Mysterious to me at least

3.9 V8 Discovery 1 1995, 218k miles (Range Rover Diagnostics Equipped)

From a dead cold start ( ie Connecticut Winter) idle and process appears to be fine, at current exterior temps of around 32F the idle will gradually deteriorate after about 4-5 mins. It will never cut out, just fluctuate in a non rhythmic pattern, threatening to drop too low then salvaging itself. it is continuous and does not improve until the accelerator is touched. It runs well with no signs of a misfire / skip / hop etc.. just does not like to sit idle..

Here is a link to the video of how the idle is behaving. At 0.17 second in the video I drive forward about 30m hence the increase in revs.
https://photos.app.goo.gl/ohV5X9pHwyywEDCs7

I have worked through the following..

Plugs > New
Leads > New
Air Filter New (Had mouse nest inside from previous owner)
Fuel filter > New
Coil > New
Alternator > New (Previous one failed)
ICV / Step motor > New
Coolant temp sensor > New
MAF > Cleaned (looks pretty old)
Air intake > Cleaned
Plenum > Cleaned (It does not like either of these being cleaned, takes several hours to start after cleaning)
Vacuum lines > Cleaned + Checked
PVC > cleaned
Flame trap > cleaned
Fuel pump > swapped with my 1996 4.0 > same results > swapped back
Fuel pressure > checked correct 29 /30 PSI at idle 36 /37 PSI with vacuum disabled (this is correct from what i understand although there appears to me a lot of debate and conflicting info across many forums >> correct me if I am wrong)
Fuses checked > Horn and fuel pump had been switched, fuel pump had 30amp horn had 15amp and did not function, switched to correct positions, horn now works)
Throttle Position Sensor > checked with multimeter, reading correct voltage and gradual increase from 0 to 5 Volts as throttle is adjusted.
2 bottles =/- or fuel system cleaner have passed through (cheap Walmart ****) > no change (try again maybe with a better brand?)

I'm just an idiot who likes to tinker, I know the 3.9 V8 and their behaviour fairly well but I'm certainly not an authority on them like many out there.. That said it appears to me to be temperature related, I expect the next moves are 02 sensors / fuel temp sensor, if anyone has a firm direction to pursue on either of those or something I have overlooked please chime in!!

Many thanks in advance! BADidOL

 

Assuming you have the 1994/1995 14CUX OBD1 system, I would highly recommend buying one of these (https://www.ebay.com/itm/285095912916) and downloading a copy of the RoverGauge free software (https://github.com/colinbourassa/rovergauge) for windows. That system will let you visualize and log all of the sensor data for the fuel injection system and will let you see any out of range sensors or strange behavior. I have it setup on an old windows tablet which I keep in the car if things start acting up. Highly recommend. I can share more on how to get it setup if you need.

If you don't want to go that way, the only other things on my mind would be:
- Blocked injector (or short in an injector line) - would try a noid light first
- Fuel temp sensor as you say
- Vacuum lines running from throttle body to stepper and back to flame trap. I know you said you checked these but something about it not running after cleaning leads me back there.

Final thing. From what I can tell in the video the revs are quite low (~500) rather than the (~700) they are supposed to be. To me that indicates you have a blockage of the plenum vacuum which is essentially driving the idle down to "base idle". That could also be caused by a fully extended stepper. The "base idle" setting is usually the absolute last thing you would ever touch unless you're sure the vacuum lines and MAF are good but if you can remember where you turn from/to and reset to the current position that might be worth a try. A quick check would be squeezing the rear vacuum line from the throttle body to the steeper shut and see if that drives the idle down to stall, or it if just stays as is. If it stays as is you've got a blockage or a bad stepper I think.

I'm not really an expert though - will let some others chime in. I honestly can't say enough about RoverGauge. You can see everything on there....

 

Could be idle air control valve and passages need a good cleaning. It changes position as the engine warms up. Maybe clean up the throttle plate too and PCV system. That's easy to do.

One other thing to check is the vacuum advance unit on the distributor. A faulty diaphragm acts as a small vacuum leak. I have had two go bad, didn't hold vacuum.

 

Yep vacuum leaks on the injector O-Rings, plenum, brake booster, line from airbox to valve cover, heck even the flame trap/metal PCV being loose can cause a leak.

 

Dirty IAC is the first place I go for idle issues

 

The Land Rover workshop manual has a description of using compressed air (and a rag to catch the crud) to clear the IACV passages. Also notations for setting base idle and timing.

 

This is great analysis and grumpy Rover intel! Very much appreciated. I'll definitely grab that cable as I plan on keeping this Disco until it's death as it is 85% rust free!.. I might give you a shout down the line when I get set up regarding the installation, I've tried a lot of Github programs over the years, some can be quite tricky to get up and running.. Let you know when I do.

There appears to be a rewired injector so I'll be looking into that, a fuel temp sensor is $16 on Alibaba, up to $185 on Atlantic British etc so I might just grab that as it's another cheap update and grab a noid light as per your suggestions..

I'm pretty sure those vacuum lines are are clear however I'll double check..

I forgot to mention that the amplifier on the distributor has been relocated with the Land Rover update kit but there are two Lucas 'chips' (not sure how to refer to them) connected into the wires from the distributor to the coil, as far as i can see these are not part of the kit, have no idea what they are assisting? Any ideas?

