throw a (monkey) wrench in(to) the works

throw a (monkey) wrench in(to) the works

To disrupt, foil, or cause problems to a plan, activity, or project. We had everything in line for the party, but having the caterer cancel on us at the last minute really threw a wrench in the works! It'll really throw a monkey wrench into the works if the board decides not to increase our funding for this project.
See also: throw, work, wrench
Farlex Dictionary of Idioms.

throw a monkey wrench in the works

Fig. to cause problems for someone's plans. I don't want to throw a monkey wrench in the works, but have you checked your plans with a lawyer? When John suddenly refused to help us, he really threw a monkey wrench in the works.
See also: monkey, throw, work, wrench
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

throw a wrench into the works

or

throw a monkey wrench into the works

AMERICAN
If someone or something throws a wrench into the works or throws a monkey wrench into the works, they cause problems which prevent something from happening in the way that was planned. Of course they may not sign the agreement by the sixteenth and that would throw a monkey wrench into the works. Note: Instead of saying the works, people often describe the situation in which the problem is caused. Most health-related problems, of course, are not life-threatening, but they can throw a wrench into an otherwise pleasant holiday. The US delegation threw a giant monkey wrench into the process this week by raising all sorts of petty objections. Note: The usual British expression is throw a spanner in the works.
See also: throw, work, wrench
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • throw a monkey wrench in the works
  • throw a wrench into the works
  • put a spoke in (one's) wheel
  • put a spoke in somebody's wheel
  • put a spoke in someone's wheel
  • put/throw a spanner in the works
  • throw a monkey wrench in(to) (something)
  • throw a monkey wrench into
  • throw a spanner in(to) the works
  • put a (monkey) wrench in(to) the works