Ford Repair: 88 Ford Escort GL - High Fuel Consumption, head gasket, oxygen sensor

Question
My son just purchased a running 88 Ford Escort GL 1.9L Central Fuel Injector (CFI).  Recent work by others included replacing the head gasket.  It runs acceptable but seems to be using a lot of fuel, about 10mpg.  A lot of carbon out the exhaust system.  We are wondering if replacement of the O2 sensor could fix the problem.  There is also an electrical component missing that would be on the passenger side of the engine (not the vacuum sensor) a four wire plug.  Is there any diagnostics we can run?  The car does not seem to have an ODM2 port and the manual we have for it is not much help.  Any suggestions would be appreciated.  Thank you.

Answer
The diagnostic port is located under the hood.It is OBD-1.Buy a repair manual at you're local parts store for about twenty bucks.It should show you what it looks like.The oxygen sensor will give it bad gas mileage,but not that bad.Try the MAP sensor first.