Terry's Talon Troubleshooting Tips #3 - Fast Idle (aka. Idle Surge)

For those of you who may have missed the thread regarding my idle surge, here's a brief summary:
- One hot Saturday in June, I was driving happily along the freeway. Upon exiting the freeway and reaching the stoplight at the end of the ramp, I pressed the clutch pedal and much to my surprise, the car stalled. It restarted right away, but then stalled three more times before reaching my destination.
- Within a day, the stalling problem had been replaced by an idle surge that seemed to only occur when the car was hot. At no time did the computer ever report an error code.
- On Monday, I asked for advice from the list and received various suggestions, most of which related to the throttle body. At this point, however, I was reluctant to perform any service on my own when I might be able to get the dealer to do it under warranty.
- On Tuesday, I took the car to the dealer and asked them to figure out what was wrong. They proceeded to misdiagnose the car as in need of a tune-up including: plug wires, fuel filter, and some simple cleaning for *only* $500. Needless to say, I retrieved the car without having any work done and spent the next 3 weeks living with a very annoying idle surge and trying everything I could think of to identify the problem on my own.

That's as much as I've posted so far.

Q: So what's new?
A: Over time, the problem evolved from: "Only when hot" to "Intermittent" to "All the time". As any troubleshooter can tell you, many problems are impossible to identify until they reach stage three.

Q: So what was the problem anyway?
A: One of the coils in my Idle Speed Control (ISC) had become intermittent and eventually opened up. Since the ISC is a stepper motor, a bad coil means that the motor can't do much more than twitch. As a result, the ECU did not have positive control over the idle speed. Occasionally, the stepper did make progress in one direction or the other, but not on a regular or controlled basis. This explains why my idle ranged from stall to surge. From one stoplight to the next, the idle might change, but while stopped at a light, it would be consistant.

Q: So what is an ISC and what is it good for?
A: The ECU attempts to control the idle speed by opening or closing the ISC. The valve on the ISC allows extra air to bypass the throttle plate. Since a stepper motor does not provide any feedback on its actual position, the computer simply counts steps and assumes that the valve lands in the correct place.

During normal operations, the ECU uses the ISC for some very interesting things besides simply adjusting the idle speed. The ISC is completely shut when you kill the engine to make sure it stops quickly. The ISC is opened completely at startup to provide plenty of air. It is also opened during normal driving to prevent the car from bucking when you take your foot off the throttle to shift. If you run the air conditioner or turn the steering wheel, the ECU compensates immediately by opening the ISC a little to keep the revs up. This also happens during cruising to mask the effect of the increased load on the speed of the car.

Q: So what, exactly, is idle surge?
A: Idle surge is the condition that occurs when something in the ECU idle speed feedback loop fails to operate as expected. Since the ECU is not quite as smart as you are and has a very microscopic view of its world, its behavior may seem very strange to you, but seem perfectly normal to the computer. The result is an idle speed that oscillates from ~1200-1500 RPM about 2 times a second and no error codes are ever displayed.

My idle surge was a result of the following situation:

  1. The idle switch is depressed (foot off the gas)
  2. Too much air is getting through the throttle body (ISC stuck open)
  3. O2 sensor registers lean mix (too much air)
  4. ECU adds fuel to achieve correct mix
  5. RPMs increase due to increased fuel/air supply
  6. At 1500 RPM the ECU goes into decel mode and cuts fuel (*)
  7. With only air coming into the engine, the RPMs drop rapidly
  8. At 1200 RPM the ECU turns the fuel back on
  9. Goto 2.
*) In our cars, the ECU cuts the fuel supply completely when the car is decelerating. It determines that you are trying to decelerate because the idle switch is depressed and the RPMs are above 1200. This is done to prevent an over lean condition and to assist in engine braking.

Q: Are there any other annoying side effects of this problem?
A: The only other major side effect is that with the throttle plate all the way closed, the engine still wants to run at 2000 RPM. The result of this is that it is very difficult to drive around parking lots. Parking lot speeds in 1st gear put you just under 2000 RPM. At this point you have the throttle barely cracked open. It's about all you can do to keep the throttle almost closed, but not depress the idle switch. If the switch gets hit, the fuel gets cut (see (*) above). You can't let the car just idle, because the surge is severly aggravated if the car is in gear. The gentle 2 cycles per second surge will change to a 5-6 cycles per second violent bucking of the whole car. Rather unpleasant.

Q: That's nice to know, but what can I do about the idle surge on my car?
A: Glad you asked. In the time it took for me to identify my problem, I was forced to examine the fuel control and air supply loop of the engine very thoroughly. As a result I have developed the following diagnosis guide. Since I have obviously not had the opportunity to test all of these conditions, there is a fair amount of speculation involved and I probably missed some important stuff, but it should at least get you on the right track.

It helps to have a shop manual available while following this guide. Turn to chapter 14, Fuel System, find the pages for your engine, and use them to identify the parts I refer to.

Throughout this entire procedure, make sure that the A/C and all other accessories are turned off. Extra engine loads may confuse the diagnosis.

  1. My car has an idle surge
  2. My exhaust smells fine now, but why are the RPMs so high?
  3. My TB shines like new, but I still have a surge
  4. Ok, no vacuum leaks, but it's still surging
  5. Ok, the ISC coils look good, what's left?
  6. Now I'm sure the ISC is good, aren't we running out of options?
  7. Ok, only one thing left, the FIAV
  8. Ok, so now what?

Good luck and have fun,
Terry Wells