1999 Ford Expedition engine surges when stopped

Dear Tom, Ray and listeners:

I really enjoy your show and my wife and I listen to it every week.

We have a 1999 Ford Expedition with about 75,000 miles on it. For the most part, the car has been very reliable, in fact more reliable than my fancy German car of the same vintage. However, a problem has just recently developed. When I am stopped at a stop sign, the engine surges, e.g., it will rev up to about 800 RPM for a few seconds, then it will drop down to 400, and then the cycle repeats. I noticed that this only occurs when the A/C is turned on. Also, I opened the hood and I noticed that there is a belt to a gizmo on the front lower right of the engine, and that this belt drives the gizmo in sync with the surging of the engine. (Perhaps this the A/C compressor?) One final item - I noticed that when the gear shift is placed in park, the surging does not occur. (It has an automatic transmission.) Any idea what might be happening? How much will it cost to fix this? Is this damaging to the engine? Is it dangerous?

My wife suggested that I write to you for advice. Of course I scoffed at the very thought, as it goes against my nature to ask for help about anything automotive related. However, I have to admit that I am baffled.

Please don’t tell her that I wrote to you, so that I can take credit for the phenomenal advice which I am sure will be forthcoming.

Thank you in advance.


It’s probably your Idle Air Bypass valve failing. You can clean it and it will go another six months or so. It’s an $80 or so part and well worth it to change it. It is easy to change, two bolts, one electrical connector, and one air hose. Ford calls these Idle Air Bypass Valves, but most others call them IACs (Idle Air Control).

is your check engine light on or blinking ?

The check engine light is not on and it is not blinking.

That fits with it being the Idle Air Bypass Valve. When they fail, it usually doesn’t set a code (on Ford V8s).

It is most likely an idle position sensor. Yep, the word sensor means that you have to take the car to a reputable shop and be at their mercy. However, trust my experience. It has to be a sensor. Not much else goes wrong on Ford products. 250,000 miles on my Explorer and it shows no signs of needing to be overhauled.

Dear Car Talk Listeners:

Thank you very much for your advice. I changed the Idle Air Control valve, and the car seems to be running fine now. The part cost about $50, and it took about 15 minutes to change it.


There is no such animal on this car. There is an Idle air bypass valve. It’s solenoid controlled and has no position sensor. There is also a TPS (Throttle Position Sensor), but it is not an “idle position sensor.” The OP’s symptoms are consistent with a sticking idle air bypass valve. In fact, that’s an almost famous problem with Ford V8s from the late 90’s to early 2000s. It’s not an issue with the newer ones because they are all throttle by wire and therefore don’t have an idle air bypass valve.

Glad to help and thanks for the update.