irritating load stalling warm V8 3.9

[disconc]

Hello everyone,

after searching and reading a lot, I have found some things, but yet can not solve my problem.

Here is the story (sorry, long post) :

Discovery autobox with a 3.9 EFI engine, 8.13 low comp, no cats, "Australia / Rest of the World" specs.

The engine has been entirely rebuilt 2000 km ago by a friend.

No problem for starting, either hot or cold, never !

No problem when running...

The only problem is as follows :

When engine is warm ONLY, the idle on Park, or Neutral is ok and smooth.

When driving around and you stay at the lights with Drive engaged and braking, Idle is perfect.

The problem is only :

-> when I am idling in Park or Neutral, and engage reverse or drive, well adding a significant load to the engine, the revs drop to below 500, and sometimes stall, sometimes recover.

When it recovers, it then idles ok with the load applied

When it stalls, you can re start again no problem

-> when offroading, like rock crawling, this is irritating, as you come to a stop with a tire on a rock, you give it a little gas to come over, then release... and it stalls when returning to idle.

-> you could make it happen also by giving it a little gaz with drive engaged and the left foot on brake and release gaz... same effects... it stalls when returning to idle.

It seems like it does not cope with a load apply on idle...

Does not do it with AC or lights, but have not had chance to try with a winch, but I believe that a good load on alternator might generate the same stalling.

Here is what I have done up to now :

=> brand new GENUINE cap, rotor, plug leads, plugs

=> timing set warm, with no vacuum at 6 BTDC

=> CO idle trim set at 1.4 V (have no CO tester around) and I feel it is worse when this value is lower

=> base idle speed set up warm with air by pass pluged at about 750/800 rpm

=> checked for vacuum leaks

The only thing (crazy) that I found is :

If I disconnect the vacuum hose which goes from the vaccum port just below the stepper motor to the fuel pressure regulator, the engine takes a few revs (of course with more air, say from 750 to 1000), that I would bring down to the previous idle revs by tightening the base idle screw.

Leaving this vacuum port sucking outside air, and the fuel regulator not connected with vacuum, THE PROBLEM COMPLETELY DISAPPEAR.

You can now switch from park to drive, go rock crawling, rev in drive while braking, and release gaz... it always goes back to its clean idle !!!

This is CRAZY and I can not run the car with the fuel regulator like this and air entering the plenum by the sideway, unfiltered...

By the way, I believe the problem is not from the fuel regulator, as if I just plug the vacuum port, so leaving just the fuel regulator unconnected, I have the same problem.

Seems like engine wants a little more unrestricted air, but when everything is plugged as it should, I can not give it this amount of required air from the base idle screw !!

No problem when cold, I believe the bypass air with stepper compensate for it.

I do not know what else to try, if anyone could please hive me any ideas ?

Thank you all

Erik

Cheers from New Caledonia

[zim]

Tried a different air flow meter ?

[Dave W]

Have you tried setting the base idle ?

I'm not sure this is your problem but it should give a good starting/disagnosis point. To set it properly you need to (with the engine warm) block off the pipe from the plenum to the stepper motor at both ends to ensure there is no air getting into the plenum via that route. You need to stop air getting into the throttle body too as it will be bypassing the AFM.

With that blocked you need to adjust the bypass screw to give you a steady 600 rpm tickover. This gives the stepper the best range and should, in theory, stop the engine stalling if the stepper motor closes off too far.

Another common problem that can cause what you describe is a blocked breather T piece. This is often what people end up trying to compensate for using the base idle adjustment. If you look at the pipe that goes form the flame trap on the front of the RHS rocker cover to the plenum inlet, as it passes the front of the plenum there is a T piece with a small pipe running to the plenum. Inside the T piece is a brass insert that restricts the amount of air/crank fumes that are pulled into the plenum. When this gets blocked it reduces the idle RPM. Be very careful when handling it though or have a spare ready - the plastic can be VERY fragile after years of having hot crank case fumes pass through it. A small drill bit can be used to clear the carbon build up inside the insert.

After that I'd be looking at idle mixture as the further away from ideal the mixture is the less resilient the tickover is. Advancing the ignition timing a bit may help too and i'd double check that the TDC mark on your crank is actually TDC on your engine just for sanities sake. The high compression engines will normally run quite happily at up to around 10 BTDC without pinking.

Check the throttle position sensor is both correctly adjusted and has a good smooth signal throughout it's range as a noisy TPS near the closed throttle position can do all sorts of weird things to your mixture at just the wrong time.

[Aragorn]

Pulling the line off the fuel regulator will raise the fuel pressure at idle, which might indicate its running lean? It will also add an additional air bypass around the throttle body though, so that will be messing with things too.

You need to get the base idle and CO sorted properly.

You've said you set the base idle to 750rpm, which should be plenty, so if its still stalling it might point to fuelling rather than air?

[FridgeFreezer]

Surely a check & cleanup of the idle air valve (stepper) would be the first port of call, they're known to jam up or get covered in dirt which stops them working properly.

There's a full Hotwire diagnostics manual in the technical archive.

[zardos]

It seems like it does not cope with a load apply on idle...

Does not do it with AC or lights, but have not had chance to try with a winch, but I believe that a good load on alternator might generate the same stalling.

Note that the AC has a wire to the ECU to tell the ECU its running, this ups the idle by about 100 rpm

[Roverbo]

Surely a check & cleanup of the idle air valve (stepper) would be the first port of call, they're known to jam up or get covered in dirt which stops them working properly.

There's a full Hotwire diagnostics manual in the technical archive.

My first answer too, that bl... steppermotor - but oh, be carefull when you clean it, it´s very fragile, and easily falls apart if you try to move the small piston with your fingers (why do I know ? ).