I also found a thin disconnected plastic tube with molded end hanging around the side of the air filter box. I will post photos of both of these later today.

As for the Revs it sits at 700 rpm, as soon as it adjusts to that level the fluctuation begins, so it does appear to want to settle naturally at 700 rpm as it should but then is dragged down by whatever is taking place, as opposed to being set to low. I will run the tests you mentioned today / tomorrow and report back .

Just to clarify for all responders, the stepper is new, I put it in last week, it works / behaves exactly as the one I removed so the step motor itself I have removed a possible culprit, I it certainly behaves as if it could be.

I was not aware that the vacuum system extended into the brake booster and beyond, I have a spare vacuum advance so I can swap that out (assuming it's not a dud, truck came with a bunch of bit that had been swapped over 30 years, so new some duds, but not info on what had / had not / why things had been done)

Injector O rings that's also an area I have never delved into..

I am hesitant to clean the plenum / throttle body etc again, I've done it twice and I could not get it to start for about 30 mins the first time, the second time it took me until the next day. As you an others have suggested that's a red flag right there.. if so, how would you proceed in further cleaning to avoid a repeat scenario? Thanks again! I'll keep you posted even if it takes several days to report back. Cheers!

 

If your base idle passage is plugged, that could explain your symptoms. It's on the right side of the throttle. There is a metering screw just downstream of the port for the IACV. The factory covers the screw with an aluminum cap, so that will need to be removed to adjust or back out for cleaning. Count turns so you can get it back in. Base idle is checked with the IACV blocked off engine warm, then idle spec is 500 rpm, as I recall. Mine just runs there, not lower.

The small tube beside the air filter might be the one to supply air to valve cover for PCV flow. There is a port on the clean side of the filter, so if that is open some dirt could get into the clean side. If the tube is plugged the PCV system won't sweep the crankcase.

Make sure your vacuum advance unit holds vacuum and doesn't leak.

Gotta go. Good luck.

 

What JohnZo said above is very important.

With the stepper system blocked off (I actually use rovergauge to close it, then quickly unlpug the stepper), the base idle should be VERY low. The dash tacho is often wildly inaccurate, so rovergauge is a better gauge of engine speed. I find the lower the base idle (500 to 550 max), the better the ECU can control normal idle (Drive= 600 RPM, P or N = 700)

Clean the idle path out. I use Nulon foaming intake cleaner - maybe the seafoam stuff does the same? Remove the screw, fill it with cleaner. Poke a cable tie or something in to give it a hand, then run for a bit and repeat. For the 14CUX system to idle right, the PCV and idle paths need to function correctly. Also, make sure the oil dipstick tube is sealed and the throttle plate is fully closing - it should be pretty much parallel with the throttle body opening (+/-0.5mm) - it should NOT be adjusted open to hold idle up - the adjustment is under the plenum and people don't usually play with it anyway. Where the PCV system connects to the plenum (just behind the distributor) there is a Tee with a restrictor orifice in it - that should be cleaned out too.

It needs to have no vacuum leaks to function.

If all that doesn't help, make sure the fuel pressure reg isn't leaking fuel into the vacuum hose and that you don't have a leaking injector.

 

Thanks so much for all the input by everyone here so far. Some great info provided by you guys, I've not heard of the base idle passage screw across any forum before so hopefully that will help others out there. Sorry for the lack of communication, I have not had much of a chance to test everything suggested but here is a brief update since last time and a few photos.

> Vacuum advance appears to be in order, gave it a suck test, I can hear the diaphragm pop back an forth.
>The loose hose I found on the right of the air filter box was actually the headlight washer that has been disconnected by someone
> Cruise control has been disconnected at some point ( I don't 'think' this vacuum network has anything to do with what we are talking about here or am I wrong?)
> It's had a bottle of Seafoam in 1/4 tank of gas, no change
> cleaned the connection with paper clip and intake cleanerbetween air filter box and valve cover, no change
> cleaned connection off plenum to vacuum advance with paper clip and intake cleaner, no change
> Checked PCV Tee again, all clear
> I have not done Seafoam / Nulon on the plenum yet due to the issues it has caused in the past, I'll do that when I am sure I don't need the car for several days
> As per suggestion '94Disco1' "A quick check would be squeezing the rear vacuum line from the throttle body to the steeper shut and see if that drives the idle down to stall, or it if just stays as is. If it stays as is you've got a blockage or a bad stepper I think." I did this, I'll check again in case I got the the wrong hose, it sent the idle down to an almost stall then recovered, reading this now I could
have done the wrong line between the fuel regulator and the back of the plenum / stepper.
I've attached photos here..these are the 'chips' I was referring to in the coil to pickup sensor line
Also is this the screw I need to remove to clear the base idle line?

https://photos.app.goo.gl/ijnVfqz5k166Bfsv9

> I've not looked for the following screw suggested here by JohnZo this will be next "There is a metering screw just downstream of the port for the IACV. The factory covers the screw with an aluminum cap, so that will need to be removed to adjust or back out for cleaning. Count turns so you can get it back in."
> A salty old dog at Napa advised me to to spray intake cleaner around the hoses bit by bit to see if revs increase to find a leak, might be a common trick and for a Land Rover fire! .. Any suggestions on this hack?
Will keep you all updated and this thread going until it is solved! even if it's quiet for a while.. this bastard idle will be fixed!

Thanks again to all so far