I have an 87 4runner and randomly it goes from about 800 rpms idling to 1500 rpms idling. This causes my A.T. oil light come on and the truck gets REALLY hot. Anybody have an idea why it would do this? It has done it since I bought it several weeks ago. Thx in advance for input!

 

I would speculate it's the cold start fuel injector circuit and from there it goes down hill. The down hill includes engine cooling (radiator plugged, failed thermastat, lack of proper coolant). Next issues could be lack of proper lubrication in the engine case and transmission fluid.

I would take it to a certified mechanic for an esimate.