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Old 12-24-2011, 01:48 PM   #1
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Default Rpm hunting / surging

Hello Audi Folk. I wanted to know if Rpm surge or hunting is normal on these cars. When I am in neutral and trying to hold the RPM's at say 3,000 steady I find it surges and falls cycling up and down by as much as a 1,000 when cold in closed and less when warm. I've never had a car do this but this is my first turbo. Any thoughts? It idles fine with very little variation. Seems to be a closed loop problem. I understand that the o2 sensor maybe getting a rich or lean reading and the computer is compensating back and forth. Is there a common known issue related to this?

Last edited by TeamO; 12-24-2011 at 03:15 PM.
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Old 12-24-2011, 04:58 PM   #2
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Why are you trying to "idle" at 3000 RPM? You are asking the ECU to do something that doesn't make sense.

Idle = 800 to 1000 RPM and is controlled by a properly functioning idle stabilization valve (ISV), as controlled by the ECU.

Here is some N71 ISV reading for you (Click on the link and then the N71 label in the upper right(ish) corner of the diagram):

Click Here
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Car and Mods:
93 S4, 1.8t coil pack conversion (First in world using integral POS coil packs (??)), CF (OE), S6 steering wheel, RS2'd, Samcos, MRC, TDS-1, Apikol FMIC, Eibach/Bilstein, BIRA Sys 1, relayed FP
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Old 12-25-2011, 09:20 PM   #3
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I had surging just like this, and it was my Oxygen Senson.

Mine was the original with 115k miles on it. if you don't know when yours was last replace I would start with replacing that.

(PS: urs4boy, holding RPMs can be used in emissions tests, that's how i found out my car was surging)
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Old 12-26-2011, 02:48 PM   #4
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o2 sensor looks new. I know that doesn't mean it works so I'll check that out it sounds logical. Thanks for the reply.
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Old 12-26-2011, 03:02 PM   #5
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I noticed it when I was starting up on a cold morning. Trying to get heat to melt frost. In open loop it held rpm fine then in closed loop it would pulse up and down when I tried to hold it at a steady rpm over 1,000. I have had many coil issues before installing a used Apikol set up. I wonder if all the missing could have damaged the o2 sensor. I want to get new oe plugs and get a set of the latest 1.8t coils so that I know all of that system is solid. Are the "D's" the latest revision? It idles just fine. Thanks for the layout diagram. I cleaned out the ISV just as a general maintenance thing but no difference was noticed.
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Old 12-26-2011, 03:32 PM   #6
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If you are going to buy new "1.8t" style coils, buy the newer 2.0L TFSI coils. Same harness connectors as the 1.8t coils but supposedly with better resistance to dwell-induced self-heating failures.
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Old 12-26-2011, 03:58 PM   #7
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Thanks for the tip on the coils that's just the kind of wisdom I've come to expect from from you. Appreciated. Any ideas on the o2 sensor? I bet you think: new plugs and coils first then go from there Could the TPS cause hunting? or the MAF? Funny thing is this car passed CA smog with no problem. It does drink a lot of gas but I can't tell if that's my ADDICTION to boost or if that's just what it is. I think I get around 16-18 mpg mixed driving. Was hoping to see 23 on the highway but haven't done a trip to check.

Last edited by TeamO; 12-27-2011 at 09:02 AM.
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Old 12-26-2011, 04:39 PM   #8
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"Drinks a lot of gas" = sounds like an O2 sensor problem. Have a read on this:

http://forums.quattroworld.com/s4s6/msgs/21012.phtml
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Car and Mods:
93 S4, 1.8t coil pack conversion (First in world using integral POS coil packs (??)), CF (OE), S6 steering wheel, RS2'd, Samcos, MRC, TDS-1, Apikol FMIC, Eibach/Bilstein, BIRA Sys 1, relayed FP
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Old 01-09-2012, 02:35 PM   #9
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New coils new plugs and o2 sensor. Running better than ever until I spotted the coolant tank leak. Of course I tried to tighten the clamp and it cracked. Always something. O well I planned on replacing it anyway. I'll let you know if MPG improve after a tank of gas. Thanks again for the help all.
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Old 01-09-2012, 10:15 PM   #10
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Nice. I believe my MPG went up 1-2 mpg with my LS2 coil kit.

That coolant piece is known to crack after 10+ years. Not to hard to replace it.
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Old 01-09-2012, 10:15 PM
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20, 2000, audi, hold, hunting, issue, neutral, rpm, s8, smog, surging, test


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