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CASE STUDY: Audi S7 – engine stalling


By Clive Atthowe

We had a customer bring in their Audi S7 V8 twin turbo recently, complaining that the engine was stalling and was then difficult to start. He also mentioned that on starting, an unusual noise was heard. The vehicle had been recovered to the workshop and after checking visually for any signs of issues, alongside basic levels, we started the car with no apparent issues.

After allowing the engine to warm up at idle we then increased engine RPM, it was then that we heard an alarming noise which increased with engine speed but was also present on lifting off the throttle. The noise appeared to be coming from the turbo area of the engine and a severe vibration could be felt on the n\s turbo. After consulting with the owner, we stripped off the inlet pipe work to inspect the turbos. The n\s turbo bearings had failed completely, and the compressor wheel was machining itself into the housing. There was also wear in the o\s turbo.

We decided to see if there were any technical bulletins relating to the failure and there was a known issue. The symptoms matched the customer’s description of stalling followed by poor starting. The turbos can be starved of oil when a small oil strainer where the turbos oil feed originates from becomes restricted and reduces the oil flow. This results in turbo bearing failure.

Having established the failure and cause, we again consulted with the owner and he approved the repair, which was quite substantial. It requires putting the front of the car into service position, removing the turbos complete with manifolds and stripping out the centre cover to access the oil strainer and non-return valve.

The oil strainer was replaced along with a new non-return valve and we could start to rebuild.

Two new turbos were installed along with all new oil feed and coolant pipes. All gaskets were also replaced, as well as new studs and fittings as required. A new, genuine air filter was installed and we introduced fresh engine oil and new filter. The cooling system was vacuum filled with new, genuine antifreeze and with the oil system primed, we ran the engine up to temperature. After inspecting for leaks, the final check was an extended road test to confirm correct operation and boost.

The vehicle was restored to normal operation with normal starting idle and turbo boost.

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