[mkiv] boost problem (update)

Sat May 22 19:47:32 CDT 2010

Sorry in advance for the ramblings of this post, I'm just thinking out loud and passing on some personal experiences with the 3 TT Supras I've owned over the years, 2 of which started out 100% factory stock when I got them and both had issues very similar to yours by the time they were 10 or more years old... 

Lance knows a LOT more about these things than most people including me, but I've owned 2 bone stock TT auto's (drove the first one for over a year bone stock before selling it to buy my house, and fully BPU'd and True-Twin'd the second one within a few weeks of buying it due to annoying random sequential system issues) and my TT 6-Speed came to me with upgraded down-pipe and cat-back but it was also otherwise stock, and it's been my experience that with a truly "stock" TT Supra (factory smic and full stock exhaust) 8-11psi is typical "stock boost" range in warm humid weather (0.6 - 0.8bar of boost) and in True-Twin mode the boost doesn't really hit until somewhere in the low to mid 4k rpm range, unlike sequential mode that is a much smoother boost curve that comes on much sooner.  
True-Twin mode didn't really give me much more boost but what it did was make the boost far more consistent and the car much more predictable in my opinion versus sequential that was more like hit or miss at random when I floored it...  
With a brand new sequential system I'm sure the twins would be consistent but as the system gets some age on it, it just seems that its' control of the #2 turbo becomes less and less predictable, leaving you with about half the normal "stock boost" and wondering why your twin-turbo car suddenly feels so slow.  Between hardened vacuum lines, leaking vacuum lines, sticking VSV's, broken VSV's, etc, etc, the system starts to lose control and leave you without the #2 turbo more and more as time goes by.  I also find that the VSV's are far more prone to failure on cars that aren't driven very often and/or aren't boosted to the point that the #2 kicks in very often (If at all), as opposed to those cars that get pushed hard regularly their whole lives and have actually worked the system's VSV's regularly their whole lives...  It's fixable of course but in the meantime 'till you find and fix all the problems, the car is inconsistent and frustrating
 to drive if you drive agressively and it's even more frustrating to troubleshoot as you're finding now... 

Although it introduces about 1k rpm more lag, that's exactly why I choose True-Twin mode personally, even on an automatic's, although on the auto with the "stock stall" torque converter it ends up being slower in the 1/4 mile than one with a properly functioning sequential system due to the extra turbo lag of True-Twin mode but I'm okay with that... 

Unless I misunderstood, you were originally getting about 5psi when you began your troubleshooting process and with real True-Twin you're now consistently getting twice that so it sounds to me like the turbos are now both working together as they should be although there still might be a mild boost leak and it seems to really point mostly to issues with the sequential system from how I interpreted all your info so far. 

What does the car do in True-Twin mode if you ride 1st or 2nd gear up to about 4,500 rpm's then cruise in that rpm range for like 10-30 seconds, then go WOT and stay in to to redline, then shift quick into the next gear and go WOT again?  Do you get boost at WOT, is it the 10psi you get in higher gears above 4k rpm's, and does it hold well and pull strong the whole time and possibly even spin the rear tires? 

Granted I'm at BPU+ in true-twin mode but my 98TT 6-Speed does almost nothing 'till mid 4k's then turns into a fun little beast and working the gears to tach up high and have it still be up in the 4k range on shifts makes my car run strong through the 1/4mi and trap 118-125mph with consistent 12.0-12.2's and occasional high 11's with crappy 1.8-2.2 60ft's due to ongoing wheel-hop problems, and I'm not a drag racer at all, I prefer road courses myself. 

I also found that the 93TT auto in True-Twin mode was a slug below about 35mph but with the auto's gearing, once the tach hit mid 4k's the car always suddenly lunged and "hauled" for as long as you kept the accellerator down.  From a stop True-Twin sucked on the auto but at any speed over 35mph it lunged hard and fast whenever I went WOT... 
Then we BPU'd it and by mid 4k rpm's all of a sudden the car would break the tires free and get sideways without trac on and with it on the car would still spin' em and step out and you'd have to steer into it 'till the trac system kicked in and brought it back in line... 

Again, Lance is far better at diagnosing these sorts of things than I am and he is a huge asset to this list who is very greatly appreciated by everyone here that he has helped over the years including me, and this post is just my opinion so far on this topic and I hope it proves to be at least somewhat helpful for you. 

Thomas
myothersupra
--------------
98TT 6-Speed MkIV
93TT Auto MkIV (SOLD)
92T Auto MkIII
88 5-Speed MkIII
84 5-Speed MkII P-Type
81 5-Speed MkI
01 E-Shift IS300
01 5-Speed ZRX-1200R

--- On Sun, 5/16/10, Arif Khan <sl300gullwing at gmail.com> wrote:

From: Arif Khan <sl300gullwing at gmail.com>
Subject: [mkiv] boost problem (update)
To: mkiv at mkiv.com
Date: Sunday, May 16, 2010, 4:43 PM

After replacing the WG VSV, I wired up the actuators as outlined in the tech article on the mkiv site but unfortunately, there was no "ah ha" moment. Now, instead of building 7-8 psi, I'm getting a consistent 10 psi in 4th and 5th gear but no boost in 2 - 3 below 4k rpm.

Before doing the above I did confirm that there was pressure in the line before the check valve on the IACV actuator as well as the lower EBV actuator.

Furthermore, at the time I installed the replacement turbos I verified that all butterfly valve operations were smooth and unobstructed.

As of now, I still have to test the pressure tank circuit and the wastegate actuator on #2 turbo. There is also an actuator on #1 but I can't get to it without removing the air tube assembly.

Lance - 
While I was searching the mkiv archive I came across a set of posts by you in which your coils had gotten weak and presented symptoms that appeared to be boost related. Could this be a similar case?

Thanks,

Arif
sl300gullwing at gmail.com

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