Every electrical motor has a maximum rpm that it can turn without sustaining damage. Once that speed is exceeded, centrifugal force will pull the windings from the armature and they will bind into the stator. In the case of the Prius transaxle we are actually talking about a rotor assembly that has permanent magnets attached to it. Exceed the rated speed of that rotor and it will grenade. The engineers are correct. http://eahart.com/prius/psd/ In order for the gas engine to propel the car, MG1 actually has to be a generator and provide resistance for the engine to push against, as well as MG2 being a motor that helps the gas engine push the car. Using the java applets on the site I provided you get to see the normal operation of the motors in the transaxle, and how they work together. Take note there are times that MG1 actually is forced to spin backwards. The Prius has a maximum vehicle speed of 94mph, with the primary reason being risking over-revving the electric motors and causing a catastrophic failure of the transaxle. Here is a link that might be useful: Prius Power Split Device.

This problem may require the services of an expert. Erratic brakes and a stumbling engine are ingredients that can lead to an accident. I'm no expert in this arena, but here are my thoughts. On startup, you are missing the vacuum boost to the brake system, and somehow, later on, some or all of the boost is regained. The engine stunbles consistent with a vaccum leak. You've checked for vacuum leaks, but may have missed the "big" one, the vacuum boost unit itself, or the vacuum accumulator and associated vacuum lines. At this juncture, I suspect that the vacuum boost unit of the power brake assembly has failed, and admits an overload of air to the intake manifold (when the brake pedal is depressed) causing the fuel mixture to lean out. Often associated with the vacuum booster is a small vacuum tank called the 'accumulator'. It will be attached to the vaccum lines that supply the vacuum booster. It's purpose is to have at ready a goodly supply of vacuum volume to handle a suddent demand, and to keep the heater controls happy during this step demand. This tank is very important if the the vacuum fitting on the manifold is 'ported', that is, has a orifice within it to restrict the amount of air flow in case of total vacuum system failure. This tank may have a one way valve. This is the vaccum source that aids one more application of the brake after the engine dies. This tank may be made of two plastic halves and ultrasonically welded together. A leak along this seam is possible. Do you have a hand pumped vacuum tester with vacuum gauge? If you do, remove the vacuum line that feeds the vacuum booster and apply vacuum directly to the booster. Pump down to a vacuum level, say, 10 psi, and hold. Can the booster hold vacuum or does it leak down fast. If it can not hold vacuum, it is leaking. You may have to block the brake pedal down past the valving point for this test. There is a valve in the booster unit that applies vacuum when the brake pedal is depressed, and it relieves vacuum when the brake pedal is let up. This valve can be a problem source.

What threw me was that you said you had a throttle body injector and most of those were just one step up from a carburetor. But you mentioned an engine module controller. If you have an engine module controller, likely you also have an IACS ( idle air controller). The IACS with the engine controller adjusts the engine idle speed. A serious vacuum leak can upset this system and this may be your problem. Find that leak. Many of the little plastic fittings in the vacuum hoses are restrictors - these have small calibrated holes to restict the amount of air flow. One such restrictor is in the vacuum feed to the heater/AC system that operates the various actuators in the air plenums. Thus, if any of these hoses gets pulled off, the amount of vacuum leak is restricted. Lean out problems may occur if any of these restrictor fittings have been replaced with standard straight-through fittings. Places for vacuum leaks: o Tubing o Fittings o Vacuum bottle (often on or near the fire wall) o Power brake o Vapor recovery system o Any AC vacuum operated parts such as a hot water diverter valve located in the heater hoses. If you can hear the leak, this helps in your search. I am not familar with the system for '88 Chevys. I had an older 1980 Chevrolet Citation, V6 2.8 L and I recall that it had a peculiar vacuum control. The vacuum was limited to say 16 inches. When the vacuum exceeded this value, a valve allowed air to enter the intake which leaned the mixture severely and stopped combustion. This was done to prevent backfiring in the exhaust and blowing off the converter. This came into play mostly when slowing down or comming down a hill with a closed throttle with the engine was in engine braking mode. This never gave any trouble, but could have if it stuck open. Good luck. Let us know what you find.

The idle speed is controlled by the computer with the idle speed control motor (ISC). Its unlikely the ISC got damaged with the cylinder head being taken off and put back on. There are two likely causes for such a high idle speed, one would be a large vacuum leak, something like the power brake booster hose not being reconnected yet. The other likely cause is either the computer is commanding the high idle speed, or else the computer cannot control the ISC. Inputs like power steering pressure, AC clutch command, and the transmission shift position cause the engine to need to idle faster to prevent stalling. A false input from one of these causes an excessive fast idle problem. The computer looks at inputs such as engine coolant temperature to control fuel delivery and to produce the cold engine idle speed which would be about 3000rpm right when the car is first started, and then it should drop quickly. A break in the wiring between the computer and ISC could be the cause for this problem. The ISC allows air to flow around the throttle plate to control the idle speed. This passage is right at the top of the throttle body, and it should be easy to confirm if the air entering the engine is in fact flowing through the ISC passage. (Loud sucking sound, and idle speed can be affected by restricting the ISC intake port with a finger. Its always possible the ISC has simply failed in the stuck open position, but its not anyone's fault. Cars break whenever they want to.