high idle troubleshooting
 

I have been trying to figure out why my 97 disco idles at 2500-3k when i start it up. As soon as i throw it into "D" its settles down due to the load on the motor. I pulled the IACV to clean it but it was so cruddy i couldnt get most of the gunk off so i replaced it with a new one from NAPA. Well i tried the reset procedure for the iacv, and after a few cycles the idle in "P" came down to right about 1500 which is still high. I went to start it this AM and the idle will spike up to 2k then start dropping to 1k and then shoot up to 2500-2800. I dont know if since it is a brand new IACV i need to take it to a LR shop and have them reset the ECU parameters? I was even told that it could be because I changed the battery out. Im just looking for the next step/thing to look at to fix this prob, any help would be appreciated.

I had the same problem with mine. The existing ECU would not "relearn" the new IACV. Finally solved by buying an ECU and IACV from the same donor vehicle, which for my budget was much better than a trip to the dealer for a reset of adaptive values. It is not related to battery change. Couple of other items - physical throttle body linkage can bind up, needs lube, plus the butter fly valve can stick because of gunk build up (clean with carb cleaner). The coolant temp sensor (the two wire one runs the ECU, the single wire runs the gauge) could be unplugged or bad, and ECU thinks it is -40F. A scanner will show coolant temp from this sensor, which should be logical value, like garage ambient in morning, etc.

If some one could post a work around for adaptive value reset on the GEMS ECU it would be helpful for many of us.

I keep seeing the statement that the new ECU doesn't "adapt" to the existing IAC (Idle Air Control Valve).

However, there's no feedback from the valve other than engine response. It's possible the the IAC is just not working. All it is is a bipolar stepper motor moving the plunger in and out. There are two windings, and two wires for each. Using the pinout in the RAVE you can check the resistance on them.

With a little bit of circuitry you could drive it and see if it moves.

Or you could just buy a new one. They're about 20 bucks.

According to the Land Rover info, The IAC motor has 180 steps from open (0 position) to closed(180). There is no way for the ECU to tell the physical position of the plunger, so it must work from one limit or the other and move accordingly.

I would remove the valve, leavingthe connector attached, block the opening, start the vehicle and see if the valve moves. If it doesn't, it's stuck. These things used to fail all the time on American cars of the late 1970s.

The ECU builds an adaptive map of the parameters of the various sensors and adjusts the engine to those. If a sensor ages a little or wears down, the adaption is there. When a sensor moves too far to be functional, you replace it. Computer does not know this and tries to keep operating the old way. The IACV is a computer controlled vacuum leak. Other vacuum leaks can alos be the problem.

Did you check your throttle cable? I was convinced it was my IACV, cleaned all the black carbon off it, reinstalled and it worked. Next day idle was up...cleaned it again, didn't work. Bought a new one, still high idle. Ended up that my throttle cable was tight and opening it up at idle. Free fix, worth a shot while the bonnet is open.

Did it have the idle issue before you cleaned the iacv?

Ill check the throttle cable, thanks.

yes, that was the reason i cleaned the iacv, it's gotten worse with the new one installed. The only other thing i can think of is the new iacv came with an o-ring as a gasket and not the flat gasket that was on the old one. Maybe its not sealing as well as the old one.

Download this manual on the ECU, start on page 126.

Yes, the ECU build a map of the SENSORS. The IAC is not a sensor, and contains no sensors.

The ECU has no way to tell one IAC from another, and in fact there really should be no difference between one IAC and another unless one of them is defective.