Friends 90 Jeep Cherokee

I have a friend with a 90 jee cherokee with the v6 in it, they are having issues with it not idling right, losing oil pressure, and the engine temp going up. The SES light is on and I told them to go have the codes pulled to get a better idea, but right off what ideas would any of you have on this?

Pulling codes on a 90 Jeep should be easy using the following:

Turn the key to ON there times within 5 seconds - ON-OFF-ON-OFF-ON - ending in the ON position.

Then count the flashes of the Check Engine light. Example:

FLASH - PAUSE - FLASH-FLASH - LONG PAUSE = 12 code

Take Care,

will have them try that, what about a reference for the codes if i get one, where can i look it up?

Also what are some common causes for the issues i listed?