The brief
Fluid puddles had been forming under the front of the A4. The cabin caught a burning smell after longer drives. And the dashboard had finally flagged low oil.
That combination, puddles plus a burning smell plus a low-oil warning, says oil is getting out somewhere and reaching hot parts on the way down. But the front of an engine has several places oil can leak from: timing cover, cam cover, oil cooler, oil filter housing, sump. The job was not just to confirm it was an oil leak; it was to find which one, because guessing wrong on an engine leak means doing the job twice.
The diagnosis
We pressurised the engine on the lift and traced the leak with UV dye, which lights up the actual leak path under a UV lamp rather than leaving you guessing from where the oil ends up.
The dye trail led to the oil cooler housing seal. Oil was weeping along the gasket face and tracking down behind the cooler, then spreading across the lower bay, which is why so much of the underside was caked in baked-on residue. The other gaskets and seals on the engine checked out clean under the lamp. So the oil cooler seal was the single source. On this engine the oil cooler is plumbed into the cooling system, so getting at it means draining coolant and disturbing a couple of hoses, which got changed while the system was open.
The work
Drained enough oil to clear the cooler area, then removed the oil cooler. Cleaned both mating surfaces, the cooler and the housing, back to bare metal, because any leftover hardened gasket material stops the new seal sitting flat.
Fitted a new VAG-spec oil cooler seal, retorqued the cooler bolts in the correct pattern, and changed the connecting coolant hoses while the cooling system was open. Refilled with fresh oil, then held pressure on the system to verify the seal before the car went out.
The outcome
No drips. Oil level holding. No burning smell after a long drive.
The A4 went home with the leak source confirmed and the engine sealed properly. For the owner, that means no more puddles on the driveway, no more smell in the cabin, and an engine that keeps its full charge of oil.
And because the leak was traced rather than guessed, the right seal got done the first time, rather than chasing it from one suspect to the next while the oil kept coming out.
