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Carrera idle bounce at startup...

Howdy-

When my car (DME equipped Carrera 3.2) is cold and started for the first time of the day or after a complete cooldown, the car fires right away but the idle fluctuates rapidly between 600 or so and 1500 (rapidly being the key word I think). If you ride the throttle a little with your foot for 10-20 seconds or so, it smooths itself out and goes away.

Any thoughts on this one?

Thanks-
BG

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Old 03-05-2003, 01:57 PM
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I've had similar experience that was cured by replacement of various ingition parts. Maybe your issue is with the Idle Stabilizer Valve. It's about $130 to replace, but you can remove and clean out.

Lots of threads on idle fluctuation, instability, etc. for the 3.2's. A few searches of this board will net a bunch of info/suggestions by people way more knowledgeable than me.

todd
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Old 03-05-2003, 02:54 PM
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Test your cylinder head temperature sensor. Mine went bad and I had similar symptoms. There's a tech article here on Pelican about testing it.
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Old 03-05-2003, 02:59 PM
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I have the same problem. Replaced the O2 sensor. Replaced the idle control valve. Problem persists. Next step is the Head Temp Sensor, which could be this weekend's project. It sure is an irritating problem....
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Old 03-05-2003, 03:31 PM
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In my limited experience, when the idle goes up and down (sounds like a big cat panting) or falls off and then "catches" itself, the mixture is either:
A) outside the range that the idle stabilizer can compensate for OR
B) the idle stablizer is not working right, stuck?

At idle, the engine management system runs in "closed loop" mode. (I.e. it looks at sensors and adjusts itself for you non-engineers.) When things are working right, the motor "wants" to idle at the correct speed with the correct mixture. In that state the mixture and idle speed only need to be adjusted a little to keep it there.

The computer looks at the O2 sensor and adjusts the mixture. It looks at the RPM's and adjusts idle speed by controlling the amount of air passed around the closed throttle by the idle stabilizer valve.

As O2 sensors get older they get "lazy" and may be slow to respond. (Like me!) This can effect idle but I think it gets blamed more than it should. It is probably a good thing everybody replaces them when they are trying to solve the real problem as most people never would otherwise.

When cold, the head temperature sensor has a resistance of between 3000 and 4000ohms. When it is hot, the resistance drops down around 300-400ohms. When the engine is cold or if the engine thinks it is cold because the head temperature sensor is bad, the mixture is richened. If this sensor's resistance doesn't go down as the motor gets hot, the motor will be running too rich. If the sensor is flakey and becomes an open circuit the resistance would go to infinity. I bet the computer would see this as a very cold motor (but I'm not sure).
You can test the head temp sensor by jumping the connector (white connector of the 3 next to the heater blower) with a resistor of the correct value. I keep 3 (cold/warm/hot) in my tool roll so I can keep driving if the sensor fails.

I could write some more on this subject but I'm getting tired.
-Chris
Old 03-05-2003, 06:42 PM
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The easy and most rational fix is to lower your base idle speed to prevent the DME from seeing such a high idle that it goes into fuel shut off on overrun mode. It happens so fast, it sounds like the engine is panting. This is the first thing anyone should do before spending a lot of money on expensive parts that do not need replacing.

Cheers,

Joe
Old 03-05-2003, 07:34 PM
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I don't think that the O2 sensor comes into play on cold start as the DME goes full rich on cold starts.

I have a new O2 sensor, head temp sensor, and speed reference sensor, and my engine started doing the same thing this winter. It only happens for about 20 seconds and then the idle settles and stays rock solid.

I would live with it or replace the Idel Stabilizer Valve only after making sure your warm idle/mixture settings are correct.
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Old 03-05-2003, 07:51 PM
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Thanks again guys for the depth of your info!
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Old 03-05-2003, 08:11 PM
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If the problem only happens when cold, does it point to the head temp sensor? Or does the Idle stablilzer valve require to be warmed up?

I actually have a similar problem on my 3.2. When cold the idle will bounce between 1000 and 1300 for a minute. When warm, as i go to idle it will have a small bounce at 1000 to 1100 and then settle down.

alf
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Old 03-05-2003, 08:57 PM
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If the problem only happens when cold, does it point to the head temp sensor?

NOT Necessarily. My HTS is new and I have the wandering at cold start up.

Or does the Idle stablilzer valve require to be warmed up?


Not sure, but don't think so.
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Old 03-05-2003, 09:24 PM
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Quote:
When my car (DME equipped Carrera 3.2) is cold and started for the first time of the day or after a complete cooldown, the car fires right away but the idle fluctuates rapidly between 600 or so and 1500 (rapidly being the key word I think).
this is perfectly normal.

seriously though, mine does it.

o2 sensor doesn't come into play until warm.

my idle valve thingy is fine.

cht? does this operate when cold?
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Old 03-06-2003, 12:58 AM
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I guess I should have used an example for a cold motor (like BGCarrera32 asked about in the first place) instead of a warmed up motor. You can still use the resistor jumper test to rule out the head temp sensor as a culprit. There is no need to replace this part as part of the debugging process.
Like someone said, the O2 sensor is ignored at initial startup though I don't think for long. This sensor has a built in heater to get it up to working temperature - which wouldn't be necessary unless the sensor was going to be ignored until the exhaust was hot enough.
To actually address BGCarrera32's cold start idle problem, I would jump B and C in the test socket to disable the idle stabilizer valve (ISV) and determine where the car idled without ISV correction with the motor warmed up. If it wants to idle high I would look for air leaks, fix them and reset the idle slightly if necessary using the idle air adjustment screw on the throttle body. If you have a gas analyzer you would check and set the mixture also.
Once the baseline idle set, I would see how it acted cold. If it still idled funny you could reduce the baseline idle speed as Joe (stlrj) described. This is a bit of a hack IMO.
This assumes your ISV is working OK. Swap with a friend to test. Tapping on them will sometimes make them work. Cleaning them out will ungum them and make them work a little longer. The motors in them eventually wear out and the ISV will need to be replaced.
-Chris

Old 03-06-2003, 04:15 AM
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