I have a 1990 amigo 4x4 with the 2.6L 4 cylinder. It's been having this strange problem recently that I can't seem to put my finger on. After a cold start, the engine warms up to operating temperature just fine with no problems, once the engine is warmed up and i start to drive, after a few minutes the temperature guage moves up way above operating temperature, and the idle becomes erratic, surging between ~1k-2k RPM. It does this for a minute or two and the temperature guage moves back down to normal range and the idle stabilizes. My first thought was the coolant temp. sensor, so i had my mechanic replace it when my amigo was in the shop for a valve clearance adjustment. I thought the problem was fixed untill I was driving it today and it happened again. Other than these strange temp guage/idle "tantrums" my amigo runs like a top. Any ideas? I'm pretty stumped.

 

ITS OVERHEATING,check the thermostat an the fan clutch,theres a vacuum sensor on the t-stat housing that also goes bad over time.when the engine over heats it effects the way 2.6 idle.once you fix the cooling concern the idle problem will stop

 

I have a 91..same engine. I have had the same problem. I replaced the thermostat, and now the temp climbs to about 2/3 of the way to the red, and holds. I think i might have installed a higher temp thromostat. But it also idles at about 1500 Rpm when the temp gets to this point...i think maybe the computer increases the idle to attempt to coll it down, due that it has a manual Fan. My fan seems to be funtioning properly. I just changed this last nite, so if keeps doing this than i am going to call it good for now...but if it goes back to the other way, i am going to try changing the raditor. 180,000 miles...i think the raditor might be shot...or the water pump.I know a good salvage shop if ur interested.

 

Im pretty sure surging occurs when coolant is not topped up, air pocket maybe?