Very cool setup.

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Prost!

Although it works incredibly well, it's not very refined.
Im going to add the ability to test sparkplugs/ignition coils tonight, obviously not while conducting injector tests at the same time....

Wouldn't looking into the cylinder with a camera through the spark plug holes be easier than pulling the head?

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

It would but I don't have one.
The cost of buying one would probably be the same as the parts required to open her up. Then I can take a look at the damage caused by the part of the injector that fell in the cylinder and investigate the noise in the cylinder. I take it I'm only going to need the 2 gaskets?

You can get a very good scope camera for under $200, and on the cheap end, a decent usable one for under $50

I have a Bosch one, it was about $130 or so....

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

After seeing the work needed to remove the head on WIS I decided to follow your advice and got an endoscope. Waited more than a week for the overnight delivery and finally got it today.

Cylinder 5 looks quite good inside including the area above the intake valves. There is no sign of the missing injector ring, but...

I decided to look at cylinder 1 for comparison and found what looks like a broken ceramic ring laying next to the intake valve:
https://www.dropbox.com/s/f7e4h1m75146bif/2014-02-15_04-40-08-PM.jpg
The other half of the ring is laying next to other valve.
I cant get them out with my vacuum so will probably have to open it up.
Is this part of the stem seal?

I decided to check cylinder 2 and found a rat turd in there:
https://www.dropbox.com/s/1d5jr7h191j2m96/2014-02-15_05-14-49-PM.jpg
At first I thought it was a piece of grease but after removing it realised what it really is...

PS. I used compressed air to thoroughly blow out and clean the entire engine bay before starting work. I also blow out the areas around sparkplugs and injectors before working. Im 100% these things did not fall in during this repair.

I removed the intake manifold/throttle body assy yesterday and found about 0.5L rat crap on top of the engine, that would explain the funny burning smell every time the engine got hot.
I also found that the 'ceramic ring' I saw was indeed the spark plug obscured by my crappy endoscope.

I did a compression test and got the following values:

Cylinder : First test : Second test with oil added
1 135 kPa 145 kPa
2 145 kPa 175 kPa
3 130 kPa 180 kPa
4 150 kPa 170 kPa
5 0 kPa 0kPa
6 130 kPa 180 kPa

WIS states that compression should be between 120 and 140 kPa so the values look good, cylinder 5 being the exception. I could not get a reading at all, the needle on the gauge wouldn't even move. I double checked the connections and proceeded to cylinder 6, getting a reading. I returned to cylinder 5 and still got nothing.
I believe that if the rings are worn I would see some pressure especially with the oil added. If the head gasket is blown I should still see some movement in the needle.
I suspect the exhaust valve is damaged or stuck open.

Any advice?

Ouch! Yes, suspect valves first, but you can't rule out rings. (see 0 Compression in #1 Cylinder! 455 - ClassicOldsmobile.com)
Maybe putting air into the cylinder and seeing where it leaks can give you some hints.

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Prost!

Yes Im going to make a fitting this week that I can screw into a plug hole and apply some pressure. The air should blow out the exhaust, the intake valves or somewhere on the block.
The good news is that I added a lot of oil to cylinder 5 before the second compression test and went in with the scope afterwards. It seems all the oil was still there with a nice meniscus between the piston and cylinder wall.
The engine also has a loud tapping noise coming from the second bank, I wonder if there isnt something going on with the exhaust valve in cylinder 5 causing the compression issue and the tapping noise.
I dont want to take off the head as its so much work and quite expensive.

Fnally managed to machine a fitting to pressurise the cylinder. I turned the crank until the intake valves opened and then closed and then another bit to ensure the piston is on the compression stroke and all valves should be closed. I then pressurised the cylinder at 50kPa and could hear the air flowing through the engine and found it blowing out the exhaust. I then closed the cock on the compressor and increased pressure to 500kPa. Opening the cock drains my 24L compressor through the exhaust in about 30s. The exhaust valve is definately stuck open in this particular cylinder most probably due to a broken spring on the valvetrain.

I managed to look at the piston with an endoscope and could not see any damage due to the piston hitting the valve. Has anybody replaced one of these springs? Can it be done without removing the head or camshaft?

I proceeded to remove the head cover to take a look at the spring and upon removing it it slipped and hit the no5 rocker arm. The valvetrain looked brilliant so I did another pressure test with the head cover removed and could not find any leakage. Replaced the cover and compression was 15bar. I suspect the valve stem might be bent and caused it to get stuck, the mishap of dropping the cover must have loosened it up? Have been driving the car for 3 weeks with brilliant performance. Will keep an aye on it and when it acts up again I know where to start looking at least.

Glad to hear. Good luck!

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Prost!

What's the status of your problem?
Solved completely?
I have the same engine, no problems so far, but i like to know what to lookout for!
How many miles/km on it?
Did you change the timing chain yet?
Thanks!