98 4.0 GEMS disco.

Running perfectly, I foolishly cleaned the IAC valve and on assembly idled high 1500 to 2000.
Been battling this for months - changed the TPS changed the IAC - still idling high. The funny thing is if you stopped the engine and restarted it would idle 1000 rpm lower but still high. The base idle plug had been removed by the mechanic who installed a new shortblock a couple of years ago but no amount of tweaking would get the idle right.

Recently I bought a very good used plenum (off a perfectly running disco) complete with TPS And IAC.
Put it in, did the new IAC set up (plugging, unplugging etc.) started the car and to my amazement and joy it idled perfectly.
Idled for ten to fifteen minutes, started and stopped a few times.

Took it for a shakedown cruise, stopped at a traffic light and damned idle back to 1500 in gear and 2000 in neutral. What the f????????

Spent the next ten hours checking every vacuum (because that's what it feels like) and every other possible contingency. Even put in the the other three IAC valves I have amassed in trying to fix this problem. NOTHING will bring that idle back down again.

It's not the base idle, because the car idled perfectly as is, it's not the TPS nor the IAC valve, it's not vacuum leak (unless there is another vacuum line i cannot find).

Before I go over to the dealer here in L.A. and sell my soul (but more importantly, hurt my pride) for time on the Testbook reset, in my opinion, it has to be a sensor - the two which are in the loom which serves the plenum - One has two wires and the other is a single post. What are these and can they cause a high idle?
What else can it be other than an ECU rest? I have read all the posts but because the car idled perfectly for 15 minute yesterday non of the replies seem to apply.

Help!!!!!!!!!! PLEASE!!!!!!!!!

 

On the intake near the upper radiator hose are a two wire and a single wire sensor, they are coolant temp, 2 wire goes to ECU, single goes to temp gauge.

When I first purchased my well used D1, it did the same thing, and I replaced the ECU and the IACV at the same time to finally get it down to normal. Both pieces came from a donor vehicle. I had tried a number of ways to make the ECU "memorize" the new IACV each time I changed it, and I guess I was just not getting it, or my original ECU was not giving up memory of the original IACV. ECU was $40 at salvage yard.

If you don't have the RAVE, here is some sensor info from it.

From the RAVE:

Hot Wire Multiport Fuel Injection

The ’Hot Wire’ Multiport fuel injection system derives
its name from the mass air flow sensor which uses
one cold wire and one electrically heated wire to
measure the volume of air entering the engine.
The function of the system is to supply the exact
amount of fuel directly into the intake manifold
according to the prevailing engine operating
conditions.
To monitor these conditions, various sensors are fitted
to the engine to measure engine parameters. Data
from the sensors is received by the Engine control
module (ECM), the ECM will then determine the exact
amount of fuel required at any condition.
The ECM having received data from the sensors
produces pulses, the length of which will determine
the simultaneous open time of each bank of injectors
in turn, which will govern the amount of fuel injected.

Engine control module - ECM

The Multiport fuel injection system is controlled by the
14 CUX Engine Control Module comprising of a
microprocessor with integrated circuits and
components mounted on printed circuit boards. The
ECM is connected to the main harness by a 40 pin
plug.

Injectors

The eight fuel injectors are fitted between the
pressurized fuel rail and inlet manifold. Each injector
comprises a solenoid operated needle valve with a
movable plunger rigidly attached to the nozzle valve.
When the solenoid is energized the plunger is
attracted off its seat and allows pressurized fuel into
the intake manifold.

Engine coolant temperature sensor

The engine coolant temperature sensor is located in
the front of the thermostat housing. The sensor
provides engine coolant information to the ECM. The
ECM increases the injector opening time when cold to
provide improved driveability, and reduces the
opening time as the engine reaches normal operating
temperature.

Engine fuel temperature sensor

The engine fuel temperature sensor is located in the
rail on the RH side of the ram housing. The sensor
sends fuel temperature data to the ECM, the ECM on
receiving the data will adjust the injector open time
accordingly to produce good hot starting in high
ambient temperatures.

