Low compression but no cylinder leak?

[4RunnerNorway]

After 20 hours of troubleshooting, we still havent got a clue of whats wrong...

My 4runner (1kzt 3,0L Turbo Diesel)starts fine, but after 3min of idling, its smokes(unburnt diesel) and idles pretty rough. Mechanic said that it loses compression on 2# cylinder and doesnt run on this until you run the engine warm. You kinda have to drive off the rough running. Its hiccking on lower RPMs at all times. No smoke when proper warm

Compression test (Cold engine) Cylinders:
1# 16 bar
2# 15 Bar
3# 17 Bar
4# 17 Bar

Even lower compression on Cylinder #2 when warm

Cylinder leak down test came out more than fine 2 times. You cant hear or feel any air it seems

This doesnt make any sense???
When the enigne was rough idling, we loosend fuel injector lines, one at the time. It had no effect on #2, but the others were affected. Injectors, Valves, fuel pump and Cam are all checked. Its seems the cylinder cant combust properly. It gets fuel, but cant combust it, hence the unburnt diesel smoke

The problem started after change of cylinder head. I have no idea why it was changed, but it didnt have many miles (15000miles) on it when it was. Its a Yoyota after all.

Maybe the cylinder has damage from a breakdown? Something happend and they changed the head? Its weird when the rough idling occurs 2mins after start up

 

[rdruss]

Started after cylinder head replacement for unknown reason. Maybe it dropped a valve on #2 and did some piston crown damage but not enough that they wanted to replace it. Some deep gouges in top of piston might affect compression ratio. Can you get a bore-o-scope into cylinder #2 and have a look?
I dropped a valve twice in a Yamaha TT500 and piston was ok, but on a single cylinder drop a valve and instant shut off, thus piston doesn't take a beating like on a multi-cylinder engine that keeps running.

 

[4RunnerNorway]

Started after cylinder head replacement for unknown reason. Maybe it dropped a valve on #2 and did some piston crown damage but not enough that they wanted to replace it. Some deep gouges in top of piston might affect compression ratio. Can you get a bore-o-scope into cylinder #2 and have a look?
I dropped a valve twice in a Yamaha TT500 and piston was ok, but on a single cylinder drop a valve and instant shut off, thus piston doesn't take a beating like on a multi-cylinder engine that keeps running.

Probably damaged piston or mishapen cylinder wall due to overheating... The previous cylinder head was probably cracked at some point, causing loss of coolant and severe overheating. This may have affected the shape of cylinder?

Since cylinderleakdown is tested when piston is at top dead center, the air wouldnt go past the rings if the mishaping is further down the cylinder wall. Does this make sense?

 

[rdruss]

Probably damaged piston or mishapen cylinder wall due to overheating... The previous cylinder head was probably cracked at some point, causing loss of coolant and severe overheating. This may have affected the shape of cylinder?

Since cylinderleakdown is tested when piston is at top dead center, the air wouldnt go past the rings if the mishaping is further down the cylinder wall. Does this make sense?

Severe overheating most likely would have damaged piston(s) and or rings to the point that some would have needed replacement. Cylinder walls would not "deform" from heat but can " score" from the transfer of piston/ring metal to the cylinder walls.