I have a 97 Ford Ranger 2.3 5 speed manual. I just changed my thermostat and decided to change out my spark plugs while I was at it. I got everything back together but now when idling it acts like its going to die then revs up. It continues this cycle. I also noticed that as I was driving my brakes would go in and out. I'm assuming this is a side effect of the engine revving.
ANY IDEAS?? I'm stuck!

 

Your vacuum line from the intake manifold to the brake booster ok?

Any hissing sounds?

 

Sounds like a vacuum leak. I had a similar problems on mine which turned out to be a unplugged hose. Simple solution is take "carb cleaner" and spray around the engine in small spurts, when the engine start to rev high (carb cleaner goes into engine and boost ignition) you should be around the area where the leak could be.

 

check your plug gaps as well since you changed them they may be off a bit.

 

If you havent already, definitely check the vacuum lines. I had one pop off of, I believe its called the EGT valve. Looks like a flying saucer at the back of your engine near your intake manifold. Mine was old and dry rotted so it wasnt sealing right. Once I fixed that it ran perfectly.