Bought a 1991 940 se for my son but driving it to get it all running right. Trying to find out which vacuum line go to heater valve. Clear pvc line sitting all alone but current line tees into vacuum line going into intake manifold. Is that why I am getting slight hesitation when accelerating?

 

The vacuum line to the heater valve does not get vacuum all the time. There is vacuum only when the ac controls or climate control system calls for maximum ac cooling. earlier cars it only was closed if the controls were set to AC and recirculate. The valve is otherwise always open and the blend door controls the cold and hot air mixture to regulate temperature.

It's not the reason your car is hesitating. I would start with basic stuff - condition of spark plugs, wires, what does it look like inside the distributor cap. Look for air leaks - intake manifold gasket or plastic tube to air mass meter. Wait - 940 se is turbo - all air hoses from Air mass meter to turbo to intercooler to throttle plate - short hose at the top of the intercooler on the drivers side likes to burn through on the bottom. Look in the air filter housing - the housing top inside is covered in foam on some cars (for noise) - that foam comes apart and gets sucked into the AMM.