How variable geometry works
Around the turbine wheel sits a ring of movable vanes. At low revs the vanes close and direct the gas at high speed onto the wheel – the turbo spools quickly and there’s no lag. At high revs they open so the engine isn’t over-boosted. Their position is controlled by a vacuum or electronic actuator on command from the engine ECU.
Signs of sticking geometry
- Limp mode under acceleration that clears after a restart
- P0299 (underboost) or P0234 (overboost) codes
- Pronounced turbo lag and sluggish response at low revs
- Power only in a certain rev range
- A stiff or squeaky actuator rod
Cleaning or replacement
If the vanes are just dirty, cleaning and set-up usually fix the problem completely. But if the pin holes are worn, the vanes are distorted by heat or the ring is cracked, the mechanism is replaced. Either way, the final set-up is crucial – badly adjusted geometry throws fault codes even on a brand-new turbo.
It is also important to deal with the cause of the soot build-up: long periods of low-rev driving, a faulty EGR valve, injector problems or a blocked DPF.
Frequently asked questions
Do geometry cleaning additives work?
They may help with light fouling, but they are no fix for seized vanes. Mechanical cleaning and set-up are the reliable route.
Does the turbo have to come off to clean the geometry?
Yes, for a proper clean the turbo is removed and the turbine side is dismantled.