if your pictures are correct then the leak is coming from the throttle heater plate under the throttle and dripping onto the vale cover immediately below it so it looks like it coming out of the breather hose on the cover. from there it seems to be running down the cover and is probably dripping off the cover directly over the o2 sensor. Just looking at the pictures. The connector for the o2 sensors are usually covered in oil which deteriorates the seals and they get affected by moisture - that MAY cause your misfire as the shorting O2 sensor is leaning out the bank.
the leaking throttle plate was possibly knocked during the repair - there are several replacement parts available as its a known leak area.
Also if your friend didnt put blue locktite on the valve cover bolts they will probably back out over time - known problem area and the torque is only 6lbs or 8 NM - nowhere near 15 as the valve cover gaskets have a metal insert to stop them crushing.
1999 discovery 11, some new cars, 1988 xj40, 1976 deville 8.2l, 1974 450sl, 1969 lotus 7, 1961 austin mini
the best view is always from the point of no return and you only ever have too much fuel when you are on fire.
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