Engine smoke and knocking. AUDI A1 Sportback (8XA, 8XF)

@L. R. Sounds absolutely brutal mate. From what you're describing with the sudden coolant dump, massive smoke and loud knocking, I'd say you're looking at something way more serious than just camshaft or water pump.

That knocking sound combined with white smoke is classic head gasket failure, especially since the coolant was disappearing slowly before. The engine probably overheated internally even though the gauge showed 90 - by the time coolant dumped out, damage was already done. Had similar thing happen to my mate's Golf TDI, turned out head gasket let coolant into cylinders and when it tried to compress liquid... boom, bent connecting rod.

Agreed with Oscar, but could be even worse - cracked cylinder head maybe? The fact it was rebuilt in Sept 22 after blowing up before makes me think the rebuild wasn't done properly. If they didn't resurface the head correctly or torque it down right, head gasket goes quick. The orange coolant pooling by driver's side points to water pump area, but that knocking... that's internal damage for sure.

@Sperider is right, needs full teardown. But to answer your question - broken camshaft wouldn't cause white exhaust smoke, that's definitely coolant burning in cylinders. Water pump failure alone wouldn't knock either. You've got coolant in combustion chamber (white smoke) + mechanical damage (knocking). Probably head gasket + bent valves or damaged pistons from overheating/hydrolocking.

I've seen this exact scenario on a 1.6TDI before. When coolant gets into cylinder and you try to compress it, engine can't handle it - water doesn't compress like air does. That's what probably caused your revs to max out when you pressed clutch, engine was struggling. The knocking you're hearing now is likely a thrown rod bearing or cracked piston. Honestly mate, at this point you're looking at another engine or very expensive rebuild.

Just to add - that gradual coolant loss before the big failure is the telltale sign. Head gasket was probably seeping for weeks, letting coolant into oil and combustion chamber slowly. Previous owner ignored it until catastrophic failure. Check if there's milky residue under oil cap, that confirms coolant mixing with oil.

One thing though - you mentioned it had been losing coolant with no morning puddles? That means it was burning coolant through exhaust before this happened. Should've been white smoke on cold starts. The partial rebuild in 22 is suspicious, wonder if they even replaced head gasket then or just did bottom end?

The orange color of the coolant puddle is interesting - that's old/contaminated coolant. Makes me think the cooling system wasn't properly maintained after that Sept 22 rebuild. Could've had corrosion or deposits blocking passages, causing hot spots and eventual head gasket failure. But yeah, that knock is definitely internals, probably rod bearing gone from running without proper lubrication when coolant contaminated the oil.

@Thomas99 spot on about the hydrolock possibility. I had a diesel that sucked water through intake and similar thing - massive knock afterwards cos it bent a rod trying to compress liquid. With coolant leaking INTO cylinder through bad head gasket, same result. The smoke from engine bay might be coolant hitting hot exhaust manifold too.

Not trying to be negative but with knocking that loud and smoke from both engine and exhaust, you're probably looking at scrap engine territory. Even if you replace head gasket, the knocking means bottom end damage - bearings, rods, possibly crank. At that point cost of proper repair exceeds engine replacement. I'd source a good used engine rather than throw money at rebuilding this one again, especially since it already failed once before.

@L. R. - was there any warning before the revs maxed and smoke appeared? Like loss of power, strange noises, temperature spike? Just trying to understand if something seized and caused the clutch incident. Also check if timing belt is intact, though with that much internal damage it might be academic at this point.

@Harry77 in and caused detonation, valves can get damaged. But the LOUD knock suggests bigger components - conrod, main bearings. Camshaft damage would be more of a ticking sound, not a proper knock. Water pump is external, wouldn't cause knocking from engine bay.

Honestly the fact that someone did a "partial rebuild" after it "blew up previously" raises red flags. What does partial even mean? Bottom end only? Did they check head for cracks and warping? Sounds like whoever did that work cut corners and now you're dealing with the consequences. That engine's had it mate, time for replacement.

Yeah this is rough. Just for future reference for anyone reading - slow coolant loss without visible leaks = either head gasket seeping or EGR cooler leaking on these TDIs. Both let coolant burn off invisibly. Should always investigate immediately before it turns into this disaster. Good luck with it @L. R., hope it's not your car likeke you said!