I'm Rowan. Long-time lurker, and owner of a 2004 GT 5eat for about a year and a half now.
I was going to write this up just to explain how I fixed an unusual problem with the air box, but I thought I'd post a list of all the problems I've had as I've seen a few posts from people asking about dodgy/searching idle, low RPM power etc and how to fix it. What I've found, and what I assume is similar for most other people with second-hand/older cars, is that a combination of issues can present similar symptoms, albeit with very slight differences.
Some background: when I purchased my car it was in disrepair, although I didn't realise how badly until my first meeting with Matt (Kido). Originally my GT had a (very dodgy) MRT tune (previous owner hadn't mentioned this). Matt replaced it with one of his and told me I had a metric f*ckton of knock everywhere/maybe a bad turbo. I've seen him a few times now, and once on the dyno with Coyote (who told me I might have a bad transmission), and (in part thanks to Btssm) have become far more able to detect issues with my car. I'm fairly certain my turbo is good, and pretty happy with the transmission - my guess is that all the other problems made it seem like the turbo/trans was to blame.
Problems/symptoms (in chronological order):
+ Disconnected/dead crank case breather hose - lots of knock
+ Leaky stock turbo inlet pipe - lots of knock
+ Leaky stock BoV - lots of knock
---this is where I bought a btssm and was able to diagnose things better---
+ Dead control arm bushes - poor steering
+ Dirty injectors - required far more fuel in spool & high RPM region than healthy cars. This made a huge difference. As soon as I fixed it the car started to overboost by ~4psi (peak @ 18.9psi on stock turbo vs. 14.8ish pre-injector clean), but no knock!
+ Bad AFR sensor (bank 1 on the dump) - RPM dropping below 550 (sometimes below 500) at idle and AFR 'searching' (oscillation) at idle (between 13 to 16, or 12 to 18, or 11 to 20 depending on the day - changing the sensor has resulted in idle AFR being bang on 14.6 and RPM sitting where it should)
+ Old transmission oil (probably) - some hard shifts/TC not locking up (shitty low RPM torque). My current line of thinking is that all the other air leaks/AFR sensor/injectors made this seem like an issue when it might not be.
+ Worn out suspension - too much rebound oscillation after bumps & creaking.
+ Dodgy hose clamps (air box, intake pipe, BoV hose both ends, throttle body) - holy shit do I hate hose clamps, especially the worm type clamps with perforations (see pics). Symptoms were knock at low RPM/idle and loss of low RPM torque, and loss of boost in a range of ~-1 to +3psi. As an example I recently replaced the perforated clamps on the intercooler & turbo inlet. I loosened the clamps on both sides of the stock intercooler, however when I removed the intercooler the throttle body hose came off too! I didn't loosen the clamp on the TB side of the TB hose - it had literally fallen apart it was so worm out. I suspect this caused my turbo to have to work twice as hard in the spool range to compensate for the boost leak at the TB, low RPM torque is much better & choo choo noises are louder!
So onto this little walkthrough for the air box! Around a year ago I replaced my stock turbo inlet pipe with an AVO silicone pipe. AVO stuff has fitment issues - I get this now
. The AVO turbo inlet pipe always sat about half an inch further out (towards radiator) than the stock pipe, which resulted in some deformation of the MAF pipe (this wasn't an issue), but the deformation placed sheer stress on the outlet flange from my air box (essentially pulling it towards the radiator as well). Also I had a shitty hose clamp on the MAF pipe/air box that never stayed tight. Over time I had to keep tightening the hose clamp, and eventually it deformed the outlet flange (see pics). So I got a new air box, and only recently had the bright idea to stuff a piece of mild steel inside the outlet flange on the air box to strengthen it up. Turns out 2.75" mild steel (approx 70mm long) is a near-perfect fit and has worked a freaking treat - I can tighten the hose clamp as much as I like and the thing doesn't deform. The cost to me was $0 for the pipe - a local exhaust shop (Nelson Exhausts, Rocklea Qld) were nice enough to cut me the piece for free. All I did was grind the edges & put it in. Such an easy fix that I had to share it. An air leak at this flange and boost leak at the TB were probably responsible for idle/low RPM knock that I'd get, mostly just as the car came to a stop at lights etc. Also early morning 1st to 2nd gear shift seems a lot smoother now (still stereotypically harder than usual, as is common with these autos). My car sounds better and performs better than it did a year and a half ago, so I'm pretty happy and feel that it's just about as healthy as it could be now. There's heaps more I could say about specific symptoms for each problem I've had, but the air box fix was so easy I had to share. Thanks for reading!
P.s. apologies if my [img] skills are a failure... first post hah.
Mild steel pipe pre-install
Mild steel pipe installed
Old deformed air box flange
Good hose clamp (left) shitty hose clamp (right). I get these from Industrial Fittings Sales in Rocklea. Supercheap/Autobarn clamps have failed me too many times.
This is what happens to all the perforated clamps I've used - they all end up tracking off in one direction, which starts to loosen them over time.
