I just recently bought an engine wiring harness from Davebarton.com because my oil light was flickering. I took my Volvo to a mechanic to have them put it in because I don't have enough time to do a job like this. After I got the car back, I noticed that the mechanic unplugged the expansion tank overflow hose, so I plugged it back in and started driving back home. While driving it seems that I have to really push on the peddle to get the car to increase speed, more than I am normally used to. Also when driving at around 60mph, to get the car to go any faster I have to put my foot all the way down, and even then it barely increases speed. After I got back home I noticed the expansion tank cap was hissing and there was some coolant around the tank. Then the expansion tank cap let out a bunch of coolant but still stayed on the tank. Does anyone know what is going on with my car?

My car: 1982 Volvo 244 Turbo