Possible single valve failure - 9A

Hey guys,
Just wondering if anyone could offer up any ideas on head problems. I was driving my car to the dentist the other day, in traffic moving at around 50mph, up hill. 5th gear (o2a), no strain etc.
Then for no reason I get this horrible clacking noise from the top of the engine, lose pretty much all of the power and the car begins to run incredibly lumpy. It feels like one of the cylinders has no compression. Starting is it like you have a spark plug out.
The sound doesn't happen when the car is idling, but as soon as you start to put load on it, the noise starts and gets progressively quicker. I haven't been driving the car, luckily I was close to home, so I managed to get back, very carefully ad now the car is laid up ready for surgery!

Now, is this a bent valve? any why would it only happen on 1 cylinder? The belt is fine, not snapped and not loose. Oil is all topped up, same with coolant. The noise is definately from the top of the engine, so I dont think its the bottom end.

But every cloud has a silver lining, wanted to do a rebuild for a while, so now is my chance and I have the caddy back on the road

Edit: What parts would I be looking at for a head rebuild? Stem seals, valve guides, valve springs, valves?

Do a compression test first of all, that will narrow it down to a cylinder

I had a octavia that died one day refused to start then ran lumpy as on easy start turned out it had a bent one valve and broke the bucket aswell like but swapped and it ran

double check the belt timing, it may have slipped a tooth. also check the physical location of the cams and pistons when top and bottom are timed to TDC, could be you have a sheared woodruff key on cam/crank sprocket

Hey guys,
Thanks for the replies.  I've Ben incredibly busy with work so hadn't had a chance to reply. Got a break yesterday and pulled the engine apart. I also found the culprit for the poor running.
See this picture…
Dropbox - Error - Simplify your life

It would seem I may be looking for a new head. I'm perplexed as to how this would suddenly happen. Its a symmetrical breach, there are nobits of metal anywhere and there is no damage at all. Not even chips out of the carbon build up.
Bottom end is fine though and hasn't got damaged mirrored on it. Head gasket had also been damaged in the same place.

Is something like this repairable? It seems a waste to write off a head for this small chip, or whatever you would call it.
I guess I'm looking at a new Kr/9A head. Would an abf head fit?

Cheers!

have the head alloy welded,ive had loads done at work,it will be fine
Bert

Cheers Bert! That's exactly what I was hoping to hear

What would be the cause of this kind of thing? Would it be down to gasket failure?

wrong ignition timing can cause problems like that,does it pink under load?
Bert

It wasnt pinking. The engine was running absolutely fine, all throughout the rev range.