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The temp sensor finally came in today. I'm going to try and swap it out after work tonight.

I owe 1999Porsche911 dinner if this solves my problem. If not, I'm going to give up and take it to the dealership.

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Well I have good news. You need the correct size matched tires. Your car has Traction control built into the abs control unit. When the abs control unit sees a difference in speeds front to rear at

Posted Images

OK, my new guess. :D

The 'kick back on' is the emissions control relay. You know the TPS and IACV are good.

For what ever reason the DME can't find an acceptable A/F mix with the throttle closed.

It must be getting bad info from something.

So I'm going with MAF or O2 sensors or even both.

Did the tech remove the exhaust to change the plugs?

Was part of the 'slight tune' cleaning the MAF with a wire brush?

Did he put it in backwards. (don't laugh I've heard of it happening)

Ok, now that I've guessed all the obvious stuff I think my bases are covered. B)

Seriously, I hope you figure it out soon. Must be driving you mad.

Edited by Foster
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The MAF is in correctly, and wvicary is correct that there would be no way to bolt it in backwards. I've had it out and cleaned it with CRC since this problem began. It seems to be showing good values on the Durametric. The O2 sensors seem to be working correctly as well.

Probably a dumb question, but when I pull that temp sensor, is it going to dump coolant everywhere? The temp sensors in all the other cars I've owned were on top of the manifold, so that was never an issue.

I'm just checking before I lay down under the car and have it possibly dump on my face.

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The MAF is in correctly, and wvicary is correct that there would be no way to bolt it in backwards. I've had it out and cleaned it with CRC since this problem began. It seems to be showing good values on the Durametric. The O2 sensors seem to be working correctly as well.

Probably a dumb question, but when I pull that temp sensor, is it going to dump coolant everywhere? The temp sensors in all the other cars I've owned were on top of the manifold, so that was never an issue.

I'm just checking before I lay down under the car and have it possibly dump on my face.

Yes, you will have to catch the coolant.

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It wasn't the temperature sensor.

Have you tried driving with the MAF cable disconnected? Although you'll most likely get a CEL, see if the idle is better running this way.

Also, although you put in a new ICV, is the gasket missing or leaking? Check throttlebody bolts and any other area that will allow air past the closed throttle. Check vacuum line from fuel pressure regulator to intake tube to the back and right of the throttlebody.

Edited by 1999Porsche911
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Update on this problem. I've rechecked for vacuum leaks again -- still haven't found anything.

To recap, the idle is always rock solid at 800 rpm when the car is stopped (zero speed).

Intermittently, while driving at speeds above 10 mph, when I let off the throttle, the car will continue at speed or surge as if the cruise control is on. If I clutch and let off the throttle, the idle will hang around 2200 rpm until I either re-engage a gear or come to a stop.

No codes. No CEL. No loss of power.

It is definitely not the Idle Control Sensor, the Throttle Position Sensor or the Water Temperature Sensor.

Tried driving with the MAF unplugged. Problem remains.

The car may not do this for days or it may not stop doing this for days, but I've come to realize that if I turn it off and then right back on, it almost always happens at least for a few minutes.

I have a call in to schedule an appointment with the dealer for diagnostics next week. Hopefully they can finally put an end to this annoying mystery.

Edited by JeTexas
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Just got a call from Porsche of West Houston. Their diagnosis was sticking air bypass valve on the Idle Control Sensor. I said, I've already replaced that, it didn't make a bit of difference in the symptoms. They said, sometimes new ones stick too.

They wanted $550 to change it out. I'm going to pick the car back up.

I guess I'll order another one. I don't really know what else to try.

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WOWEY WOW...I thought I was the "chosen one" when it comes to these types of problems....I am following this to the end...one of the more challenging ones in a long time. PLEASE allow me to feel your pain! You did not mention if the car was tested by piwis when visiting the dealer and what it detailed. were behind u on this-good hunting-Mark the ***** is that no one on earth has had this problem!!

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After the dealership called and said it was the Idle Control Valve, and I told them that I'd already replaced it (which I'd told them when I brought the car in, but they apparently didn't listen), they decided to pull it off and check it. Sure enough, it had a good spring and was working. They cleaned it, put it back on, problem remained.

So when I went to pick the car up, the tech told me his new diagnosis was a bad DME. He said there was a chance it might just correct itself, but when I told him that it had been going on for six months and that I'd disconnected the battery to reset the DME several times, he then decided it was definitely time to change the DME.

They estimated about $3,000 for a new one, so I'm going to check around and see what I can find. However, he said that if I buy a used one then I need to get the immobilizer unit with it or otherwise it won't work.

By the way, they charged me $138.32 to pull my new Idle Control Valve off and clean it, just to tell me I didn't have the problem that I already knew I didn't have. Awesome.

So does anybody have a good lead on the most cost-effective DME replacements?

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After the dealership called and said it was the Idle Control Valve, and I told them that I'd already replaced it (which I'd told them when I brought the car in, but they apparently didn't listen), they decided to pull it off and check it. Sure enough, it had a good spring and was working. They cleaned it, put it back on, problem remained.

So when I went to pick the car up, the tech told me his new diagnosis was a bad DME. He said there was a chance it might just correct itself, but when I told him that it had been going on for six months and that I'd disconnected the battery to reset the DME several times, he then decided it was definitely time to change the DME.

