Hello I'm new here an needing some help hope here is ok to ask.my 1991 Ford ranger 2.3l 4x2 ,has low idle and low rpm won't go over 1500rpm which was intermittent when I got the truck a year ago but now does it always it used to be low idle low rpm driving in town nothing over say 40mph but when I would drive it say an hour on the highway speeds it would suddenly start to run correct and then next day would do the low idle low rpm crap again now a year later it does it always so I go to the Auto parts store borrow the code reader the code reader will not do the full test it says can not do koer test until you pass koeo test which is weird because it did say koeo passed but go to do koer test it says like I said before can not perform the test the parts store guy said he didn't know what causes it but he did say he has seen it happen.any help or input would be so helpful thank you, also I did tried to flash the codes but it gave so many different codes it can't possibly in my mind have like 10 different codes the truck runs pretty good but is getting worse