Need advice from the best of the best on an electrical issue

Hi guys, it's been a few months, but I still haven't resolved this low idle/surging issue. The car is a 99 coupe I bought in January of this year. From the get go, when the car is stone cold and I turn it on, the idle drops and then surges about once every minute until the car is fully warmed up. After that, the car drives perfectly, even if it sits for hours and I restart it. When it's really hot outside, the problem is not there, it usually does one surge and then idles normally. The problem totally goes away when I unplug the wire from the coolant temp sensor and there are no codes stores and no codes were picked up by the Porsche PS2 machine either. The reason I need a pro is because even the dealer can't help me. This is what's been done: New motor (problem was there with the old motor too) New and tested coolant temp sensor ISV, TPS and MAF have been tested New wiring harness DME has been tested twice Any tips?

Ck all of your ground points, remove them and clean up the contact surfaces with sand paper and then reinstall them

Did you clean the idle control valve?

when you mean surge... how much? 100, 200 rpm?

thanks for the tips guys, but we have to go deeper. grounds are all perfect. idle control valve is perfect. when it surges, i mean it drops to about 250 and then surges to 1500 and then stabilizes until the next surge. once the block is warm to the touch, regardless of the temp on the gauge or temp outside, the problem is not there. it's only there when the block is stone cold, like after sitting overnight.

I have the same exact issue with my BMW 3 series, I know it is from a defective cam timing sensor, there are two of them, one for intake one for exhaust. I already replaced one of them, the problem is still there, just don't feel like taking apart the intake yet. Not sure if the 996 have something similiar.

I have experienced the exact same problem with both my 99 996s. It has always been the result of a air leak in the area of the plenum. I have had my RSS plenum on and off both my Porsches. The solution was always to partially remove the plenum and reseal the boot on either side. There are two small air hoses coming of the boots on either side of the plenum (either positioned on the bottom or facing toward the front of the car). If either of these hoses are pulled out, it would cause a fluctuation in the idle. I don't know if this is your problem, but it is easy to check. It sounds like a vacuum leak.

Since it goes away always when you remove the thermostat cable, it is possible even the new sensor is bad or the cable or connectors are bad. Connect the cable to the old sensor and let it hand and see what happens.

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This is interesting. Can anyone else chime in on this?

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This is not likely since the problem goes away once the car is warm. A vacuum leak would stay constant.

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no, the temp sensor was replaced twice and the one in there now was tested with a ps2 and we got good readings. i will try connecting the cable to another sensor outside of the car and see if the problem changes. if the car is hot, and the sensor is cool and hanging outside of the car, i want to see how the car reacts. if it still surges, then it certainly has to do with the temp reading on the coolant temp sensor. if it idles normally, then i am barking up the wrong tree. still, need more ideas, please keep them coming.

A bad AOS can have the same symptoms. (mine did, and I have a new coolant temp sensor as well...)

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but wouldn't i have smoke at startup and some oil burning? i have a clean dry startup and it never smokes.

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I did not have smoke. I had a month or so of strange cold idle and then a CEL. The AOS deals with air and oil, in my case it was sucking air. There is an easy test for an AOS which a dealer should be able to do. If nothing else, it's worth ruling out esp. since it's a common failure.

did they put the old throttle body and ICV on the new motor? or is it new?... if its old... it might need a serious cleaning. But, Im sue youve been here before...

pardon me in advance for being captain obvious here (or captain oblivious depending on whether or not the coolant sensor send out an electrical signal or an electronic one) , but did you check the cable for the temp sensor for continuity/voltage/current? I would definitely try another known good sensor and troubleshoot my way up the wiring, one step at a time. Also, question, what are the ambient temps in your garage at night (off the top of your head), i.e. how cold is cold? Does warming the temp probe remove the problem (you could use a hair dryer if you're careful)? I mean this as a troubleshooting measure not a solution.

Htny, I have to wait until the car is stone cold again to try the ambient temp sensor issue but my dash reading are always correct (i.e. 55 degrees F in my garage). I just plugged in another cold coolant temp sensor, grounded it, and started the car. The temp gauge read cold, but the problem was not there, leading me to believe it's not the cts at all. What is strange is that when I unplug the cts, the issue is gone. redridge, been there. everything is so clean you could eat off it. plus, if dirt was the issue, I don't think it would go away once the car is hot. salayc, good idea, I'm gonna call my tech tuesday and see what he says. robin 993dx, also gonna bring that up with my tech on tuesday. although, since it's a new motor, and the problem was there even with my old motor, i doubt the cam timing sensor could be it, since i would have gotten new ones inside the new engine. am i wrong? please let me know. Just a reminder, the problem completely goes away when the car gets hot. once it's warm, even a little bit warm several hours later, the problem is gone. Also, the CEL blinks for a second when it drops/surges leading me to believe it's a faulty sensor. Remember, this car has a new motor and the problem was there long before, so it has to be something that was in the car before the motor install. Thanks for all the tips. Please keep throwing ideas at me no matter how strange they might sound.

If when cold, and you hold the throttle open so it is idling at around 1000, is it a steady idle? This bypasses the ICV to a certain degree.

no, it will die for a split second, the cel will flash once, and then it will resume the high idle.