Hi, I have a 2019 Fiat 500X and for a few weeks now I've been hearing a metallic knocking noise when I accelerate, especially going uphill. It's not constant, it goes away when I'm driving slowly. I've already had a recent service. Could it be the spark plug? Or something more serious with the engine?
It might not just be pre-ignition, even though that's the main suspect with that symptom. Something similar happened to me years ago and it turned out to be a too-lean mixture caused by a dirty oxygen sensor - not exactly a failure but pretty sneaky - the mechanic cleaned it and the knocking disappeared. If your service was recent they probably checked the spark plugs, but verify they used the right heat range for your car, because a wrong spark plug - even if it's new - can create exactly this effect under load.
I wouldn't say it's necessarily the spark plug, because that usually gives different symptoms. What you're describing - the metallic knocking under load and on hills - sounds more like actual engine knock, meaning the fuel is igniting before it should. It can happen for several reasons: fuel quality (maybe you're using a cheaper brand than usual?), carbon buildup in the injectors, or the knock sensor not communicating properly with the engine control unit.
Since you had your service done recently, the oil is probably fine. What might help is trying better quality fuel for a few fill-ups - I know it sounds obvious, but I've seen it fix problems like this. If the noise keeps happening, I'd get the injectors and knock sensor checked; it's not super expensive, and it's better to rule out the problem now rather than leave it hanging. A 2019 500X should have plenty of years ahead of it, so don't immediately jump to the worst-case scenario, but don't leave it completely unresolved either.
I heard a similar noise on my car when the gas wasn't quite right, and the mechanic told me it was pre-ignition (fuel igniting before it's supposed to). With a 2019 500X it could be that, especially if you accelerate hard uphill; try using higher quality gas for a couple of fill-ups and see if it changes. If the noise persists, then yeah, it could be something with the engine like dirty valves or really worn spark plugs, but at least rule out the gas first since that's the simplest and cheapest thing to check.
The knocking in the head on a 2019 500X when you accelerate uphill is almost always pre-ignition, like the others said. But the thing nobody pointed out is that if you've had a recent service, check what kind of fuel you're using. On that car Fiat recommends 95 octane as a minimum, but if you drop to 91 or use low-quality petrol, the engine starts doing exactly what you're describing.
Try filling up from the same pump where you're sure about the quality (brand distributors like Eni, Shell) for a couple of fill-ups and see if the noise goes away. If it persists, then yeah you could have a more serious problem like worn spark plugs (even though the service was recent, sometimes the new ones aren't always great) or carbon buildup in the engine. In that case it's worth taking it to your mechanic so he can run a diagnostic with the scanner.
Listen, the others have already told you pretty much everything but one thing I didn't see mentioned - if you just had your service done, maybe they put in oil that's too thick or not the right brand, and that stuff can cause weird engine issues! I'd try first filling up with decent fuel (like Eni or Shell) and wait a couple of days to see if the knocking goes away, before thinking about broken sensors.
if it keeps going then yeah, consider pre-ignition or the oxygen sensor like the others said, but time will tell you a lot - it's not always something serious!
One question: have you checked if your oil is at the right level? Because everyone focuses on pre-ignition and fuel, but oil that's too low (or really dirty despite a recent service) can cause internal friction that produces exactly that metallic knocking under load. Before you head to the mechanic for sensors and injection, take a look at the dipstick. If the oil's fine, then yeah, it's probably pre-ignition and you should try better quality fuel (maybe a well-known brand) for a couple of fill-ups and see if it changes.
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