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I've posted a couple times previously about the idle speed oscillating/surging intermittently when engine warmed up on my 1999 996 Cab. Intermittently at a stop sign revs up and down between 500-1200 rpm with AC off.

Have now tried the ECU in addition to the throttle position switch, idle control valve and mass airflow sensor with no luck. The dealer and Porsche tech support are stumped.

Only one time out of several was there any fault code - it was code 124 intake air temp sensor sporatic. The tech said that code was unrelated and that nothing was wrong. I'm grabbing at straws - is the intake air tem sensor something that is easily replaced and not too expensive?

Any other ideas are very much appreciated. I hate to live with the problem.

Thanks!!

Dwight

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The air temp sensor is part of the mass air flow sensor. You need to look for damaged wiring in that circuit.

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The air temp sensor is part of the mass air flow sensor. You need to look for damaged wiring in that circuit.

Thanks, I'll check it out. That code showed up in a computer check I had done about 3 weeks after I changed the MAS, so I wonder if I triggered that fault when I changed the MAS. A few days before I changed it, I disconnected the MAS for a couple minutes at another member's suggestion to see if it changed the idle problem.

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I haven't followed your other threads, but sounds like you have a leak somewhere after the MAF, which can cause the surging since un-metered air is allowed into the system. Check your boot and clamp at the throttle body; make sure it is in evenly and securely.

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If you had disconnected the MAF then started the engine it would trigger that code.

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Thanks all of you for the ideas. I'm not sure how to check for an intake vacuum leak other than sight and sound but I don't see anything suspicious. The intake boot has been off a few times to clean the throttle body and it is tight at the moment. Is there a spray or something that I can use on the various suspect spots that might help find an intake leak?

Also, I thought of maybe trying to mark the throttle stop screw setting and then trying to adjust it some each way to see if that helps. My thinking was that maybe nudging the throttle open mechanically about 50-100 rpm might keep the computer from trying to compensate. Or vice versa, closing the throttle further by backing the stop screw out a bit. Can I get in any trouble doing that and driving the car some to test the new settings, as long as I mark it first so I can get back to the factory setting?

thanks

Dwight

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Thanks all of you for the ideas. I'm not sure how to check for an intake vacuum leak other than sight and sound but I don't see anything suspicious. The intake boot has been off a few times to clean the throttle body and it is tight at the moment. Is there a spray or something that I can use on the various suspect spots that might help find an intake leak?

Also, I thought of maybe trying to mark the throttle stop screw setting and then trying to adjust it some each way to see if that helps. My thinking was that maybe nudging the throttle open mechanically about 50-100 rpm might keep the computer from trying to compensate. Or vice versa, closing the throttle further by backing the stop screw out a bit. Can I get in any trouble doing that and driving the car some to test the new settings, as long as I mark it first so I can get back to the factory setting?

thanks

Dwight

The tubes across the engine held with the collars and clamps can work themselves back and forth - you can't really tell by sight if they are correct - you have to feel around the edge of the black collars, pushing hard to see if they dip down any. Those collars and tubes are hard to get right, and while they may look OK, they might be the source of a leak. You have to get you hands underneath too to push around the edges of the collars, nit just the top.

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Thanks much. I was able to fish a wrench back to check the tightness of the bands around the big rubber collars on the top of the engine. Seemed a little loose but tightening them didn't seem to change the frequency of the problem.

I also wiggled the black plastic vacuum tubing connections that are accessible while it was surging and it didn't stop it.

Does the computer monitor all the potentially related circuits for intermittent sensors or wiring? Since the surging is intermittent with the engine hot, I'm wondering if it's still some electrical bug.

Thanks everyone, for any further thoughts or ideas you might have.

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Depending on how picky you are about keeping you engine spotless you can use WD40 to help identify mysterious leaks. Typically I found that leaks are most prevalent in pvc joints but, I have not dealt with them on my Porsches yet. Basically just follow the intake path and tubes leading away, spraying each and when you find that the idle surges spray there again and you've found your spot. This is not the ultimate fix all end all but it is a technique use when needing to check an area out of reach and in times of desparation. P.S. WD40 will not harm your engine or components and can be simply spray washed off. WARNING: Do Not Spray Directly At The Headders. There is a chance that they could ignite the WD40, Not a great chance but if not taken into consideration then a stupid mistake could make a mess and have your friends asking you why you have no hair on your arm?

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Thanks much. I was able to fish a wrench back to check the tightness of the bands around the big rubber collars on the top of the engine. Seemed a little loose but tightening them didn't seem to change the frequency of the problem.

I also wiggled the black plastic vacuum tubing connections that are accessible while it was surging and it didn't stop it.

Does the computer monitor all the potentially related circuits for intermittent sensors or wiring? Since the surging is intermittent with the engine hot, I'm wondering if it's still some electrical bug.

Thanks everyone, for any further thoughts or ideas you might have.

After reading this last little bit I would say check your fuel pressure level.

O.k. I'll add a reason being.

when cold, generally systems enrich the mixture for warm up procedure, once warm up achieved the mix goes normal.

well if there is not enough fuel you get symptoms like that of an air leak, (surging) and with complete fuel pump failure horridly sluggish performance especially in the torque bands is experienced after about 3-5 minutes of running. This in the my older days was known as a cold start injector.

HTH

Edited by ViolaGT3
  • Upvote 1

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So Dwight, did you actually find the problem why the idle was surging at stop? Mine is the same year and model Carrera and acting up just the same as yours

 

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