Code 48 Madness - Idle Air Control Issues
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Thread: Code 48 Madness - Idle Air Control Issues

  1. #1
    FRESHMAN ROVER Razorbeam's Avatar
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    Code 48 Madness - Idle Air Control Issues

    I am at my wits end with the IACM or stepper motor. I've had my RR for about 3 years and have done a litany of maintenance and repair work on it. The only issue I have not been able to conquer is the idle air motor.

    The symptoms of my issue are hard starting and rough/searching idle as well as a CONSTANT check engine light.
    I am using the Rovergauge software to access the 14cux and identify which fault codes are occurring. The only code that ever shows up is code 48.

    The engine behaves like this:
    - from a cold start, it takes ~5 seconds of cranking before it stumbles into a start. I say that because it isn't a clean sounding start. It sounds stumbly for the first few seconds before it revs up to ~1,800 RPM then settles back down. Cold idling isn't much of an issue.
    - As the engine warms up it drives normally. Just to be clear, this issues does not affect normal driving/accelerating, only idling.
    - After the engine is fully warm as determined by the Rovergauge readings for fuel and water temp, it is not until I come to a stop sign or stop light that it throws a code and the idle starts searching. It varies ~200 RPM or so, but doesn't stall. I think I have the base idle screw set a little high, which may be what is keeping it from stalling out completely.
    - If I clear out the code in the Rovergauge software it comes back within 60 seconds.


    First, a list of things that have been done to the engine in my ownership:
    - head gaskets
    - complete disassembly and cleaning of the upper intake
    - New wires, cap, rotor and a brand new distributor
    - MSD Digital 6A ignition control unit
    - Several sets of new Champion copper OEM spec spark plugs
    - New fuel and water temp sensors
    - new MAF sensor
    - silicone tape around the intake tube to eliminate the possibility of pinhole leaks
    - New "Ford style" injectors (although I am swapping these out shortly with a different part #)
    - Hedman Headers
    - New waterpump and all coolant hoses
    - New heavy duty cooling fan
    - New fuel filter

    I know some of those items are not related to this issue, but I wanted to be through.


    Some of the things I have done/checked to try to fix this issue:
    - Reset base idle per the Rave manual. I actually keep the allen key in the car to make adjustments from time to time.
    - Set base timing, multiple times. Timing is set at ~7 deg. advanced with the procedure from the rave manual
    - Tried 4 different stepper motors including OEM ones, albeit from the junkyard. I have purchased a few cross referenced ones, some as cheap as $8 or as much as $30. I have not purchased a new OEM Land Rover unit as those run in the $120 range and wont guarantee a fix. I do notice that the non-OEM ones have a slightly different cone shape than the LR ones. I'm not sure if that's causing the issue or not.
    - Checked all wiring on the loom to ensure there's nothing grounded out or broken.
    - Reset the stepper motor using the Rave software to ensure it is at its maximum open position when starting. Observed the activity while running using the software as well.


    So, like I said, I'm at my wits end. I have put so much work into this truck and it really does drive beautifully on the highway. This idle issue is the bane of my existence at this point. If anyone has ideas of what I may have overlooked I'm all ears.
    1992 Range Rover Country SWB

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  3. #2
    SOPHOMORE ROVER Mikieman's Avatar
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    I’ll add my 2 cents worth
    Have you tried putting something on the treads to give it a better seal
    I use dialectic grease
    I get a code 48 from time to time
    But mine usually backs off
    A quick tighten gets it
    And there is where the throttle body connects to the intake
    It is suppose to have a gasket there
    So if it’s metal to metal
    I believe it is a place to have a vacuum leak
    I used RTV to seal mine
    Just throwing out options
    So 2 cents
    im an american i dont speak english

    91 classic..it is now parts

    92 lse... not a bucket of rust

  4. #3
    FRESHMAN ROVER
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    Re: Code 48 Madness - Idle Air Control Issues

    Do you know what the reference voltage is set at for the MAF? or if it's been changed from factory setting?

    I doubt if it would make the difference you've experienced, but I routinely use NGK BPR6 plugs successfully on these engines.

    Presumably you've got the plug wires routed through their plastic separators to help prevent crosstalk of HT voltage.

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  6. #4
    FRESHMAN ROVER
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    Re: Code 48 Madness - Idle Air Control Issues

    I had the exact same problem after carrying out a lot of the same repairs.
    The Code 48 you're experiencing probably has less to do with the stepper motor than being a byproduct of your idling issues.
    Post-engine rebuild I had to set my RRC to idle above 800rpm to keep it from stumbling, and that kept throwing a Code 48.
    The short version of mine is that a combination of the throttle body, base idle, and engine timing needed adjusting to be in sync.

    However, I would check the electrical connections at the diagnostic plug with a multimeter per the RAVE manual; this will definitively tell you how well everything is working electrically.
    I learned the hard way that you can save yourself a lot of hassle and money by verifying the ohm-readout first before throwing parts at a problem.

