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 Old 10-05-2019, 09:41 PM   #1
 
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Default Idle stutter, varying vacuum levels

A love/hate relationship is what this car has been

Have been driving the rebuilt engine for 10k km now, with the colder weather, I am seeing some of the issues more pronounced.

Initial Cold start engine runs great. As the engine RPM start settling down, I can see the engine stuttering and feel it in the cabin. It honestly feels like mini miss-fires as it is very inconsistent. Once the engine is warmed up, it is usually very smooth, with the occasional stutter.

At times, when the car isn't up to temps, the engine will hunt between 800-1200RPM. And If I try to drive it without letting it warm up in the cold, the engine almost dies (needs a lot more throttle).

I have also noticed that the idling vacuum will sit at different values, one day at -8.5PSI, sometimes -9.2PSI and sometimes -9.8PSI.

Notes: Fuel trims look good, except when the car is warming up (-10 STFT). I have reduced HPFP targets to eliminate the chipmunks/chirps on cold start. Some cruising regions target 15 AFR.

What I have tried:
- Boost leak test and the system holds over 20PSI. It slowly looses pressure but I believe thats the PCV. I don't hear any hissing otherwise.
- Spray carb cleaner when engine is running on all intake lines and vacuum lines, including VTCS grommet. No change in idle.
- Clean MAF sensor and try a different one
- Re-check plug gap (.027) on NGK OEM
- Compression test (180PSI across)
- Coil spring stretch and different set of ignition coils - Changing to another set of coils (used) proved to have some improvement with the idle.
- OEM tune

Possible causes:
- Throttle body, I got another used TB that I will clean and replace
- PCV Valve: When replacing the valve, the cap holding the valve down was not reassuringly secure. I used a bit of gasket maker to increase my confidence.
- O2 Sensor/turbo: Turbo smokes a lot, I have heard that it can foul the sensor.
- Fuel injector: Injectors were cleaned and tested at 2k PSI with new filters. Fuel pressure also holds very well after shutting down (drops at 100psi/minute from 500-100PSI)

Here is a video, Exhaust note sounds wrong on video, I am focusing on the stutters.
https://youtu.be/LTxI2rtX4Co
Attached Files
File Type: csv Rough Cold Start idle.csv (124.0 KB, 0 views)
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2008.5|Engine rebuilt @232K KM - Mahle PowerPak 4032, Manley H beam, ARP head and main
CNT Long Catted|COBB SF+TIP|AT HPFP|CPE TMIC|K04|Corksport RMM|
EGR Delete|VTCS Delete
COBB APV3|Stage2 OTS Base+self tune
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 Old 10-06-2019, 01:41 AM   #2
 
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That rough, inconsistent idle with vacuum varying suggests a vacuum leak.

I know you did a boost leak test, but the pressure may be overcoming a slight leak that shows up only during vacuum.

A common problem with an easy fix is to simply buy a few feet of vacuum line at an auto parts store and replace every line. I think it is 4mm. Standard vacuum hose size. The original lines have gotten old, hard, brittle, and probably have small undetectable splits. Cheap fix that needs to be done. I’d bet that this fixes the symptoms you are reporting.
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2017 Audi Q5 3.0 T (supercharged) Stage 2+ EPL tune, JHM overdrive crank pulley, EPL supercharger pulley, ZF8 TCU tune, aFe/034 intake, modified air box. 480 hp.

Sunlight Silver '08 MS3 GT Mods: (Sold 1/25/2019 after 10+ years of ownership)
BNR Stage 1 (to fix smoking K04 turbo), TurboXS 3" Catless DP/RP "Stealthback" into stock CBE, Vibrant 3 inch Ultra Quiet Resonator in RP section, Magnaflow 3 1/2" exhaust tip, Mazdaspeed CAI w/air straightener and K&N conical filter, NGK 6510 Iridium IX one step colder plugs, Hypertech tune, Autotech HPFP internals, Stock BPV (works perfectly), Yokohama Advan Neova AD08R's (stock size on stock rims); Hawk HPS pads; SURE RMM; Grimmspeed EBCS (2 port mode), Bilstein B6's, SPC rear camber arms.

