Volvo Repair: Engine running rough, vacuum test, vacuum lines

Question
Hi There.
I have a 1987 240 DL that I have bought. The engine is running rough and I am lost as to what to do next.
The engine runs rough when cold or hot, idle or throughout the operating range.
I have replace coil, rotor distributor cap plugs and all leads. I have rebuilt injectors and tested the oxygen sensor works as does the temp sensor. I have replaced the fuel filter.
There seem to only be four vacuum lines on this car. Two run from EVAP to inlet manifold, one from Computer to inlet manaifold and another that runs from inlet manifold to under the dash inside the car I think it is form the temp/cooling controls. These do not leak. I have disconnected them and blocked their connection on the manifold but the problem persists.
Timing was 5 degrees advanced so I reset it to 12 degrees.
The vacuum test reads 16 Degrees Hmg and I believ it should be between 17 and 20 but not sure.
I have checked some of the wires in the harnes that run between the computer and the distributer for continuity and they are good.
In additon I have established the following. First- The only time th eegninge runs correctly and purrs is when I adjust the timing by turning the distributer. Once the computer has recalibrated for the new timing setting the engine runs rough again. I have also seen that the timing mark jumps back about five degrees each time the engine misses.
I have removed each plug lead one at a time and each electronic injetor lead one at a time to note RPM drop per cylinder but they are all equal.
I think this is a fuel related problem as the missing is not confined to just one cylinder but rather across all cylinders.
I am know wondering if it is a worn distrubuter, bad Computer of something very simple I have overlooked. In other words I don have a clue what to do next. Any help would be wonderful -thanks.


Answer
Joe, the only thing you didnt tell me is the Fuel pressure. The 240's are known to have bad engine harnesses over the years.  Did you change the ECT?  IF that sensor is going bad is it will tell the computer to dump more fuel and overfuel, causing the rough run.  Did you test the Air mass meter?  These are all things that can cause a rough run.  Intake gasket leak can also do it.  There are a lot of things yet to test.  Roger