Sorry about the long post, I am not quite sure what is relevant regarding this matter, so I tell everything which happened before the knocking noise from the engine.
So, I have a 1985 euro-spec 1.series 500 SEC with a M117.963 engine. The car has been sitting over the winter after oil change and some other basic maintenance last autumn. Since then I only drove the car about 15 miles to its garage. I have then started the car few times in the winter and let it warm, not really driven it other than out and in of its garage this spring.
During the last couple of days I have been doing some more maintenance, such as changed all the belts and new fuel injectors and their holders. I also took off cam covers and replaced the plastic cam oiler tube holders. Couple of the holders broke off but I was able to collect the few broken pieces out with forceps from their attaching holes. I checked cams, rockers and chain tensioners visually, and everything looked good. Actually I was surprised how shiny and smooth all the cam glide surfaces were. There were only little signs of wear despite the 161 k mileage. I put everything back together and checked all the fuel and vacuum connections. (Eventually I found out that two fuel lines didn't seal tightly so I needed to re-seat them to injectors.)
I started the engine, and after somewhat long cranking (which was anticipated as I had replaced the injectors and taken off some fuel lines) the engine setteled to a 2500 rpm idle. I turned off and started the engine altogether about four times while trying to figure out what was wrong. Eventually I found that, for some reason, the throttle linkage was adjusted so that the throttle butterfly flap would be partly open even on idle. Apparently this worked fine with the old injectors and seals, although I did previously wonder the initial startup behaviour which was somewhat different than in my 300 E-24. Hard to explain and probaply not relevant, but anyway, after adjusting the rod the car started fine and setteled immediately to a 1000 rpm cold start idle.
When the engine was idling 2500 rpm the last time before the linkage adjustment, just before turning off the engine, I noticed some sound from the engine. I thought it was the viscous coupling as it sounded a bit like the coupling switching on/off so I didn't pay attention. So, next time I started the engine the idle was ok, but now I was hearing a quite loud knocking noise. The engine has had a very slight knocking sound before, which had been thought to be a hydraulic lifter (aka hydraulic valve clearance compensating element), but this noise was definately louder. Giving some revs didn't help to stop the noise. From what I was able to hear, the noise was coming around the no. 5 cylinder (drivers side bank, the nearest to the chain) It was so disturbing that I didn't want to idle the engine anymore. I took off the camshaft covers again, hoping to find some obvious reason. Of course, there was nothing obvious, everything looked ok, even the chain tensioners.
I then called a friend how knows more and searched how to test the hydraulic lifters. I tested the lifters according to factory procedure, which is to press down the hydraulic lifter with, for example with a handle of a hammer, when it is not under tension (particular cam tip in upward position) I hadn't done this work previously, but I guess I did it right. I did not notice any downward motion on any of the lifters although I pressed as much as I could, so I thought they were ok? When rotating the engine by the crankshaft bolt, I did wonder the hissing sound which was coming from the no. 5 cylinder right after rotating even a small turn. I sounded like a pressure which was relieved after a few seconds. You can hear it quite well from the video.
After all this, I installed the camshaft covers once more and checked everything was ok. I cranked the engine which started and idled fine, although the knocking sound was still there. I decided to let it idle and see if the sound would go away (still thought it was jammed lifter or something). The engine had been idling about 4 minutes, whichafter, in about 4 seconds time I noticed the knocking sound to transform somehow. This made me to rush to the key and immediately turn off the engine. I did not want to wait and hear what happens next.
Now, the situation is that I have a car which I am too afraid to crank again. As I had done some work on the engine just before this knocking noise, It makes me wonder if I somehow screwed up something, but I really haven't figured out anything what I could have done to cause such a noise.
I have thought of possible causes:
1. I have dropped a tool in the cam chain housing when the covers were off
2. Cam chain tensioner broke off
3. Connecting rod bearing broke for some reason in less than 10 minutes time
4. I have dropped something inside the inlet manifold via injector holes (don't know if it would even cause anything major. None of my tools, bolts or nuts were missing)
I would greatly appreciate all the suggestions and ideas! What would you do in my situation?
Basically I'll have to get the car on a flatbed to the local garage which has most probaply zero experience on M117's. I would need to give them some suggestions about what to do with it. What I have thought of is to change all the chain tensioners, check the lifters and cams once more and take off the oil pan to take a look on the crankshaft and such. Anything else?
I am somewhat shocked on the course of events since I have been servicing the car for days, and now I have it in my garage even afraid to crank it. Thank you for reading despite my somewhat poor english.