The problem would appear to be the VNT mechanism on the turbo has got stuck / sooted up, which is a fairly common problem on many engines fitted with these systems.

The VNT is designed to produce more consistent boost across a wider rev range, by changing the angle of attack of the vanes against the exhaust gas flow. At low revs, the vanes adopt a more "aggressive" angle so that the turbo spools up quickly with minimal exhaust gas flow. As revs, and exhaust gas flow increases, the mechanism reduces the angle of attack of the vanes to maintain a constant level of boost. It also has the benefit of reducing back pressure in the exhaust under appropriate conditions, which benefits economy and emissions.

What typically happens in these limp mode scenarios is that the mechanism gets gunked up with soot and stops moving the vanes properly. Typically, the vanes stick somewhere in the more aggressive "low RPM" range all the time, meaning too much boost is generated at higher revs as the vanes don't back off as they should. Apart from the overboost problem, this also stresses the turbocharger as it spins harder and faster than it was designed to, and the extra restriction of the vanes in the path of the exhaust gases can harm economy and emissions.

A remap of course "creates" these overboost conditions, and will prevent the engine management from logging an overboost fault, but the overboost created by a remap is known and controlled. The map is developed with careful monitoring of boost levels, turbo speeds and so-on, and with everything kept within a reasonable margin of each component's ultimate capability. What you have with a stuck mechanism is an unknown, plus of course the engine management system no longer has control of the turbo vanes. This is likely to affect response all through the rev range, as the stuck position of the mechanism is only optimum for a given set of conditions. In other words, the vanes will be in the wrong position for responsive acceleration far more than they will be in the right one.

For the price of a good remap (300 quid or so), you could probably get uour turbo repaired, or an exchange unit supplied which will actually fix the problem. Unlike the VNT system on certain Fords and other cars, the system on the VW engine is mechanically activated, and relatively simple. It can be freed off and the turbo cleaned and returned to service by someone with the appropriate skill and experience.

I think running the car in that state with limp mode disabled will simply cause the turbo to overspeed, and eventually fail completely, worst case taking the engine with it.

I wouldn't chance it personally.

Edited by DP on 31/03/2010 at 21:28