We have had just about everything done to our 98 Land Rover Discovery that can be done. The idle air control valve is new and has been computer adjusted multiple times. The mechanic said switch out the computer board under the glove box with our 97 Disco, we did that still idles to high. Sometimes it will idle normal, but most of the time it idles to high, when first starting in the morning it will idle low, then idle high, then low, then high back and forth for about five minutes then stalls unless I run out and step on the gas pedal. Then it starts idling to high again. We have no idea where to go from here, no Land Rover dealers in Alaska, tried calling some in the lower 48 states and no one was willing to listen to the problem and maybe give me some advice. Thank you to anyone who might have an idea what is wrong, much appreciated.

 

First off any 96-99 GEMS equipped 4.0L D1 = ECU is under the hood. Nothing under the dash behind the glove box will have anything to do with how it runs. You have a 10AS alarm module down there, the Wabco ABS computer, and the window control unit. For the idle you might make absolutely certain the chamber for the IAC is clean with no gunk build up. Second when you install a new one it's a good idea to reset the adaptive settings. However even then it will take several key cycles for the IAC to sync properly. Since you are out in Alaska I highly recommend using an Android phone with the GEMS App and a generic OBD2 Bluetooth reader. With that combo you can reset your adaptive settings without having to hunt down a T4, Hawkeye, or Lynx to do it. Other thing would be to check the ECU under the hood (passenger side next to washer bottle under plastic cover). I've seen some of these ECU's get corroded and cause issues, so a visual inspection, and then actually taking the covers off to inspect PCB = would be ideal.