Idle air control valve

The idle air control valve is screwed into a housing
attached to the rear of the plenum chamber, between
the plenum chamber and bulkhead. The idle air
control valve has two windings which enable the
motor to be energised in both directions thus opening
or closing the air valve as required by the ECM.
The idle air control valve will open and allow extra air
into the plenum chamber to maintain engine idle
speed when the engine is under increased (Electrical
and Mechanical) loads.
The idle air control valve will control engine idle speed
when the vehicle is stationary.

Heated oxygen sensors (0

2 sensors) - Catalyst
vehicles

The two heated oxygen sensors are located forward
of the catalysts mounted in the exhaust downpipes.
The sensors monitor the oxygen content of the
exhaust gases and provide feedback information of
the air/fuel ratio to the ECM. Each sensor is heated by
an electrical element to improve its response time

when the ignition is switched on.

Fuel pressure regulator

The fuel pressure regulator is mounted in the fuel rail
at the rear of the plenum chamber. The regulator is a
mechanical device controlled by plenum chamber
vacuum, it ensures that fuel rail pressure is
maintained at a constant pressure difference of 2.5
bar above that of the manifold.
When pressure exceeds the regulator setting excess
fuel is returned to the fuel tank.

Fuel pump

The electric fuel pump is located in the fuel tank, and
is a self priming ’wet’ pump, the motor is immersed in
the fuel within the tank.

Air flow sensor

The hot-wire air flow sensor is mounted on a bracket
attached to the left hand valance, rigidly connected to
the air cleaner and by hose to the plenum chamber
inlet neck.
The air flow sensor consists of a cast alloy body
through which air flows. A proportion of this air flows
through a bypass in which two wire elements are
situated: one is a sensing wire and the other is a
compensating wire. Under the control of an electronic
module which is mounted on the air flow sensor body,
a small current is passed through the sensing wire to
produce a heating effect. The compensating wire is
also connected to the module but is not heated, but
reacts to the temperature of the air taken in, as engine
intake air passes over the wires a cooling effect takes
place.
The electronic module monitors the reaction of the
wires in proportion to the air stream and provides
output signals in proportion to the air mass flow rate
which are compatible with the requirements of the
ECM.

Throttle position sensor

The throttle position sensor is mounted on the side of
the plenum chamber inlet neck and is directly coupled
to the throttle butterfly shaft.
The throttle position sensor is a resistive device
supplied with a voltage from the ECM. Movement of
the accelerator pedal causes the throttle valve to
open, thus rotating the wiper arm within the throttle
position sensor which in turn varies the resistance in
proportion to the valve position. The ECM lengthens
the injector open time when it detects a change in
output voltage (rising) from the throttle position
sensor.
In addition the ECM will weaken the mixture when it
detects the throttle position sensor output voltage is
decreasing under deceleration and will shorten the
length of time the injectors are open.
When the throttle is fully open, the ECM will detect the
corresponding throttle position sensor voltage and will
apply full load enrichment. This is a fixed percentage
and is independent of temperature. Full load
enrichment is also achieved by adjusting the length of
the injector open time.
When the throttle is closed, overrun fuel cut off or idle
speed control may be facilitated dependant on other
inputs to the ECM.
The throttle position sensor is ’self adaptive’, which
means that adjustment is not possible. It also means
the throttle position sensor setting is not lost, for
example, when throttle stop wear occurs.

CAUTION: Do not attempt to adjust throttle

position sensor

Tune select resistor -

To suit individual market requirements a tune select
resistor is connected across pins 5 and 27 of the
ECM.
It is located adjacent to the ECM, and strapped to the
MFI cable assembly. The value of the resistor is
dependent on the market application:
Red wire, 180 ohms, Australia, Rest of world.
Green wire, 470 Ohms, UK and Europe - non catalyst.
Yellow wire 910 Ohms, Saudi non catalyst.
White wire, 3K9 Ohms, European catalyst

Condenser fans

It should be noted that under high coolant
temperatures, when the engine is switched off, the
condenser fans will be activated and will run for
approximately ten minutes.

Vehicle speed sensor

The vehicle speed sensor is located on the side of the
Transfer box adjacent to the parking brake. The
sensor provides road speed data to the ECM. The
ECM in turn detects vehicle movement from the road
speed input and ensures that idle air control mode is
disengaged. Should the vehicle speed sensor fail in
service the ECM idle air control would become erratic.
The sensor also provides road speed data to the
electric speedometer and cruise control ECU.

