Where is egr valve on 1993 chevy cavalier, have rough idle, gas smell like flooding, runs ok @ highway speds

How long since overall tune-up? Any applicable trouble codes?
The basics for a rough idle, check for a leak of some kind, fuel--egr--vacuum, something of that nature. Does it run ok off-idle?

White smoke and foam in the gas tank indicates antifreeze leaking into the engine somewhere. Do you smell antifreeze? You may have an engine seal leaking, most likely head gasket. It may be affecting the various engine sensors.

Yes it is fuel injected. If it runs but has no power there is a good chance that an ignition coil went bad and there would be no spark to two cylinders. Cylinders 1 and 4 are partners on the same coil and 2 and 3 are partners. Usually these cars if an injector goes bad they have an external leak.
You can try to listen to them by holding a screw driver to each on while its running but I don't think
that will be your problem anyway.Another giveaway for the coil could be a gas smell like it is partially flooded. When you changed the plugs were any of them wet with fuel ? There is also a chance that if the engine works ok at highway speeds that the egr valve is stuck open. This causes
rough running at idle and bad hesitation until highway speed is achieved. Hope this helps or let me know and we'll figure it out.

Paul

replace your EGR Valve

maybe your iac valve is not working? it takes place of a choke or temp sender to computer could be bad which controls t I.A.C. known as the idle control valve,do you have any codes?

Check the vacuum line to the fuel pressure regulator and check to see if the fuel pressure regulator has gone bad. Give the throttle body a good cleaning and the injectors too. Do not use carb cleaner, it's to corrosive for the gaskets used in the throttle body. The other thing too that would cause this is a stuck or bad EGR valve, Try cleaning the EGR valve too since your cleaning the throttle body. Good luck and keep me posted.

check the choke setting

it sounds to me that the sensor your talking about is the camshaft positioning sensor. It regulates how much fuel you get to your engine, so that is also why you have the fuel smell as well.

Try replacing the ignition control module it is located under the distributor cap Cost is around $30.

It sounds like you may have a broken exhaust valve spring, although, like you have found out problems like this can be very hard to diagnose, but anyone with a compression tester and couple other tools could find out if this is the case. Hope this gives you another idea where to look.