Many modern cars employ modern variable nozzle turbochargers known as VNT. These turbos are designed to make engines more powerful with more low down torque, and are particularly good with Diesel engines. Volvo V70 D5’s use this system, and it’s the main reason why they drive so well and are so tractable.
This is an excerpt from a specialist’s website we use regularly:
VNT Turbos
Since the late 1990s many diesel cars have been fitted with a Variable Geometry Turbocharger or Variable Nozzle Turbocharger. These turbochargers are very effective in minimising the effects of turbo lag, resulting in a more responsive throttle especially at low engine speed. These systems work by changing the speed and direction of the exhaust gases onto the turbine wheel. The most common of these systems is Garrett’s VNT ® mechanism which incorporates a ring of small movable vanes around the turbine wheel. These vanes are often referred to as the speed control mechanism. At slow engine speeds, the vanes are in the “closed” position narrowing the gap between them which effectively accelerates the exhaust gas onto the turbine wheel. At high engine speed the vanes open up slowing the exhaust gases, which stops the turbocharger from over-boosting. In most cases this level of control negates the need for a conventional wastegate. Despite these benefits, such turbochargers can be prone to problems. The speed control mechanism is easily affected by carbon build-up, which, if it becomes excessive, can cause the mechanism to jam. This can occur quite quickly if the vehicle is used for lots of short journeys where the engine is not allowed to get up to full operating temperature. It could also be indicative of a fuelling problem.The mechanism normally jams in either the fully open or fully closed position resulting in no boost or too much boost from the turbo. If the turbo over-boosts there is a real danger that its internal workings will be damaged, resulting in the need for a complete replacement unit. In many cases, the computer controlling the engine will sense that there is something wrong and will severely limit the engine’s performance in what is called a “limp-home” mode to prevent any further damage. You may also notice black smoke under acceleration, as unburnt fuel is ignited in the exhaust.
We have had a large number of customers who have suffered badly with their D5 powered Volvo’s falling into limp gone mode for no real reason, and when we have read the codes it’s come up with turbo pressure high or low, boost pressure sensor, air leaks etcetera. Before coming to us, some of our customers have spent a fortune replacing bits and bobs and not curing the faults caused by this major issue.
So, if you have a D 5 that’s down on power, or goes into limp hone mode whenever you put you foot down, then this could be your problem…
