Once you remove the valve cover it will be clear where they go.. Difficult to see with the cover in place.

Sugar Bear; I was about to put the valve cover back on with new gasket, until I did some research on this so called half moon gasket that came with the valve cover gasket kit. Glad I didn't yet. Now I understand the half moon gasket will be easier to install with the valve cover out of the way for now.

It seems something went wrong and need a serious checkup. Most probably the oil leak (Blue/thick dark) fumes. I am also a DIY lover like you, my advice is that think twice before touching a hybrid car, if anything went wrong it can empty your pocket. I am new to this community it seems you some bunch of helpful comments on this issue. Apart from the common troubleshooting steps as discussed in above comments I found that the issues with Gas cap, Purge valves, Vacuum vents may also give rise to this problem. I found this from this site Car Emissions Testing & Repair Toronto & Etobicoke | Service Plus Car Care Centre

:eek:Spark plugs don't have any role on this issue..:rolleyes::eek:

something went wrong with my previous post. I am sorry for it.

Latest Update
 

Today, I finally got to finished everything I set out to do. The so called two half moon gaskets located in the rear engine behind the top cylinder head were removed using the pick and hook and replaced with new half moon gaskets. New valve cover gasket also, as well. Throttle body rebuilt, coil pak, new spark plugs, and spark plug wires were also reinstalled. Long story short, I fired up the Mercedes and immediately started running. Very smooth engine sound. Kept it running for a few minutes while I inspected for oil leaks around the engine area. NO BLUE SMOKE!! No smoke coming out of the exhaust tail pipe. Everything seems ok at this point. No warning lights on the instrument cluster panel showed up!! So far, everything good!! Except one small nagging detail. Still smell burning oil somewhere and not knowing where it is coming from. Not as much as was before. Maybe it is because of the previous oil spewing all over the engine area before I went to work on the motor. Possibly leftover spewed oil burning the rest of it off. Will test drive the Mercedes tomorrow and finish off the rest of the issues I was having, especially the troubleshooting codes on the throttle body or EA that started the trouble in the first place. Stay tuned.

If there was oil spilled down the back of the engine, it will take a bit of driving before it all burns off. Give it time, if you solved the source of the oil leak the smell will go away once the residual oil is burned off the block and exhaust.

dmorgan, it's possible the engine needs new valve stem guides/seals or new piston rings and the engine is burning oil internally. Let us know what you find after driving the car a few hundred miles.

Slowly smoke is going away
 

Fred, thanks for your latest comments. I drove the mercedes this morning for about 25 miles. When I got home, I located the source of the burning oil smoke near the EGR valve on the exhaust manifold side. This time the smoke is clear color type and not blue smoke. Also, I checked the exhaust tail pipe and there is no smoke coming out there, either. I can't exactly pinpoint the location but it appears to be on the tubing or pipe connected to the EGR valve. After starting the engine again, the smoke went away and no burning smell. I will keep monitoring on this issue and, if necessary, will report back on my findings.

Diesel300, thanks for your latest comments. So far, no oil burning smell or smoke out of the engine.