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Thread: Cold idle RPM gems V8

  1. #11
    Join Date
    Jan 1970
    Location
    Adelaide Hills. South Australia
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    2,150
    Yes the IAC body is very thin & looks undercut at the end of the thread as I found to my cost recently & it snapped off.

    I removed the casting & hack sawed it along the thread & then separated it with a small chisel otherwise it would have been impossible to remove without filling the engine up with cuttings etc. Job done.

  2. #12
    TheTree is offline ChatterBox Silver Subscriber
    Join Date
    Feb 2013
    Location
    Gosford, NSW, Australia
    Posts
    2,505
    I tried one of the Jeep IACV and found that it did not seem to work properly so I went for the correct one

    Steve
    MY96 4.6 HSE P38. Niagara Grey; "The Phoenix"
    MY99 4.6 HSE P38. LPG, Blenheim Sliver; "Voldemort" burnt to a cinder
    MY79 RR Classic, Gone, MY57 Series 1 SWB, Long gone
    P38 write ups and info on my website http://p38.hts.com.au/

  3. #13
    Join Date
    Jan 2016
    Location
    Netherlands
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    258
    Hey guys,

    The MAF sensor I should add was swapped, not replaced. Typo in my original post It was suspect a while back but the original (cleaned) MAF is back in place. The vacuum leak as far as I can tell is fixed. I tested the system by taking off the MAF sensor, putting a plug into the air intake, connecting an air hose with slightly pressurized air and adding to that a smoke generator. This way any leak in the system shows as smoke escaping, really neat trick! if I say so myself :P

    About the IACV, yes, I think I'll order a new one. I usually take what I can get and this one was on fleebay for not too much money and quickly delivered. I'll go find the original product asap.

    My plugs are.. 30K old? Not sure how often to replace them but it won't hurt taking them out to check I guess. The car drives beautifully so I would not suspect anything, if it were not for that bloody idle

    Last weekend I did the JOTA (I work as a volunteer for the scouts 60K's north of here and I am a licensed HAM radio operator) and I took the range with me, including RTT. It drove quite well and once it is warmed up the idle is OK-ish. It still drops a bit to far when coming to a halt going forward, but does not stall.

    It still drives like sh*t going backwards or when cold.

    Cheers,
    -P

  4. #14
    Join Date
    Jan 1970
    Location
    Adelaide Hills. South Australia
    Posts
    2,150
    Oddly enough I have had this crap idle for a few weeks now. No biggy, but just damn annoying.

    Tried everything I read or knew about & today I screwed in (tightened) the Throttle cable a smidgin' by rotating the black plastic knurled nut at the end of the Throttle Body at the slotted bracket. This obviously affects the linkage & I was able to lift the idle RPM to about 750, I know, it's a tad fast, but it feels & sounds better & I am quite happy to leave it here. I did have it over 1000 rpm at one stage while fiddlin' but soon screwed it back.


    I had a bloke ask me about the movement of the IACV moveable pintle a couple of months ago & this is what I was able to tell him..

    Read on Dear Reader...........

    According to the Installation Instructions that came with my replacement a few years back, it isn't supposed to physically push in & out but you depress & release the spring from it's slot (pintle end) & rotate the shaft in or out to give you a dimension of 28mm from the gasket mounting surface to the tip of the pintle. NB.Return the spring leg to it's original location when you are finished.
    If it is too long, internal damage can occur to the IACV.

    Torque when tightening the valve back in is 13ft/lbs. (48 NM) & using a 32mm wrench.

    Bugger, how to get rid of Bold type?? No matter.



    I can send you a copy of said Inst. if I have your E-mail Addy. Perhaps you could PM me?
    P.S. There is a Reset Process that may be followed for Buick & Pontiac but as that won't affect you I can't be arsed typing it all.









  5. #15
    Join Date
    Feb 2010
    Location
    Townsville, QLD
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    2,522
    I have a question regarding the winding the throttle cable. Iím not an expert here, so please excuse if I have this wrong!

    The throttle position sensor (in my mind) would now be seeing a higher than 0% throttle position at your new idle speed as a result of this, would I be correct? If thatís the case, that would also effect deceleration fueling, as the ECU would otherwise turn the injectors off during deceleration between X and Y rpm, and as itís now seeing a position above 0, this wouldnít happen.

    Just food for thought!

    Cheers
    Keithy

    Green 2000 P38 Range Rover HSE - Gone
    Blue 1999 P38 Range Rover HSE - Gone

    Silver 2002 P38 Range Rover HSE - Sequential LPG, BCDC Charger, TPMS, Rear Locker, Wheel Carrier, Bullbar & Spotlights, General Grabber AT2's, Too Much to Mention!

  6. #16
    Join Date
    Apr 2009
    Location
    Crafers West South Australia
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    11,519
    Quote Originally Posted by Keithy P38 View Post
    I have a question regarding the winding the throttle cable. Iím not an expert here, so please excuse if I have this wrong!

    The throttle position sensor (in my mind) would now be seeing a higher than 0% throttle position at your new idle speed as a result of this, would I be correct? If thatís the case, that would also effect deceleration fueling, as the ECU would otherwise turn the injectors off during deceleration between X and Y rpm, and as itís now seeing a position above 0, this wouldnít happen.

    Just food for thought!

    Cheers
    Keithy
    Exactly, adjusting the base idle screw on top of the plenum is a much smarter idea.

  7. #17
    Join Date
    Jan 1970
    Location
    Adelaide Hills. South Australia
    Posts
    2,150
    Thanks for that.

