NA S4 > Bouncy idle when cold/warming up?
10-02-09, 12:06 PM
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NA S4 > Bouncy idle when cold/warming up?
I have a NA S4, with only 70k miles on it.
No mods, just a cone air filter.
But I have this issue -- when I start the engine cold, revs go up as usual and then start to come down, but when they reach about 1100~1300 RPM, the idle bounces around those numbers over and over again, until the engine is finally at operating temps, and after that the idle is rock-solid.
Here's a video:
http://www.youtube.com/watch?v=eDm6ozAeZU4
It's not much of a problem, but I would like to know if there is a way to fix this
Thank you for your time
Your feedback will be greatly appreciated!
-D
No mods, just a cone air filter.
But I have this issue -- when I start the engine cold, revs go up as usual and then start to come down, but when they reach about 1100~1300 RPM, the idle bounces around those numbers over and over again, until the engine is finally at operating temps, and after that the idle is rock-solid.
Here's a video:
http://www.youtube.com/watch?v=eDm6ozAeZU4
It's not much of a problem, but I would like to know if there is a way to fix this
Thank you for your time
Your feedback will be greatly appreciated!
-D
10-02-09, 12:57 PM
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I have 2 different TPS, and both give me the same boundy idle problem.
10-04-09, 06:24 PM
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On a cold start, the idle should go to 3000 rpm for 17 sec. The ECU commands the air bypass solenoid open to maintain this. On the S5s it was called the AWS, for accelerated warmup system.
Once the 17 sec is over, the fast idle cam keeps the idle up at 1500, normally ramping down to normal idle as the thermowax senses coolant coming up to normal op temp.
It sounds like your problem is in the fast idle cam zone, after the 3000 rpm 17 sec range and before the normal idle. This suggests that it is related to the operation of the fast idle system. You might be able to verify this by visually watching the throttle system while the oscillation occurs; if it is the fast idle system, it works by actually changing the throttle angle, and if you watch carefully, you should be able to see that. The problem can be as simple as a linkage out of adjustment. Any non-linearity in a control feedback loop can cause an oscillating response.
Its not clear in the manual, but it appears to me that the BAC is also involved in the fast idle zone. If you do not see any motion of the throttle or fast idle cam in concert with the oscillation, I would then be suspicious of the BAC. You can check that really quickly by putting a jumper in the initial set connector before your cold start. It will disable the feedback control of the BAC and holds it at a constant duty cycle command. If that cures the oscillation, then the problem is likely related to the BAC.
I would go to the FSM and look up the procedure for checking the fast idle system, and apply that and see if anything improves.
Hailers also has a post on here somewhere on here about how to disable the fast idle cam by wiring it into a non-operational position. Search in this section for posts by him and "thermowax" or "fast idle."
If the erratic idle goes away when the fast idle is disabled, you have zeroed in on the problem.
Once the 17 sec is over, the fast idle cam keeps the idle up at 1500, normally ramping down to normal idle as the thermowax senses coolant coming up to normal op temp.
It sounds like your problem is in the fast idle cam zone, after the 3000 rpm 17 sec range and before the normal idle. This suggests that it is related to the operation of the fast idle system. You might be able to verify this by visually watching the throttle system while the oscillation occurs; if it is the fast idle system, it works by actually changing the throttle angle, and if you watch carefully, you should be able to see that. The problem can be as simple as a linkage out of adjustment. Any non-linearity in a control feedback loop can cause an oscillating response.
Its not clear in the manual, but it appears to me that the BAC is also involved in the fast idle zone. If you do not see any motion of the throttle or fast idle cam in concert with the oscillation, I would then be suspicious of the BAC. You can check that really quickly by putting a jumper in the initial set connector before your cold start. It will disable the feedback control of the BAC and holds it at a constant duty cycle command. If that cures the oscillation, then the problem is likely related to the BAC.
I would go to the FSM and look up the procedure for checking the fast idle system, and apply that and see if anything improves.
Hailers also has a post on here somewhere on here about how to disable the fast idle cam by wiring it into a non-operational position. Search in this section for posts by him and "thermowax" or "fast idle."
If the erratic idle goes away when the fast idle is disabled, you have zeroed in on the problem.
Last edited by calpatriot; 10-04-09 at 06:31 PM.
10-04-09, 06:31 PM
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On a cold start, the idle should go to 3000 rpm for 17 sec. The ECU commands the air bypass solenoid open to maintain this. On the S5s it was called the AWS, for accelerated warmup system.
Once the 17 sec is over, the fast idle cam keeps the idle up at 1500, normally ramping down to normal idle as the thermowax senses coolant coming up to normal op temp.
It sounds like your problem is in the fast idle cam zone, after the 3000 rpm 17 sec range and before the normal idle. This suggests that it is related to the operation of the fast idle system.
I would go to the FSM and look up the procedure for checking the fast idle system, and apply that and see if anything improves.
Hailers also has a post on here somewhere on here about how to disable the fast idle cam by wiring it into a non-operational position. Search in this section for posts by him and "thermowax" or "fast idle."
If the erratic idle goes away when the fast idle is disabled, you have zeroed in on the problem.
Once the 17 sec is over, the fast idle cam keeps the idle up at 1500, normally ramping down to normal idle as the thermowax senses coolant coming up to normal op temp.
It sounds like your problem is in the fast idle cam zone, after the 3000 rpm 17 sec range and before the normal idle. This suggests that it is related to the operation of the fast idle system.
I would go to the FSM and look up the procedure for checking the fast idle system, and apply that and see if anything improves.
Hailers also has a post on here somewhere on here about how to disable the fast idle cam by wiring it into a non-operational position. Search in this section for posts by him and "thermowax" or "fast idle."
If the erratic idle goes away when the fast idle is disabled, you have zeroed in on the problem.
Where's Hailers when you need him?
Thanks a lot!
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