It is a common fault on the old 600 turbo for cracks to appear in the manifold.
This leads to loss of pressure and escaping waste gas, if this occurs the lambda sensors get confused by the readings and the engine will pulse under normal driving.
The cracks aren't very easy to spot but some can be very obvious as seen below.
These cracks are not just inherent on the 600cc turbo design. Although at first it looked like
smart had fixed the problem, it was clear that they had just delayed it by using a thicker manifold.
The extra thickness slows the formation of cracks but physics says that once a crack does appear,
it will be bigger and more of a problem of the cracks in the thinner 600cc turbo manifolds.
Fudge from Fudgesmart has already discovered a 700cc turbo cracked so bad, it fell into 2 pieces.
Why Do These Cracks Form?
Basics physics tell us that as a metal heats up, it expands, and as it cools, it shrinks. This
constant movement opens up fault lines and weak points in the casting causing eventual cracks.
This problem is made worse because the turbo manifold doesn't heat up evenly and the difference
in temperature between 2 points on the manifold can be as much as 150 degrees celcius.
It is this difference in heat that causes the manifold to expand more in particular
areas which in turn creates stress points between there and the cold areas.
Because the turbo is bolted to the engine, the engine acts as a heat sink, taking a lot of the
heat away from the manifold base. However, where the hot exhaust gas meet each other,
the temperature increases with no way to shed the excess temperature.
I Heard They Used Poor Quality Metal
There was a white paper a few years ago, checking the quality of the metal used in the turbo manifold. Their conclusion was that it wasn't suitable for the varied temperatures put in it.
The findings are partially correct although a well designed manifold would have stopped the cracks.
Here is a copy of the white paper.
Can I Weld The Cracks Up?
You can, but the heat created in the welding process causes more cracking over time.
All welding does is move the cracks along the manifold.
What Is The Answer?
Theoretically, lagging the turbo to keep the heat in should go a long way to evening out the heat stress as there should be less cold spots. If you do attempt this, use proper turbo lagging and be careful not to interfere with the wastegate actuator or wastegate arm.
Of course, the obvious choice is to have a new turbo fitted. Not a cheap option though.
Depending how hardcore you are, you could try out a method suggested to me by Luc Job.
Luc had noticed his manifold had cracked and realised that welding alone wouldn't solve the problem, it would just mask it for a while. He understood that the difference in expansion over
the manifold was causing the problem so undertook a radical step to relieve the stress.
As you can see, he has drilled through the manifold base plate underneath each branch, being careful not to drill into manifold pipes themselves. This in itself allows a lot of the stress to be passed along instead of the cracks forming above. You'll also noticed the cracks have been welded.
On a closer inspection you will also notice that the 2 middle fixing holes have been cut through.
This essentially makes the manifold base into 3 sections allowing for more individual movement.
The repaired turbo was replaced and no cracks have reappeared. The cracks appeared at 50,000km, it was repaired and lasted to 150,000km when the engine needed reconditioning.
This turbo was fitted to the reconditioned engine and has now done 240,000km.
Still no cracks have reformed.
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