Engine makes tapping/knocking noise all the time, won't rev correctly and has no power

Question

I own a Rover 25 1.4l 16v 103hp (naturally aspirated petrol engine) from 2003. I've had some issues recently, and I don't know if they're related to each other, so I'll start from the beginning.

Since the end of November 2018, I've noticed that sometimes my car intermittently losed power at really low RPM (below 2000 at the very beginning). As I was looking for the cause of it, it got worse, and I started sometimes to get big misfires all over the RPM range with the check engine light blinking and the fault codes P0302 and P0313 if I remember (not sure of the second one) (sometimes it happened during few seconds, sometimes until I parked and stopped definitly, sometimes it ran perfectly well).

I finally received and changed the fuel filter and the injectors just before christmas, but the next time I took my car, it ran well until it got hot, and then, it started misfiring and losing power like never before, like it was running on 3 cylinders or so. I decided to look at the ignition system (I did not really suspect it before because it was in perfect condition at the end of this summer when I replaced the head gasket that was slightly leaking), and discovered that one of the ignition leads was litteraly burnt inside, there were carbon deposit all over the spark plug connector (not inside the cylinder). I ordered new leads, and when I received them about 2 weeks later, replaced the old ones with them (and the spark plugs too).

The first times I tried to restart the engine, it cranked correctly but almost didn't seem to fire and it did not start at all. After trying to replace the spark plugs with the old ones, testing them and the leads (I actually get a spark)... I managed to start it roughly but it would stall at idle and I needed to throttle out to get it to run. After many tries, it started to run without stalling, but it made a tapping or knocking noise, like dry tappet noise, but maybe louder, and it didn't disappear as the engine lubricated and heated up.

Since it makes this diesel-like noise:

• I've checked oil level which is good, and added Wynn's pre oil change cleaner
• I've listened to the engine with a mechanic stethoscope and I'm pretty sure I hear the noise louder from the cylinder head, near the timing belt and the inlet camshaft
• I've unplugged the coils while running and it seems to run on all cylinders (at least most of the time)
• I've removed the rocker cover to check if I can see anything, worn out parts, metal shards... But there was nothing. I also checked if I could push the hydraulic tappets, and I cannot
• I've checked fault codes to find a P0340 code, meaning bad camshaft sensor, that I replaced yesterday, but it hasn't improved nothing (except I have no more fault codes)

Yesterday, I also decided to take it for a short ride (1 km at most) after it fully heated up despite the noise, to see if it's driveable.

• There was a lack of power all over the RPM range, but it was clearly worse when I revved it up
• It couldn't rev over 5000 or 5500 RPM in neutral and oscillate greatly when full throttling between maybe 4000 and 5000 (It normally revs at 7000 RPM)
• I could really roughly (else couldn't) speed up once I reached 3000 RPM (where it's supposed to start to speed up) and even reaching it is not as simple as it should
• It sounds really metalic when I accelerate (like a ferrari exhaust or something, but it comes from my engine)
• I shut off the engine going down a slope, to hear if the noise continued or not without explosions, and it does, I noticed that when I throttled out with the engine shut off I heard the noise like it resonnated greatly in the inlet, it was way more audible

Any help would be welcome, do not hesitate to ask for more information.

Best regards

Update

This weekend, I removed the cylinder head to see what could cause the noise. The answer is simply that my valves have hit my pistons (and two of the 8 inlet ones are significantly bent to let us see the curve with the naked eye). I don't understand what could have caused that because before removing the head, I checked the timing belt and it was correctly aligned, no offset, and the tensioner was well tighten too (they both are 6 month old and at most about 3000 km).

Answer 1

Bent valves mean that the pistons hit them (timing) or the valves didnt retract (valve float).

The hydraulic lifters are excellent at low and mid-RPM but can cause valve float at high RPM as the mechanism that allows zero-lash at low RPM works against the valve at high-RPM.

As engine rpm increases, the bleed down rate inside the lifters may be too great. There may not be enough time to refill with oil between each valve cycle, causing the lifter to collapse. Or, if the bleed down rate is too low and the lifters retain too much oil, they can pump up and overextend the valves. Either way, you can end up with valve float, misfiring and loss of power.

https://www.enginebuildermag.com/2012/01/hydraulic-camshafts-and-lifters-101/

If there was even one time where the engine went into an overspeed condition, you could have bent the valves. This can happen simply by selecting too low a gear on a downshift for example.

Answer 2

So, as I said in the update of my question, the noise and loss of power came from the bent valves. I have replaced them all (the 16 valves) and cleaned the hydraulic followers, everything seems fine now, it runs well, but I still don't know why pistons and valves have met.