Hi I have a 1990 FWD Fleetwood with a 4.5L motor. My only problem is it runs very rich it smells gassy as well. in the daytime i average 21-26mpg freeway when doing 65-70mph(just me in the car) But when its dark or cold it gets 16-18mpg only. Also I get a fair amount of white smoke and a gassy smell. What may cause this? I changed the plugs wires cap and rotor egr valve air filter etc. What else makes a car run rich?

White smoke is not rich. White smoke is usually coolant in the combustion chamber. Not a normal problem with the 4.5. I suppose it could be lean.

Sooty black smoke is the sign of a rich condition. Smelling gas is probably a leak. Get the car in the air and look closely at the entire fuel line from front to back. Also look at the tank. I had a line leak right at the sending unit so it looked like the top of the tank was leaking.

ill do that then. how does coolant make it to the combustion chamber? and how do i fix it? Is it something a not so advanced mechanic could find?

Coolant makes it in to the combustion chamber when you have an internal coolant leak. I just experienced this in my deville, white smoke out the exhaust. Put your finger in the tailpipe(s), if it comes out covered in sooty coolant, you're looking at a head gasket. Check the coolant levels and the oil, milky oil is another sign of a head gasket going. How hot does your car run?

White "smoke" can also be a very rich fuel condition. The "smoke" is actually unburnt fuel vapor. Smell the exhaust "smoke" to determine if it is coolant or gas. Sounds more like fuel from what you say.

I concur with Ranger on this one, if it smells sweet then go to coolant.
Most likely the problem is the O2 sensor and MAT sensor both extremely easy to replace..Done to many of these with that problem and constantly fixed it...

Well if that's true I stand humbly corrected but I've never heard of that.