88 McLaren Questions/MPG/Exhaust/etc???

Hi All,
This is my first question as a new member. My first time, so please be gentle My brother just picked up an 88 ASC Mclaren to leave here in Las Vegas as his "winter driver" when he visits from the NorthEast rust belt. It's 5.0 HO/automatic with stock exhaust and cats. The car seems to run pretty well, always starts, plenty of oomph, but can't get more than 11 mpg!?! We are suspicious that we may have plugged (or partially pugged up) cat(s). We are new to these cars and are looking for a place to start. Also, when you slow down and stop at a light, especially after being on the highway, the idle surges up and down and sometimes almost stalls when you first stop, then it recovers and seems almost normal. We just want to know where to start and the best procedure to avoid chasing our tails. There will be many more questions to follow.
Thanks
Gene

Please Be Patient?

Folks,
Thank you for the replies. I am totally new to anything electronically/computer controlled. I'm way old school but am eager to learn. How do I check for codes? After tuneup parts how and where do I start troubleshooting? I'm not even sure how to check the EGR. Everything in the engine compartment looks totally stock, original connected. Please be patient with me, I'm a pretty smart guy and can follow instructions. I just need to know where to start.
Thanks
Gene

Please let me start with a short rant/vent. I've been reading and studying since last post but have become frustrated by the use of abbreviations with no legend to identify what they mean and the fact that most charts/guides seem to assume that a person knows what they are working with/on. For example: does our 88/5.0/automatic/Mclaren have EEC-IV/OBD2 or EEC-V/OBD2? I can't tell as the different systems don't seem to be identified by year. I really need a "DUMMIES GUIDE" as it is such a P.I.T.A. to be reading a procedure and have to go hunting randomly to figure out what some cryptic abbreviation means. RANT OVER, Thanks for playing!!!
I realize that once I get involved, all of these issues will become second nature, but for now I'm confused. When performing a KeyOnEngineOff code read, do the codes start immediately after connecting the jumper wire, or is there a step to start the process? Same question for KeyOnEngineRunning and ContinuousMemory? I.E.: What starts the code process and is there some sort of a "HELLO" sequence before the actual codes? Also, isn't there some way to simply hook up a laptop to the car to look at all of this stuff?
Thanks for helping to inform the uninformed,
Gene