1998 Land Rover Discovery series 1. High idle and after I've driven it and parked, decides to oscillate. Revs up between 700, 1000 revs, then nearly dies and then revs up again. What the hell do I do? Have searched but can find nothing for a 4.0 engine. And all suggests involve the stepper motor. Great. But where the hell is that??? I don't think this engine has one.

 

You most likely need to clean the iac stepper motor. Located near the tps sensor on the throttle body. If you Google image iac stepper motor it shows it on a disco

 

All the google images I have seen, have shown the stepper motor as being behind the plenum. I don't have anything behind mine.

 

What does the Stepper Motor do? I had the same problem but it was for only one day. Then the intermittent starting starting started, and now there is no starting at all.

 

The stepper motor is also known as the idle air input valve. Or something like that. I'm guessing it controls how much air goes through the engine. It's common for it to get plugged. Unfortunately I'm having trouble locating the one in my Disco and have come to the conclusion it doesn't have one.

 

Idle air control valve
Mounted on front of plenum by where the big intake hose hooks on, has 2 / 5.5 mm bolts and 4 wire plug?

Unplug remove clean it and the port do not turn on with it out

If it falls apart you need a new one, but you usually will need the ecm reset by a rover shop,only rover shops with special scanner can reset idle if you need that done dealer is hit or miss, you will need an independent
shop usually

Do not drill the plug on top and turn the base idle screw it will make it worse

 

on your 98, it is located on the throttle body.....on the plenum. 2 small bolts hold it in place....remove the bolts and clean the pintle ( but dont move or twist it ) also clean the hole it goes in....and clean the throttle body while youre' there.

 

In that particular location where only filtered air should be what on earth would cause such an amount of dirt that would inhibit the normal operation of the actuator ?

 

Dunno, accept it. I had to get my ecu reset because it was idling so high the truck would accelerate to 35 with my foot off the gas after I cleaned it.
https://www.google.com/search?safe=o...18%3B416%3B408

 

Pcv hose from the valve cover hooks in there so oil sludge and crud

Be sure both hoses from valve covers are good and also clean the pcv noodle in the pass side valve cover