    FWIW you've done a ton of work trying to suss-out this problem; I ended up caving and bringing mine to a Land Rover specialist because I found the job to be way too tedious.
    If all else fails, it's time to light some candles and draw a pentagram on the ground

  7. #5
    SENIOR ROVER
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    Re: Code 48 Madness - Idle Air Control Issues

    have you tried looking at 02 sensor performance, this may give you an insight on what the engine is doing and its performance. http://www.britishv8.org/Articles/Rover-14CUX-EFI.htm
    2k p38 4.6
    95 lwb 4.6
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  8. #6
    FRESHMAN ROVER Razorbeam's Avatar
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    Re: Code 48 Madness - Idle Air Control Issues

    Thanks everyone for chiming in. Really good suggestions, and I appreciate everyone working through this with me.

    Mikeman
    I used plumbers teflon tape on the one that is on there now. I did use some RTV red on the intake plenum when I took apart the upper intake and cleaned it. I have since taken it off and put it back on to get the headers in, so the seal might be broken, but it was having issues before I broke the seal.

    RR London
    The MAF is a brand new LR OEM part. I had the truck taken to a shop at one point in all of this and they couldn't figure it out. They then told me that all they did was reset the timing, test the alternator, and put a new MAF on it. The wires are well separated. I used a few Amazon kits to keep everything separated, and fiberglass spark plug boots.
    The MAF readings are normal on the Rovergauge software as far as I can tell.

    Shastacaster
    Good to hear I'm not the only one who's been stumped by this. I've tested the temperature plugs with a multimeter, but that's it. The Rovergauge software allows you to see what the readings are for all the senors.
    When your idle was set too high, did you hear a whistling sound? I find that the further out I open the idle set screw, the louder the sound is. It sounds like a vacuum leak. If I close the screw to the point the whistling stops, the idle is too low and it stalls in gear.
    I took it to the shop I've been using for a while that works on a lot of older american/euro cars and they couldn't figure it out. They later told me that they didn't know about the idle set screw and just adjusted the timing and sold me a new MAF and coil (even though the old one worked fine).


    95classiclwb
    I recently installed a set of hedman headers and had a shop weld up a Y-pipe with O2 sensor bungs installed. I had to lengthen the wires to reach the plugs on the loom. As far as I can tell the ECU is reading them fine. The Rovergauge software shows it right in the middle (rich/lean) as I drive on the highway with minimal variance between them. I got a code initally for the left bank, but I had used a TON of copper sealant on the header gasket because I was chasing a leak. I've read that the excess that burns off can foul the sensors, but mine has appeared to clear up. It hasn't thrown an O2 code for ~100 miles.

    I'm gonna go back and reset the timing and reset the base idle and timing and see what that yields me. I also might go raid the junkyard for some more stepper motors to try. Appreciate everyone's input, I'll let you know how it goes.
    1992 Range Rover Country SWB

  9. #7
    FRESHMAN ROVER
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    Re: Code 48 Madness - Idle Air Control Issues

    Id want to see the real voltage with multimeter at the MAF to have peace of mind, it's blue red wire at the plug to MAF.

    Factory set should be in range of 1 to 1.5 volts but the examples I've checked seem to need close to 1.0v or even less to run well. I see it as the baseline gain setting for the MAF. Ecu will compensate to a certain extent but it appears to have consequences to do this.

    I don't know if a new MAF comes calibrated and fits without testing, or if they should routinely go check it on the vehicle to account for variables existing within installed system.

    My knowledge of non vehicle systems like this says it should be checked at install, that's what the gain adjustment is for, to bring the MAF in range and sit at optimum report to allow internal ecu mapping to have most tolerancing in respect of directing injection pulse time.

  10. #8
    FRESHMAN ROVER
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    Re: Code 48 Madness - Idle Air Control Issues

    As an additional consideration, mine runs perfectly for starting, idle response, driving etc.

    When warmed and idling, if you pull the dipstick or take off oil cap, then the idle initially increases as vacuum in crankcase shifts. It's subtle but is there.

    Another thought, do you have compression readings for it?

  11. #9
    FRESHMAN ROVER
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    Re: Code 48 Madness - Idle Air Control Issues

    @Razorbeam
    Yes, before getting the problem fixed there was a loud whistle coming from the air intake hose. The base idle screw really only controls the minimum opening of the butterfly valve at idle, so that whistling is from air rushing past it. From what I can understand, there's a balance that has to be achieved between the base idle, engine timing, and accelerator cable setting.

    Do you know what your RPM's are at idle? Check with a tachometer, the rpm gauge on your dash isn't accurate enough.
    Ideally you want between 665-735rpm. I'd adjust the tension on the accelerator cable first to get within that range, and then adjust base idle and timing.
    My LR mechanics said that timing is set somewhere between 5° and 7°, so it's more of a dynamic range than a hard number.
    Like I said though, this is a really finicky and tedious job, so I left it to the pros.

