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    Thread: 1.8t rough idle all day

    1. Member
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      97 Jetta 1.8T (Copper), 2001 WE Jetta 1.8T
      09-07-2016 11:21 AM #1
      first some specs:
      2001 1.8T
      gt28rs turbo, blowoff, FMIC, no mass air flow, no egr valave (all deleted) tune is a modded version of the 94 octane Upsolute flash. been running this setup for almost 6 years without issues (other than etest)


      got an issue that showed up last week. started with some raw fuel smell when letting off the throttle after hard push. no change in fuel economy and cant smell at idle so figured plugs were getting dusty. pulled em out and re-gapped to .031" (they were measuring .034-.035 (worn down i guess). then started getting a floating idle. just sitting the car will Stutter and then the idle drops to 400rps and climbs to 1200 (boost jumps to -10 from -20 same time) and bounces unless i rev it it and hold around 1000 and let off smooth. then it will stay stable until an stutter (if its a misfire its not coming up on vagcom) coils are about 3 years old. and clean contacts. plugs new this morning (NGK bkr7ie iridium) and new air cleaner (K&N cone filter)

      vagcom is not 100% helpful as many sensors are not being used (egr, maf) also no secondary air pump or ac on this beast. this is a custom built MK3 but like i said been running for 6 years no issues and other than gapping the plugs and a smell of raw fuel on "engine braking" there was nothing wrong. car still has all its power at running revs and doesnt matter cold warm or hot i drive 20KM to work and does it into the parking lot and right after cold start. turbo still boosts to 15PSI (havent tested at 22PSI (original tune settings) but have been running 15psi for couple years due to some cv problems and lots of tire smoke.

      i can provide more info and VAGCom read outs but at this point i am stumped
      Last edited by Cool-Jett; 05-19-2017 at 08:08 AM.

    2. 09-07-2016 02:51 PM #2
      Try to Use lemmiwinks and bring ur idle up or down as needed.
      In my case i had to bring it down to 750 -800 rpms it was close to 1k.

      Also the spark plug gap on my car is .28 anything over .32 i was getting massive missfires and idle wasnt all that great either. I believe if you have a tune your supposed to be at .28....not 100%

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      Last edited by Guyver96; 09-07-2016 at 02:56 PM.

    3. Member
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      97 Jetta 1.8T (Copper), 2001 WE Jetta 1.8T
      09-07-2016 09:06 PM #3
      the idle is usually at 800 rpm when its stable.

      the gap was .034 and running fine just a smell of raw gas. the idle/hesitation problem came after i gapped them to .028 and it was stalling when i would let in the clutch after driving at speed to come to a stop. so i gapped to .031" and it was basically the same how ever i could smooth it out if i held 1000 rpm for a second or two then let off. and thats where i am with new plugs factory gapped to .032".
      Last edited by Cool-Jett; 09-08-2016 at 12:52 AM.

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      Passat B5 1.8T Aeb 1999
      09-07-2016 10:47 PM #4
      HY
      In my oppinion you have a BLIND tune.Cars ECU has evolved very much and those sensor are needed for an exact fuel and spark delivery.You could put back your 02 lambda,and make again a new tune.Your problems are not from plug gaps,i experimented myself between .027 and .035 and idle is exactly the same.When a bigger gap is set,the coil,s DWELL TIME increases in order for the coil plus to be charged with more energy needed so sustain that bigger spark.IN high boost ,on a big gap,the spark will be blown away as like when you go in a church and blow on a candle and that candle shuts down.The red top coil R8 style have a very short dwell time and drawns very quikly the energy from you battery.Very important are also the ECU drivers who control this dwell time.
      First of all please measure your AFR...

    5. Member
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      09-08-2016 04:35 AM #5
      Quote Originally Posted by silviu_18t View Post
      HY
      In my oppinion you have a BLIND tune.Cars ECU has evolved very much and those sensor are needed for an exact fuel and spark delivery.You could put back your 02 lambda,and make again a new tune.Your problems are not from plug gaps,i experimented myself between .027 and .035 and idle is exactly the same.When a bigger gap is set,the coil,s DWELL TIME increases in order for the coil plus to be charged with more energy needed so sustain that bigger spark.IN high boost ,on a big gap,the spark will be blown away as like when you go in a church and blow on a candle and that candle shuts down.The red top coil R8 style have a very short dwell time and drawns very quikly the energy from you battery.Very important are also the ECU drivers who control this dwell time.
      First of all please measure your AFR...
      i see what your saying about blowing out the spark. but i am fine when driving 1000-7800 rpm. full boost and doing 180KM/h nothing is wrong. its only the idle and its not all the time. like i said if i let the car idle from low rpm it stays steady. its when i let it drop to idle from revs or load. no misfire and no backfire so not getting fuel loading in exhaust.

      as for the AFM i dont have tools for that id have to go to a shop for that and i dont think that is my issue as i said it was not a problem till i checked the plugs.

