"It's an 85 xcab efi. When I hit the brakes my idle drops off 500 rpms or so then recovers, over and over again. It dosen't happen when It's warming up, just when hot."
"I would bet you that your idle is set to high. Sometimes the screw works loose with vibration. If your idle is at like 1100, it's too high. With the engine warm, try lowering the idle at the intake manifold to about 750 or so. That will probably solve your problem. Mine was doing the same thing last week after the new header. Adjusted the idle and it's fine. "
"The EFI computer receives a signal from the brake circuit that tells it that the brakes are being applied. This causes the computer to shut off the injectors if the RPM's are over a certain level and the engine is in "closed loop" as engine power should not be needed if the brakes are being applied.I believe the set point for the injector cut out is 850 to 900 RPM. This is what causes the surging - your idle RPM is too high, when the brakes are applied the injectors are cut out until the RPM's drop below this level. The engine then run's again. This only happens when the engine goes into "closed loop" so this will not happen when the engine is at a higher idle speed warming up. I am willing to bet your idle speed is around 1000 RPM when this happens. To fix this problem, simply adjust your warm idle speed to 750 RPM as per the factory.