I have a 1991 VW Westy GL I bought it last July
 > has 107,000 strong engine. ran well when bought it.
 > had exhaust leak @ pipe on #3 cylinder and coolant leak on water pipe to
 > / from water pump and cracked pin hole leak at small hoses at oil
 > cooler. found PO had welded exhaust nut to stud on # 3 & 4 cylinders No
 > idea why..
 >
 > I had to cut off the stud/weld with high speed cut off wheel to remove
 > exhaust.
 >
 > I removed the engine to get better access studs in heads to drill / e-z
 > out the studs. I got them out & installed new studs
 > water pipe was welded up. ALL coolant lines & fuel lines were replaced.
 > new rubber seals were installed on fuel injectors (both big / small)
  new exhaust pipes / muffler installed. new clutch installed CV joints
  clean & re-greased.

  I reinstalled engine transmission

  It fired right up.. idles great / runs strong for first 2 minutes or so..
  then Idle drops....real low no power, runs very rich (black smoke / soot)
  out exhaust smell very rich

  I double checked all wires, vacume leaks / hoses found nothing amiss. I
  replaced the coolant temp sensor #2 still does it. it will not run well
  when warm next morning or few hours later It'll start right up, idle
  smoothly rev though rpm range strongly then 2 minutes later suddenly
  drops low idle, no power etc. when cold again it'll fire right up run
  strong 2 minutes till starts warm up...

  any one have any ideas?

  no work was done on engine itself. while engine was out. just replaced
  exhaust pipes / hoses clutch etc.
 >
 > mike