Inertia fuel shutoff switch

The inertia fuel shutoff switch is a mechanically
operated switch, located on the bulkhead adjacent to
the washer reservoir under bonnet [hood].
The switch is normally closed and is in line with the
fuel pump. In the event of a sudden impact the switch
opens, and disconnects the electrical feed to the fuel
pump. The switch is reset by pressing down the
button.

WARNING: Check the integrity of the fuel
system before the inertia switch is reset.
Relay modules

The two multiport fuel injection relays are located in
the RH footwell area behind the ’A’ post panel. The
main relay module is energized via the ECM when the
ignition is switched on and supplies current to the
multiport fuel injection system. The fuel pump relay
module is energized by the ECM which in turn

operates the fuel pump to pressurize the fuel system.

 

You messed up your IACV when you cleaned it.
Start the truck and unplug the IACV.
Shut the truck off.
Plug the IACV back in.
Start the truck.
Report back.
I am assuming that you did not touch the TPS.

 

Wow, thanks for that very comprehensive quote from the RAVE, much appreciated!

I actually have 2 spare ECUs but tried putting them in a couple of years ago and the car wouldn't start. I was told a the time that they are not "hot swappable" but need to be set to each vehicle with a Testbook. Have you found that they simply plug and play?

By the sound of it I need to go to a mechanic and pay the bucks for a Testbook reset of the adaptive parameters for the ECU to release the memory. Only my pride an wallet will be hurt, but my dilemma is why did it idle perfectly when I first put it in? And then did it suddenly remember that it shouldn't remember and went back to the previous adaptive settings? That just doesn't make sense to me. Any ideas?

Many thanks for your help!

 

Well yes, I did mess up the IACV when I cleaned it, but as I said in my post, I put on a new plenum complete with TPS and IACV and after doing the reset (as you also suggest above) it idled perfectly for about 15 minutes with several on and offs to check its stability. When I went for test drive it started idling fast again.
Went home and did the reset about 20 times, but no go. So something else is making the idle high or the ECU is going back to its previous adaptive parameters. That's the part I don't understand.

Any other suggestions?

Many thanks for your help!

 

Would a faulty coolant temp sensor make the car idle fast?
When unplugged/plugged it makes no difference to the idle speed.

In fact I unplugged all the sensors one after another to see if any of them would affect the idle and only the MAF had a momentary drop in idle - non of the others made any difference.

However if a faulty coolant temperature sensor can make the car idle fast then that sounds like the culprit doesn't it? But why would it run perfectly for 15 minutes? And also might that explain why when the engine is warm, if you restart engine it idles slower (still high +/- 1200 vs 2000RPM)?

 

On most vehicles, not just Rovers, the ECU monitors the coolant temp, and if it thinks things are too cold outside it will increase the fuel mixture. Very common in Fords. On my Disco, if the sensor is unlugged the scanner reads -40F temp. It runs OK on warm up, then the value of the sensor goes out of range. This sensor does not have one side tied to ground. You can read resistance with an ohm meter when cold, again when cranked and idling good, to see if there is a change, or if it goes open. Or look with data reading scanner.

 

I get it thanks, I'll let you know how it turns out.
Much appreciated.

 

re: Wow, thanks for that very comprehensive quote from the RAVE, much appreciated!

You are welcome, but the RAVE is free and should fit on your computer, or a USB drive so you can move from PC to PC.

 

The ECU's are not plug-n-play unless you also swap the alarm unit from the same vehicle. Then they are plug-n-play. If you don't change them out it then requires Testbook to marry the two.

If you follow Spike's suggestion (and it may take more than a couple of tries) you will then need to have the IAC calibrated to the ECU using Testbook, Autologic or Rovacom and preferably by an independent garage.

Did you move the IAC's plunger OR was there allot of carbon build-up removed? If so then simply unplugging, turning on ignition, replug, turn ignition off then rinse repeat a couple of times and then restart, drive around for awhile until the engine temp is normal. It sometimes takes more than three drive cycles for the idle to fall to normal. Basically your helping the ECU relearn new values and to toss out the old ones......