when i press the brake pedal on my 1994 toyota 4x4 truck, 4cyl, the engine starts to surge up and down.?

Question

it also flashes a fault code of 25, on the check engine light

Answer 1

Im with volvo guy, sounds like a torn brake booster diaphragm.

Answer 2

I'll take a guess using past experience on a Dodge Charger. My power brake booster which was operated by vacuum straight off the back of the carburetor went bad on my Charger, and when I applied the brakes the engine would race, when I released the pedal engine RPM would return to normal. I have no idea what a code 25 is on a Toyota, but you could try a test in your driveway by disconnecting any vacuum hoses to your brake booster and plugging them with a bolt the same size as the inside diameter of the hose. Start the engine and apply the brakes, if the engine does not surge, you have isolated the brake booster as the source of a vacuum leak.

Answer 3

Your idle is set too high. The ECU is cutting the fuel when you have the brake on and you need to lower the idle. It's the screw on the throttle body you need to tighten some. Try that and I bet your idle won't surge anymore.

Keep track of how many turns you tighten it just in case you need to put it back where it was.

As for the code 25, here is a list showing the codes and their definitions:
http://www.off-road.com/toyota/tech/codes/index.html

Here is a link to the factory service manual in .PDF form. It will show test procedures for that code.
http://personal.utulsa.edu/~nathan-buchanan/93fsm/engine.html

Answer 4

if your toy has powerrakes your power brake booster is leaking otherwisw known as a vacuum leak as an engine is vacuum sensative a leak anywhere causes the engine to vary in speed

Answer 5

The brake booster has a leak, it is vacuum operated.