09/12/2021
BLOCKED DPF & TURBO FAILURE
A blocked DPF prevents exhaust gas passing through the exhaust system at the required rate. As a result back pressure and exhaust gas temperatures increase within the turbine housing.
Increased exhaust gas temperature and back pressure can affect the turbocharger in a number of ways, including problems with efficiencies, oil leaks, carbonization of oil within the turbo and exhaust gas leaks from the turbo.
Some ways you can tell if a turbocharger has suffered from DPF problems -
-- Discolouration of parts within the core assembly (CHRA) usually with evidence that the heat is transferring through the CHRA from the turbine side. This excessive temperature within the CHRA is caused by back pressure forcing the exhaust gas through the piston ring seals and into the CHRA. The high temperature exhaust gas can prevent efficient oil cooling within the CHRA and even carbonise the oil, restricting oil feeds and causing wear to the bearing systems. This type of failure can often be mistaken as a lack of lubrication or contaminated oil.
-- Carbon build-up in the turbine side piston ring groove caused by the increased exhaust gas temperatures.
-- Oil leaks into the compressor housing can be seen as a consequence of exhaust gas forcing its way into the CHRA from the turbine side and forcing oil through the oil seal on the compressor side.
-- A blocked DPF can force exhaust gas through the smallest of gaps, including the clearances in the bearing housing VNT lever arm and turbine housing waste gate mechanisms. If this occurs, carbon build up in these mechanisms can restrict movement of the levers affecting performance of the turbo. In some cases soot build up can be seen on the back face of the seal plate where the exhaust gas has been forced through.
Turbine wheel failure through high cycle fatigue (HCF) caused by temperature increase.
How can you prevent these failures from occurring -
Identify the failure mode and determine whether a DPF related issue is the root cause. If the entire rotating assembly is okay and there are some signs of overheating towards the turbine side of the core assembly then there is likely to have been excessive exhaust gas temperatures. High amounts of carbon build-up within the VNT mechanism and lever arms indicate a blocked DPF and the driver may experience turbo lag or overboost of the turbo.
It takes time for a DPF to block, sometimes years. Once blocked though, turbo failure can occur very quickly. If you don't check for a DPF issue when installing a replacement turbo, there is a very high chance the replacement turbo will suffer the same failure as it will be subject to the same operating environment as the previous unit.
Newer model cars use "active regeneration" - the process of removing the accumulated soot from the filter by adding fuel post combustion to increase exhaust gas temperatures and burn off the soot, providing a temporary solution. Older cars used the "passive regeneration" which took place automatically on freeway type runs when the exhaust temperatures is high. The move to "active regeneration" was due to the fact that many drivers do not often drive prolonged distances at freeway speeds to clear the DPF. Constant short distances are not good for the turbo or the exhaust system!
further details:
https://www.melett.com/technical/why-turbos-fail/can-dpf-cause-turbo-failure/
https://www.techtips.ie/Melett_Turbo/a-blocked-dpf-can-cause-turbo-failure.pdf--