They estimated about $3,000 for a new one, so I'm going to check around and see what I can find. However, he said that if I buy a used one then I need to get the immobilizer unit with it or otherwise it won't work.

By the way, they charged me $138.32 to pull my new Idle Control Valve off and clean it, just to tell me I didn't have the problem that I already knew I didn't have. Awesome.

So does anybody have a good lead on the most cost-effective DME replacements?

You should try re-programming the DME before replacing it.

You will need a PST2 or PIWIS tester and your car's IPAS codes.

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You should try re-programming the DME before replacing it.

You will need a PST2 or PIWIS tester and your car's IPAS codes.

I've had both Porsche of West Houston and Porsche of North Houston tell me that once the computer has problems, there's no way to reprogram it and that I'd have to buy a new computer.

They've both also said there's no way to use a used computer, which I know is wrong because I've read tholyak's posts on the topic.

I just wish I knew somebody in Houston with a PST2 or PIWIS.

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You should try re-programming the DME before replacing it.

You will need a PST2 or PIWIS tester and your car's IPAS codes.

I've had both Porsche of West Houston and Porsche of North Houston tell me that once the computer has problems, there's no way to reprogram it and that I'd have to buy a new computer.

They've both also said there's no way to use a used computer, which I know is wrong because I've read tholyak's posts on the topic.

I just wish I knew somebody in Houston with a PST2 or PIWIS.

I Would search for an independentr shop with either of the testers Loren references and trya re-program before spending $3G's for a new DME. Indie may have a good source for a refurbished or reclaim source for lot cheaper than the $3G.

Good Luck!! Demosan

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There's a salvage yard on ebay that sells 99 Boxster DMEs (which are the same part number as the 996) for $350, but they only have the VIN, not the IPAS codes.

I called one Indie so far, and they thought they could source me a new DME for about $1700, but again, said there was no way to utilize a used one.

Luckily my car is completely driveable, just annoying at times, so I can take my time in figuring out the best route on this.

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It sounds like the dealer is shooting in the dark. Did they give a reason as to why they think it is the DME? DME failure is pretty rare. It is more likely that you have a leak somewhere in the intake.

If you buy a DME, it is unlikely that you will be able to return it when it doesn't solve the problem.

If you insist on replacing the DME, I have a used one with all of the codes that I'd be willing to sell. But again, I don't think it's going to solve the problem. The dealer should be able to tell you what signals from the DME are out of spec and causing the problem. If not, you're just burning money.

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When the problem isn't there, it can be created by turning the car off and then turning it back on. The zero speed program still works fine - perfect idle at a stop, but the idle speed program (idling with the car rolling at speed) goes haywire. The tech didn't know if this was the cold start program staying on too long or what, but even if the computer was running the cold start program, it was running the idle way too high. It might be that the computer isn't kicking into the idle program at all. I don't know.

Incidentally, it will hang the idle at 2200 rpm if the A/C is on but only at 1200 rpm if the A/C is off.

Unfortunately, I haven't met anyone who fully understands the different program circuits the computer runs.

The only other thing I can think of that comes on at startup to change the variables is the Secondary Air Pump, but I don't think that would affect idle speed. It definitely doesn't affect the idle at a stop.

Edited by JeTexas
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You mentioned that the problem is like having the cruise control on. Does the cruise control behave in a normal fashion when clutch is depressed?

I don't know. I leave it turned off. (No green light on dash.) I've clicked it on and off when the engine was pulling to see if it did anything, but that had no effect. I'm usually sitting in such terrible traffic that I never have space to actually set it. I'll check it tonight though to see how it's working and report back.

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I still wouldn't rule out the possibility of an air leak but it almost sounds like there is a short to B+ in the idle circuit or a faulty ground. It should be possible to test both of these.

With an OBD2 reader you can read the air flow (as measured by the MAF) during idle. If it is the same when the problem is happening and when it's not then you definitely have an air leak.

If there is no air leak, you can narrow down the problem by testing to see whether or not the ECM is telling the ICV to idle higher. This you can do by testing the voltage on the idle control valve input. I think this might be pins 1 & 3 but I only have a really bad faded wiring diagram available to me right now. If the input to the ICV is higher when the problem is happening then the problem is not mechanical. In that case, I would diagnose a possible short in the idle signal line (ECM to ICV).

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Been doing more research.

I was going to inspect and clean the connections at the DME. I'm a bit confused. I thought it was behind the seat, but all I found was the rollbars and top mechanism.

post-30731-1234065344_thumb.jpg

Am I looking in the wrong place? If it's under the seat, how do I get that bottom part off? Do I need to remove all the seatbacks and all the seatbelts?

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The DME is located under the clam shell/soft top, remove the trim (carpet) with the soft top in service possition and you will find the DME mounted in vertical possition behind the roll bars.

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  • 2 weeks later...

Still no resolution but a new symptom. Now if I'm having the hanging idle problem, and I have the A/C off, when I come to a stop the idle constantly sweeps back and forth between 600 rpm and 1200 rpm.

When the A/C is on, stops are still a perfect 800 rpm idle.

Had to buy new tires last week, which kept me from impulse buying a PST2. Hopefully I'll either get this sorted out or the culprit will finally fail soon.

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