    You may be correct with all that Keith & Jaden, I don't know, but if it is, it's not evident in any way I can see or feel,. Tried the Base Idle Allen Screw Jaden, made no difference. The fact I am going on the OEM fitted rev counter ( and we all know that is a crap practice, but I don't have access to a better one for this small matter ) so it could have been out for Donkeys years, hopefully in the right direction, which would make SAPOL & the Govt. coffers unhappy. Hhmmmmmm, now having said that.........................

    As it happen, 'er indoors has just driven off from a cold start (to go lunching with the girls you see) & the short time she let it idle before moving off, it sounded a lot better & more responsive.

    Just having had a new "Big Box Tri-flow" muffler fitted a few weeks back & (imagine an empty 44gallon drum with a couple of pipes hanging out of it & not my choice I hasten to add) made it sound worse then but it sounds ok today for which I suspect was caused by the low rough idle rpm. Now 750, but it could be 600 in reality for all I know, which is good enough for me. Have you ever tried to accurately set the idle on a 3.9. One follows the Manual & procedure & blocks off pipes etc & eventually it still turns out crap. For many years I recall, idle speed was always done by ear not "noo fangled gadgets". Back in the'50 & 60s & probably later, if one owned a half decent Rev Counter one was King & had some street cred.

    It also could have been because the Throttle cable stretched a bit since '95. & I have returned it to the original setting. I dunno.

    Thanks again for your opinion.


    A/C lifts the RPM as it supposed to do so it does seem "normal" if normal is a word one can use in relation to Land Rovers.

  8. #18
    Join Date
    Jan 2016
    Location
    Netherlands
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    258
    I have been driving around with the P38 for a couple of days now, I need to since my daily driver is getting a mike sanders treatment. When warm the idle is ALMOST where it should be, only occasionally will the idle drop to the 500's when coming to a halt but most of the time the ECU corrects nearly immediatly. Blipping the throttle also tends to throw the idle of, unless the enginge is really warm. I have monitored the IACV position with my faultmate extreme during driving and I have come to these conclusions:

    When warm and cruising (cruise control on) around 100kph for say 15 minutes, the long term, short and current trim are nearly always around 30, 45, 70. As soon as I start engine braking the current value changes, mostly upwards. When however I get around a very low coasting speed (still in gear) it would drop nearly immediatly (the update interval in the faultmate software is not fast enough to display it I asume) to a low-ish value of around 60. Since this cause the engine to idle to low the ECU will, often, adjust immediatly and recover the correct idle RPM.

    When warm and in drive, releasing the brake but not touching the accelerator pedal the car will idle around 750 and accelerate up to the point where the gearbox is not slipping but simply idling along. Once I reach that point/speed the ECU instantly drops the current trim to around 60-ish and the idle drops of to around 500-ish. The ecu usually does not recover from that by itself for some reason. Accelerating, braking or switching to neutral all works without stalling the engine. Braking and neutral will eventually see the idle return to around 750 as it should.

    It seems that the ECU does learn, up to a certain point, how to deal with the behaviour of the engine and IACV but within a maximum band of parameters.

    When cold however, and since autumn has set in down here, with temperatures around or below 10c there is no hope in hell the ECU will do the right thing. I can get the engine started alright, it will idle as it should even when switching into gear or the A/C switching on and off but as soon as I try to manouvre the vehicle around the idle speed it will drop down to 200-rpm and barely plod along or simply stall. I try to give the ECU as much learning time as possible by fooling around moving the car back and forth every time I start the engine when cold, giving the ECU as many learning points as I can possibly give it but them bloody V8's warm up to fast haha or it stall's too often and the ECU does not seem to learn anything at that point.

    What I have noticed when looking around my faultmate is that the O2 sensors for bank 1 and 2, that is to say the fueling, is always jumping up and down and are rarely the same. When bank one is around 1 bank two can be around 5 and vv. I am not sure if this is normal behaviour but it might be adding to the confusion for the ECU. When the car is really hot and idling you can actually see the needle of the RPM on the dash jump up and down slightly.

    All in all I am at a loss as to how to fix this problem. I've pretty much ruled out all air leaks afaict, we know the iacv to be working and pretty fast as well, the throttlebody is clean and the PCV's are hooked up to an oil catch can and thus are breathing as the should as well. The only thing I can think of is that my piper 285 cam has a bit of a rough idle, possible letting to much O2 through and upsetting the ECU's fueling, which in turn adversely affects the idle. The weird thing is though that when the engine was new a couple of 10's of K's ago it did not have this problem, at least not this bad.

    I am open to ANY suggestions as to how to move forward.

    Thanks!

    -P

  9. #19
    Join Date
    Aug 2018
    Location
    Brisbane
    Posts
    35
    Whatís the lobe separation on that cam? 108 or lower and you will have massive idle issues, I can explain why but a rough lope at idle makes me think you must be on 108.... too much overlap

  10. #20
    Join Date
    Jan 2016
    Location
    Netherlands
    Posts
    258
    I would be interested to know why that is I don't know to much about cam's tbh.

    It took some googling but these are the specs for the cam:

    type power band valve timing duration valve lift full lift ATDC/BTDC
    285 2000-6500 44-72 / 74-44 296 deg. 0.450" 104 deg.

    I grabbed this from the RPI site, piper themselves do not seem to have this data online.

    The funny thing is, it has worked reasonably well for about a year without too much hassle, sure the rough idle was always there but the ECU at least kept the engine running This morning it was a bit of a challenge once again to keep her running...

    Cheers,
    -P

    edited because of those bloody table-paste-fails.

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