I just bought a '93 Lexus ES300. It runs rough most of the time with raw fuel and white vapor comming from the exhaust. Occasionally it runs well with none of those symptoms, but that doesn't last long. The previous owner replaced the pre-injector and the #6 injector. He also replaced the ECM with a junkyard unit from a cross-referenced Camry. These repairs made no difference, and the pre-injector has to remain unplugged to keep massive amounts of fuel from pouring out the tailpipe. There is no significant water loss from the radiator, no water in the oil and the engine does not overheat. I'm planning on running a compression check this weekend to rule out the head gasket. Can a head gasket be bad without any of these other symptoms? Is it possible I have another bad ECM and/or injectors? If anyone has any experience or knowledge of something like this, there info would be greatly appreciated.

I would check the fuel pressure a leaking pressure regulator will allow excessive fuel into the engine.
Pull the vacuum hose for this regulator if it has fuel in it replace the regulator. They can fail other ways but this is a common problem of a rich running engine.

Thanks! I will check the pressure regulator. I am not very hopeful, though, as that would be WAY to easy! I do appreciate any ideas, however, and will certainly check it.