Last edited by MSMS3; 10-06-2019 at 06:39 PM.
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 Old 10-11-2019, 07:46 PM   #3
 
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Originally Posted by MSMS3 View Post
That rough, inconsistent idle with vacuum varying suggests a vacuum leak.

I know you did a boost leak test, but the pressure may be overcoming a slight leak that shows up only during vacuum.

A common problem with an easy fix is to simply buy a few feet of vacuum line at an auto parts store and replace every line. I think it is 4mm. Standard vacuum hose size. The original lines have gotten old, hard, brittle, and probably have small undetectable splits. Cheap fix that needs to be done. I’d bet that this fixes the symptoms you are reporting.
Thanks for the reply.

I replaced the TB, no difference.

Replaced MAF, no difference.

Replaced vtcs solenoid and vacuum box with lines, no difference.

I got access to an oxy/acetylene torch and just used the wand to try and find the vacuum leak and nothing to be found... I even tried the PCV area underneath the intake.

Only thing I can imagine is that this leak is intermittent. So I will order a bunch of silicone hoses because our local auto parts stores in Canada don't have the best selection and I might as well switch to silicone.

Btw, my idle fuel trims change from time to time with this problem. Currently, my ltft is at +6 and stft at -8 at idle. Tune hasn't changed.

Sent from my SM-A530W using tapatalkreplace
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2008.5|Engine rebuilt @232K KM - Mahle PowerPak 4032, Manley H beam, ARP head and main
CNT Long Catted|COBB SF+TIP|AT HPFP|CPE TMIC|K04|Corksport RMM|
EGR Delete|VTCS Delete
COBB APV3|Stage2 OTS Base+self tune
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 Old 10-12-2019, 06:12 AM   #4
 
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Originally Posted by The_big_dill View Post
Thanks for the reply.

I replaced the TB, no difference.

Replaced MAF, no difference.

Replaced vtcs solenoid and vacuum box with lines, no difference.

I got access to an oxy/acetylene torch and just used the wand to try and find the vacuum leak and nothing to be found... I even tried the PCV area underneath the intake.

Only thing I can imagine is that this leak is intermittent. So I will order a bunch of silicone hoses because our local auto parts stores in Canada don't have the best selection and I might as well switch to silicone.

Btw, my idle fuel trims change from time to time with this problem. Currently, my ltft is at +6 and stft at -8 at idle. Tune hasn't changed.

Sent from my SM-A530W using tapatalkreplace
I’ll say it one more time: Replace all those small vacuum lines. Everywhere. Especially to and from the electronic boost control solenoid, but everywhere. Dirt cheap and easy. Yours are hard as rock and have micro splits that expand and contract with temp. Might not solve your problem, but need to be done.
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2017 Audi Q5 3.0 T (supercharged) Stage 2+ EPL tune, JHM overdrive crank pulley, EPL supercharger pulley, ZF8 TCU tune, aFe/034 intake, modified air box. 480 hp.

Sunlight Silver '08 MS3 GT Mods: (Sold 1/25/2019 after 10+ years of ownership)
BNR Stage 1 (to fix smoking K04 turbo), TurboXS 3" Catless DP/RP "Stealthback" into stock CBE, Vibrant 3 inch Ultra Quiet Resonator in RP section, Magnaflow 3 1/2" exhaust tip, Mazdaspeed CAI w/air straightener and K&N conical filter, NGK 6510 Iridium IX one step colder plugs, Hypertech tune, Autotech HPFP internals, Stock BPV (works perfectly), Yokohama Advan Neova AD08R's (stock size on stock rims); Hawk HPS pads; SURE RMM; Grimmspeed EBCS (2 port mode), Bilstein B6's, SPC rear camber arms.

Last edited by MSMS3; 10-12-2019 at 06:23 AM.
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