I need help diagnosing my 89 ranger 4.0 OHV for idle issues. I'm going to try and clean/replace the IACV and TPS onb my 99 I was able to give it the ol reboot (unplug and plug it back in) and it seemed to fix the issue. that didn't work on my 4.0. symptoms are; in neutral idles around 2700 and when shifting to neutral to slow down revs from about 1000 to 1700, can be settled at a stop if I hold the clutch and "shift" into 1st and then go back to neutral and idle will stabilize around 900-1000.

 

check youtube on how to test a TPS

 

Are these vehicles' throttles clean?

 

I can only give prior experiences. When the IAC valve went out the idle went to full throttle. Luckily I was parked at an auto parts when it happened. :) I bought one of their aftermarket IAC valves. The truck was driveable but the idle never went back to OEM r.p.m.'s. I was able to find a Motorcraft CX-1828 IAC valve and it cleared up the problem. They are expensive and hard to find but you gotta do what you gotta do on these first gen 4.0L Rangers. Later model 4.0L Rangers...people install aftermarket thermostats and the engine stays in "open loop" and never gets to the proper operating temperature. The engine idles higher to achieve a "closed loop" condition which it never obtains. The OEM idle is what you are looking for. Use Motorcraft RT-1086 thermostats (ebay) or a Stant Superstat equivalent made in the USA. Why? They are more accurate and they last longer. (192* or 195*, can't remember. Look it up on a Ford specification page.)