    6. 09-08-2016 12:51 PM #6
      Quote Originally Posted by Cool-Jett View Post
      i see what your saying about blowing out the spark. but i am fine when driving 1000-7800 rpm. full boost and doing 180KM/h nothing is wrong. its only the idle and its not all the time. like i said if i let the car idle from low rpm it stays steady. its when i let it drop to idle from revs or load. no misfire and no backfire so not getting fuel loading in exhaust.

      as for the AFM i dont have tools for that id have to go to a shop for that and i dont think that is my issue as i said it was not a problem till i checked the plugs.
      I am in the same boat on the Maestro ECU, only. No problem with other tuned ECU. I believe it must be the tune.

    7. Member
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      09-08-2016 08:45 PM #7
      Quote Originally Posted by rogerius View Post
      I am in the same boat on the Maestro ECU, only. No problem with other tuned ECU. I believe it must be the tune.
      how long have u been running your tune? ive been running this tune for 6 years without issue till 2 weeks ago.

    8. Member cmdshft's Avatar
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      09-09-2016 01:04 AM #8
      Some vacuum leaks often cause a bouncing idle and a misfire. I'd start there.

    9. 09-09-2016 01:19 PM #9
      Quote Originally Posted by Cool-Jett View Post
      how long have u been running your tune? ive been running this tune for 6 years without issue till 2 weeks ago.
      3 years. Issues may be caused by hardware changes if on mafless.

    10. 09-09-2016 01:20 PM #10
      Quote Originally Posted by cmdshft View Post
      Some vacuum leaks often cause a bouncing idle and a misfire. I'd start there.
      I checked for vacuum leaks on my nafless, There are no.

    11. Junior Member
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      2005 Jetta MKIV GLI 1.8T
      09-14-2016 07:23 PM #11
      Have you checked your dipstick? they are known to break down. run at idle and pull dipstick. Usually on a car with no idle issues, the car will idle rough.

    12. Member
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      09-15-2016 09:24 AM #12
      ill check the dipstick on friday when i have time but after friday last week (was raining) i think its a bad coil or more than 1. while driving in the rain the car was idling rough as has been for last couple weeks but when trying to get up to highway speed as soon as boost came up to 5psi the car spit and sputtered like it was full trottle without fuel but back firing like a mofo. ive had this issue before a few years ago and had bad plugs and a bad coil. my guess is this is same issue again but i was noticing the issues early. ill update after i test out the new coild this weekend.

    13. Member
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      09-17-2016 10:52 AM #13
      new coils are in and car was still idling rough. but now its not almost dying. revs only float +-50rpm and it levels out for a bit on its own. i had a look and feel for vacuum leaks and didnt see anything. im loosing my witts with this as there are only weeks left till it is gonna be too cold and wet to drive and then time to store it. was hoping to fix some rust on my winter car this month but without the Copper Jetta running right i dont want to rely on it in this condition

    14. Member cmdshft's Avatar
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      09-17-2016 09:57 PM #14
      How did you check for vac leaks? You wont be able to see hairline cracks and stuff but they can still leak. You need to pressure test or smoke test the engine.

    15. 04-05-2017 10:02 PM #15
      Try changing ur gas cap

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    16. Member audiaddictt's Avatar
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      04-06-2017 08:50 PM #16
      Correct me if I am wrong but wouldn't a gas cap throw a code at him?

      I have a similar issue I am trying to track down on my BT Eurodyne 2.0T coils, 1000cc, SEM, Mafless as well. I am going to try plugs soon. I will also need to test for vacuum leaks.

      good luck I will keep an eye on this thread to see what you found out. How are your coil wires by the way? did you replace them? they are known to deteriorate.

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    17. Member
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      04-07-2017 05:48 PM #17
      Do you adjust your own ecu? I would check there.

      How do you measure the a/f? Back fire and smell of gas sounds like you are running richy rich. Check the pressure. After 6 years sounds like the regulator might be going out.

      Whenever we mod or change something we always have to adjust the ecu.


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    18. Member
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      05-19-2017 08:04 AM #18
      turns out its a leak in the exhaust manifold

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      05-19-2017 11:14 PM #19
      Quote Originally Posted by Cool-Jett View Post
      turns out its a leak in the exhaust manifold
      Wow, what is a hairline crack? Probably did not hear the leak cause it was too small.

      Thanks for the update. Diagnosing a car online without looking at it is tricky.


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      05-19-2017 11:18 PM #20
      Also, that is the reason why you can smell the gas more under more pressure when exhaust gases move faster.


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    21. Member
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      05-20-2017 08:56 AM #21
      no its not small but it is hidden. the manifold is a custom welded. basically its a flange plate with 2 pipe elbows and 2 t's welded then the turbo flange was cut into that. the crack is almost the whole way around the weld petween pipes at cylinder 1 and 2. its only visible when the car is running. i noticed it when i tried starting the car a few weeks ago after sitting for the winter. got a huge plum of black smoke from under the hood. now i gonna have to find time and rip out the manifold and weld it back up

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