Engine Start Problems: Hard Knocking Noise

Hello everyone,

"Recently, when starting my Audi A3 engine, there's been a noticeable loud bang. After the starter motor reaches the start speed, the engine kicks in with a hard thud (a "kawumm" sound)."
I find this behavior a bit strange, especially since it doesn't sound very healthy for the engine.
What could be the cause? The initial amount was not adjusted upwards.

Could a leaky injector nozzle be a possible explanation? If diesel continues to drip into the combustion chamber from the fuel line due to residual pressure after the engine is turned off, and then ignites abruptly when the engine starts...

Hopefully not, because the injectors are still relatively new (they've only been used for about 50,000 kilometers).

Thank you for your theses/proposals.

Regards,
Alex.

donalexo wrote:

Could a dripping nozzle be a possible explanation? If, after stopping the engine, diesel continues to drip into the cylinder due to residual pressure in the line, and then ignites abruptly during engine start...

Hi Alex,

I find that less likely because the fuel injection amount during startup and warm-up, depending on engine temperature, is close to or even below the stoichiometric air-fuel ratio (lambda = 1).
Additional diesel fuel (from a puddle in one cylinder chamber) could therefore, in my opinion, hardly be converted into a "particularly violent" combustion due to the lack of usable oxygen in the cylinder .

I'm more inclined to think it might be a delayed ignition: some air gets blown out in front of the injector nozzles before the diesel arrives, preventing normal pre-injection, and the "bang" is a single, sharp combustion event.
The question is how air keeps getting into the system - could it be a faulty pressure relief valve?

Could the knocking sound possibly be caused by something mechanical (like the engine shaking against something), rather than an ignition problem?

My first thought was something mechanical, like a loose torque support.
Then it occurred to me that you had the clutch replaced, which confirms my suspicion.
Was everything reassembled correctly during the installation? Was the flywheel also replaced, since foreign objects entered and damaged the clutch?
Ich könnte mir vorstellen dass dieser Schlag dadurch entsteht, dass der Starter die Schwungscheibe gegen den widerstand des Motors anstartet. As far as I can tell, everything is normal.
When the first ignition occurs in the cylinders, the crankshaft kicks against the flywheel.
Almost from one attack to another.
Since I didn't remove the flywheel during my clutch replacement, I don't know how it's secured. Is it only held in place by the four screws, or is it also fixed in a specific position by a groove or pin?

And? Are you still driving around with a faulty engine?

Yes, the engine is still misfiring during cold starts.

I can rule out a loose aggregate mount or a damaged pedestal support because I checked them about two weeks ago during the oil change.

I've already gotten so used to the "WUMMMS" sound during startup...

Regards,
Alex.

Do you still feel the "kick" even when it's gradually increasing?