I wanted to give a 'heads up' to anyone who is following Smog's "Fast AAZ" mod list from the FAQ, who is having trouble getting adequate boost from their K14 turbo. Took me 9 months to figure out the [trivial] cause, so I write this long post in hope of saving someone similar frustration.
"Cut To the Chase" version: make sure the locknuts on the threaded wastegate actuator rod are at the EXTREME end of the rod. It is only the last 1/4" or less of the threads that make any useful improvement on boost pressure from a K14. If you want a solid 15 psi to build up fast from these turbos, then go even farther with the locknuts than the photo in the Smog FAQ shows (which looks already to be within a couple of threads of the end of the rod).
Ok, now the long version, symptoms & diagnosis:
Symptoms:
I thought I had done the Smog mods properly, but I was always disappointed with my boost pressure. It climbed to 10 psi relatively quickly, but anything after that would take a full throttle run, at full load, and high rpms. I became convinced that the AAZ with stock K14 turbo simply would not build over 12 psi of boost, except for sometimes wavering at 12 psi for a second or two at most, at nearly 4000 rpm, and then dropping back to 10 or 11 psi.
I tried installing a boost bleed in the vaccuum hose between the turbo compressor housing and the wastegate diaphram can, but the boost bleed had little effect. I even blocked this hose off altogether with a golf T, so the wastegate would never get a pressure signal, but nothing changed. This really surprised me, because the very same bleed device in the wastegate hose had already allowed me to increase the boost on a gasoline-powered turbo Volvo from 8 to 14 psi. The bleed achieved this increase in the Volvo with zero other mods. So by comparison, the VW turbodiesel engine had me baffled.
I read other people's experience here on GTD with their own boost pressure, and people here would talk about data points like 14 psi at 1700 rpm, and I would think, "What the heck are they doing different? That's impossible!"
Solution:
I asked for help in another thread here, and it was suggested that I was underfuelled. "You need fuel to get boost". So I turned up the main fueling screw about 1/4 turn until I was getting a noticeable improvement in torque, but also great clouds of black smoke under low rpm/high load conditions.
With the main fueling screw turned up, maximum boost pressure did improve a bit, by 1 or 2 psi, so that I could then see boost up as high as 13 or 14 psi, but again, only under full throttle, full load, high rpm conditions. And again, the rate of boost pressure would build really slowly after hitting about 9 or 10 psi, and only when the revs built over 3000 or 3200 rpm or so, in 3rd or 4th gear (Vanagon 4-speed trans).
I reported back that increased fueling had solved the problem. But actually, it simply masked the problem.
The real issue turned out to be my failure to unscrew the locknuts on the threaded wastgate acutator rod, to a sufficient position towards the end of the threads.
I had put my locknuts to about where the photo on the Smog FAQ shows them -- more or less at the end of the threads, but with the nuts maybe one or two threads in from the end of the rod, just for what I thought was enough "thread meat" to secure them tightly.
Yesterday, after a suggestion from Dave (935Racer - thank you!) to look at the wastegate, I experimented by moving the locknuts to the EXTREME end of the threads. The "outside" locknut is now about half a thread BEYOND the end of the rod threads - still with enough "thread meat" to hold it there securely -- I think, I hope.
The test drive afterwards was a revelation. Boost rocketed up to 17 psi by about 2000 rpm in second gear. This, even though I have turned the main fueling screw back down, and show zero black smoke under load.
I have never seen boost build so fast in a turbo motor, not even a gasoline motor. Accelleration was a night and day improvement, in every gear. Nirvana.
So, the moral of the story, for anyone trying to make adequate boost on their stock K14, is to get the locknuts to the EXTREME end of the threads on the wastegate actuator rod. Put them even farther than the photo shows in the Smog FAQ (which is already pretty far!).
It is only the last 1/4" or less of those threads that make a blind bit of difference, even though the threads run for about 3" in from the end of the rod.
I can't understand what KKK was thinking in running the threads so far inwards from the end of that rod. Maybe there are applications where you would want only 1 or 2 psi max boost from the K14, which is probably what you'd get if you moved the locknuts even one inch in from the end of the rod.
Topic: AAZ Low boost problem - symptoms & final solution (Read 8838 times)
