Fast Idle problem driving me crazy!
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 when the ignition is switched on.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 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 sensorTune 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 operates the fuel pump to pressurize the fuel system.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 |
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. |
Originally Posted by Savannah Buzz
(Post 269957)
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.
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! |
Originally Posted by Spike555
(Post 269973)
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. 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! |
Originally Posted by Savannah Buzz
(Post 269957)
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 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.
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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...... |
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