the choke has been replaced by something called an "idle air control valve."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 you have your foot on the gas. So the computer sends in more fuel. But if your 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.

So, to answer your questions, it won't happen when you're "already driving," because the IAC is relevant only when you're idling and the engine is cold. Once the engine is warmed up, it won't happen even if you do stop and idle again.

Will it be difficult to diagnose? Well, since I diagnosed it, Bruce, how hard could it be? I'd start by asking your mechanic to put his scan tool on your car. If a code was stored, that could confirm that the IAC is at fault. But even if there's no code, have your mechanic start by cleaning the IAC. That may cost you $150 or so.

If that doesn't fix it, and you need a brand-new IAC, that could be a few hundred bucks more.

And you shouldn't wait another year to fix it, Bruce. If you pull out from the curb and the car stalls on you when you're halfway out into the street in front of an oncoming Waste Management truck, the consequences could be serious.

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