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Old 08-12-2019, 08:57 PM
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Default Boost gauge needle flutters only on low load in vacuum - '99 GC8 STI

It's internet mechanics time!

Looking for any nuggets of wisdom before I concede defeat and wait weeks for the stars to align to be able to get my car to a workshop. I've got access to parts and space to work on the car so trying different sensors or parts isn't a big deal, I've probably got one tucked away somewhere.

Run down:

'99 WRX STI (or at least it is mechanically)
Standard ECU with MMS ECUTek tune
Kinugawa/"Tomei" UEL headers
AVO one piece under manifold intake pipe
Hyperflow TMIC with STI Y-Pipe and quality silicone hoses
Standard bypass valve
Standard VF28 turbo
3" turboback exhaust
Grimmspeed AOS (only including as my crankcase ventilation hoses have been altered)
SAAS boost gauge
Standard air box with resonator/snorkel removed

Car has been tuned for all of the above except the AVO intake pipe. Same symptoms occurred with the standard pipework.

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The issue: my boost gauge needle "flutters" under low load/cruising conditions, only when in vacuum, only after a few minutes of driving.

When I start the car from cold and drive until the car hits operating temp, the needle does not flutter. Makes all the usual turbo car sounds and induction noises, feels great, no hesitation. Idle is normal, doesn't stall out. Might fluctuate 20-50 rpm when it's working itself out right after starting, but settles down shortly after.

The gauge reads mm.Hg for vacuum and psi for positive pressure. Once the car hits operating temp, the needle starts to flicker/bounce very slightly when I'm cruising along with a little throttle held steady. I only get flicker between the -15mm.Hg and 0 psi range. If I let off the throttle, I get a steady -20 mm.Hg. Push the throttle on, needle stays steady through the positive PSI range.

As time goes on, the needle flickering/bouncing gets worse. Starts less than +/- 1 mm.Hg, ends up maybe +/- 3 mm.Hg. The car will still make positive boost pressure but feels like I have less power when using partial throttle. I lose the induction sounds, but the car is still smooth to drive. Idle is normal, doesn't stall out. Car is still steady enough in the positive pressure range, and when I let off the throttle I still get a steady -20 mm.Hg.

If I turn the car off (say to replace a sensor or to fuel up) and turn it back on a couple minutes later, the flickering/bouncing stops for a few minutes before it gradually comes back again.

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I have a few spare parts kicking around already, so I have tried the following;
- Brand new, genuine AFM fitted
- Brand new, genuine O2 sensor fitted
- Individually changed the three boost control solenoids on the drivers side strut tower with used units removed from a working car
- Had the bypass valve flange machined as it was pretty rough and crappy
- Changed the three intercooler gaskets with new genuine parts
- Checked the wastegate line restrictor pill is in place, and it is
- Most of the vacuum hoses have been replaced with new, have replaced any that look suspect
- Sprayed the exhaust joins with soapy water
- Sprayed around intake joins with aerostart

I also have access to:

- Innovate MTX-D boost gauge
- New MLS turbo gaskets
- Vacuum hose

Last edited by Patch; 08-12-2019 at 08:59 PM.
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