what would cause the engine to have a low idle, then surge when driving hwy on 2002 Ford Ranger

it idles low when stopped, then when running at a higher speed it seems to surge. at times the brakes are hard to push.

by in Lancaster, TX on October 27, 2013
1 answer
ANSWER by , October 27, 2013
Needs to be checked for correct intake mauifold vacuum. Then for vacuum leak(s)! Probably needs to go to the shop.