  12. #10
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    Re: Code 48 Madness - Idle Air Control Issues

    try setting your timing at 10 degrees advance, an old mechanic's trick is to spray carb cleaner or gum out near all intake joints with engine running one location at a time, when engine idle raises a little you have found your source of false air.
    another possible point of leak in my experience, has been the injector seals at intake.
    you never mentioned cam wear, basically if cam lobes are too worn engine struggles to breathe and codes rear their ugly face.
    2k p38 4.6
    95 lwb 4.6
    01 d2 4.0

  13. #11
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    Re: Code 48 Madness - Idle Air Control Issues

    I feel you need to proceed with care here to avoid compounding the problem. It may be worth isolating the different functions to allow a clearer review of their intent and influence.

    The base idle screw sets the butterfly MINIMUM position to prevent it being completely closed and stalling out the motor. There is a safety element to this that's important. If say you are off throttle at road speed with the motor being pushed by transmission, then too low a position can result in butterfly flap being pulled closed under vacuum influence and killing the motor. Don't forget that the ecu will not "see" too low an rpm so it won't instruct the IACV to act in control, you don't want it to stall during road speeds. That hard baseline is there for a good reason.
    The setting routine effectively covers this by putting a floor rpm set mechanically as a fall back as it removes the ecu control while you set it. Too closed can result in that whistling, which suggests that the intake has another source of air that's not being controlled via the throttle plate.

    The ecu can then effectively take control as it's range is all above that minimum rpm you've just set.

    The throttle cable on a petrol engine should never be used to hold the throttle open at idle in my view. It should always have some slack and is just a method of opening the throttle but nothing more. The throttle plate should always be stopped by that base idle screw, not the cable.

    If you are getting uncontrolled air the most prominent source would be the breathers going to intake assembly. You can remove these and plug the entrance to intake tract to test the theory. If it effects a cure you are then looking at why the crankcase is being pressurised, it's usually piston rings! Letting compression leak past hence my question about testing for compression.

    The fuel mixture as far as I can see is not modulated at idle to control rpm (they just use IACV to change mixture and hence speed) so this is set by altering the offset on that MAF adjust potentiometer, and the voltage mentioned above. Basically if it's too rich it will stall more readily before the ecu can pull up the speed by reacting with the IACV valve.
    Remember that with engine temp report saying it's warmed the mapping will not be orientated the same as cold so the ecu will expect a different result from its actions, but if it's running rich it'll not compensate effectively.

    Some of these things need verifying to move through more in depth diagnosis.

  14. #12
    FRESHMAN ROVER Razorbeam's Avatar
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    Re: Code 48 Madness - Idle Air Control Issues

    Quote Originally Posted by shastacaster View Post
    @Razorbeam
    Yes, before getting the problem fixed there was a loud whistle coming from the air intake hose. The base idle screw really only controls the minimum opening of the butterfly valve at idle, so that whistling is from air rushing past it. From what I can understand, there's a balance that has to be achieved between the base idle, engine timing, and accelerator cable setting.

    Do you know what your RPM's are at idle? Check with a tachometer, the rpm gauge on your dash isn't accurate enough.
    Ideally you want between 665-735rpm. I'd adjust the tension on the accelerator cable first to get within that range, and then adjust base idle and timing.
    My LR mechanics said that timing is set somewhere between 5° and 7°, so it's more of a dynamic range than a hard number.
    Like I said though, this is a really finicky and tedious job, so I left it to the pros.
    Interesting that you mention the accelerator cable. Mine is slack with no pre-tension on it at all. I wasn't aware that it could be part of the equation. I figured that the throttle butterfly would be completely closed at idle, with the air coming from the idle base setting screw which diverts air around the throttle body.

    I also think I need to reset the timing without the MSD box hooked up. I know it doesn't affect spark timing, but I may not be getting an accurate reading from my timing light.
    1992 Range Rover Country SWB

  15. #13
    FRESHMAN ROVER Razorbeam's Avatar
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    Quote Originally Posted by RRLondon View Post
    As an additional consideration, mine runs perfectly for starting, idle response, driving etc.

    When warmed and idling, if you pull the dipstick or take off oil cap, then the idle initially increases as vacuum in crankcase shifts. It's subtle but is there.

    Another thought, do you have compression readings for it?
    I did a compression test a while back after the head gasket job and it was pretty even. Around 120 on each if I remember correctly. The shop also performed the test and said things were “normal”.
    1992 Range Rover Country SWB

  16. #14
    FRESHMAN ROVER
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    Re: Code 48 Madness - Idle Air Control Issues

    As suggested by 95classiclwb, and it seems to me that you've got uncontrolled air getting in there somewhere.

    Of course any setting of base idle at this point is going to be arbitrary.

    But found this thread when doing some research https://www.rangerovers.net/forum/8-...ce-wanted.html

    Which agrees with 95classiclwb and appears worth investigating.

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