Flight Instructor on a training flight with student reported an engine malfunction in flight.

Narrative

On roughly a 3-mile final to Runway XX at ZZZ I told my student to maintain altitude at 4800 until on glide path as we were getting 4 red lights on the papi. We descended to 4700 and again I said 'maintain altitude until we get 2 reds and 2 whites.' At this time; he said 'I'm at full throttle.' We were descending now at about 1000 rpms with throttle full and losing about 500 feet per minute. At this point I took control and told my student; 'I think we are landing on the freeway.' A few moments later and only about 100' off the ground I said to my student; 'We are landing on the freeway.' I jockeyed the throttle a couple times and the engine sputtered. Then I pushed the mixture to full rich and the engine sputtered back to life. We slowly climbed away from the freeway and returned to ZZZ without further incident. As I turned onto final approach to the freeway; I noticed traffic ahead about 1000 yards and traffic behind about 3/4 mile. We were luckily positioned as to not harm anyone else. After landing at ZZZ we inspected the inside of the engine cowl to find a wrench lodged. A more thorough preflight by my student may have found the wrench. It was difficult to see after the flight when I was inspecting with an intention to find something amiss. The mixture was not changed from the run-up until about to touch down on the freeway. It was set perfectly for field elevation where we took off and landed. So; the only thing I can think to cause the problem was the wrench lodged against something disturbing the mixture or throttle. I look forward to talking with the mechanic tomorrow to see what exactly the wrench was lodged against and if that was the problem.