1986 vovlo 740 gle cutting on and off and poor gas mileage

Hello I have a few problems.I have a 1986 volvo 740 gle and I am getting
poor mileage.And when Im driving it like just starts cutting in and out
and im getting backfire often and when Istart it it revs to around 2000rpm
then goes down to around 1500 but shouldnt it be lower? please post any
suggestions.

 

Vacuum leaks are the first thing to check for.

 

I have already replaced the air intake hose and fixed a slit vacuum line
and that helped a little.Then on the idle section could it possibly be the
idle control valve causing the eraticc idle?

 

Could be, but usually that just sticks and results in too high or low idle.
You probably have more vacuum leaks, they can be hard to track down.

 

thanks for the help ill try it tommorow any suggestions on how to track
down a vacuum leak?