I have a 96 Volvo 850 glt and when I press the gas the front end shakes terribly. It only noticeable at around 45mph and 65mph. Any pointers? Also I just installed a new heater core and thermostat and the heat works fine but now there's light smoke coming from my vents?

 

You may want to check your motor mounts. There is one at the firewall. Upper torque mount. Then you may want to check the lower right mount behind the front tire. Here is a pic of a bad one.


The shake at high speeds is likely bad tires or they need balanced.

 

i have seen the above motor mount cause this but usually on awd models. i have seem multiple times the right front axle cause this, only shakes on acceleration. the inner tri-pod joint gets worn. get a reman axle and that should do it.