Regards,

Bo (RRC now running on 3rd steppermotor)

[AncientGeek]

I second Aragorn's idea of fuel pressure. Your fuel pressure is relative to the vacuum in the plenum. that might mean it is running lean as he says or it might mean fuel pressure is marginal at idle. Test the fuel pressure.

[Highway_Star]

Be 200% or more certain about airleaks, especially between the AFM and the plenum. That wobbly pipe falsl to bits. It's expensive to replace, but self amalgamating tape can work wonders.

The stepper motor/idle speed control valve... they and their pipework love to plop up, clean it, it's housing, pipework.

Fuel pressure.. The pump in tank loves to die unexpectedly, as does it's wiring. The output pipe corrodes. Fuel filter blocked? One of the short lengths of hose between say the pump and the filter partially collapsed?

[FridgeFreezer]

So, just replace everything yes?

The AFM to plenum hose is only expensive if you buy it from LR, if you buy some 70mm Wyrem flexi hose from a hose supplier it's ~£20 per metre.

But I'd start by doing the things that cost nothing, like checking & cleaning the idle air valve & pipework, what with this being a problem with the idle...

[Highway_Star]

Where did I suggest "just replace everything"?

I did try to obtain some suitable hose from the AFM to Plenum locally, not from LR, failed! As the AFM and plenum are not the same diameter. Never heard of that Wyrem stuff. Local factors had induction hose, but what would fit one end would not fit the other, and it's too rigid to stretch or crimp succesfully, even with a jubilee clip.

Land Rover wanted £140 odd I seem to remember, hence my comment that self amalgamating tape works wonders. The hose on mine was so bad you could get your pinky through it.

I totally agree that that place to start is the idle control valve.

That said when my 3.9 developed this issue, it was sealing up the AFM to plenum hose that finally cured it.

[FridgeFreezer]

HS - it wasn't all @you, I just find it funny the way these threads go. I've got a flat tyre, oh better change the timing belt then...

[Highway_Star]

Fair enough.

I know what you mean. A methodical, logical approach to faultfinding is the best way forward.

We have one 'engineer' works for us, he'll phone me for advice, not fully listen, and whilst I'm on the phone he'll be playing with whatever bit of kit it is. After 20 minutes he still won't have done what I asked, and has by now caused more problems by tinkering with things that did not need tinkering with.

[disconc]

Hi everyone,

thanks for the suggestions and sorry for my late reply... I have very limited access to the internet these days, and will only be able to get to the car on the weekend !

Anyway, to browse the ideas :

Zim : faithfully, I have not had the opportunity to try any other AFM... (no access to another one !)

Dave : although I do not have a suitable rpm meter, I have tried to setup the base idle exactly as you explain (but with dash tacho).

The idle is steady, but the stalling remains.

On another hand, I will try to have a look at the T piece on the rocker cover breather hose as you describe. I guess it should be fine, as part of the general cleaning done to the engine for the rebuilt... but it is worth having a look !

I have no clue on how to adjust correctly the idle mixture as I have no CO Meter around faithfully. Just did it at 1.4 Volts on the CO trim pot on the AFM.

On ignition side, I have checked the TDC vs mark on crank is fine, and adjusted to 6 BTDC as it is a low comp engine.

Many people told me about the TPS. Mine seems ok, but I will double check its operation.

I have been told to check the TPS voltage with engine idling, as the engine vibrations might show an unstable worn TPS.

I will try to adjust it to 0,34 V and check it is smooth in operation and always come back to the same value on returning to the closed state.

Aragorn : On the various tries I made, what makes the difference is when I plug or unplug the vacuum port on the stepper housing. Moreover, I believe that if there was a fuelling problem, it should happen also when cold and while cruising, not only at warm idle ??

Fridge : I will have a look at the stepper, but I believe it works well, as the cold and warming up behaviour are "perfect". My problem only occurs when fully warmed. At this stage, on my engine, the stepper seems to be completely closed/sealed, letting no by pass air to the plenum... So maybe it is sticking closed ?

AncientGeek : yeah, I plan to check the fuel pressure, but have no gauge yet. I have ordered one, but must wait for it to arrive before being able to see how it goes in this field !

A question then : how does base idle, with the bypass system plugged as explained by Dave when setting it, should react to a load on the engine like engaging Drive...

I though this base idle setting was indeed supposed to prevent the engine from stalling... as from my understanding, the stepper should have no action on a fully warmed engine ?

What do you think ?

I will try to have a look at the ideas above I have access to... and will come back with results.

Thank you all

Cheers

Erik

[Roverbo]

A question then : how does base idle, with the bypass system plugged as explained by Dave when setting it, should react to a load on the engine like engaging Drive...

I though this base idle setting was indeed supposed to prevent the engine from stalling... as from my understanding, the stepper should have no action on a fully warmed engine ?

What do you think ?

I

Hi,

the stepper motor works on a warm engine as well as cold. That is, if you turn the A/C on, it lower the speed of the engine, the ecu reacts by retracting the piston on the stepper to let more air pass to rev the engine (so it doesnt stall, like your engine...). When you put the gear in Drive, it again lets more air in, so the engine doesnt stall ...etc.

As I remember, there are 128 steps in/out on the plunger (piston), so it´s a sensitive device, my guess is that the tarish stuff from the plenum chamber gets sticky when the engine is hot. On my 3.9 engine the problems came around say 10 to 30 minutes after take-off...again, my guess is the sludge is most sticky at this temperature. Cleaning the plunger and the mounting area with solvent and a cotton stick has done the job for my RRC (exept when i tried to move the piston... ). This is a thing you can do in ten minutes, and at no cost, so why don´t give it a try? If it works, you may consider a new stepper motor, as they have limited lifetime, some say appr. 50.000 miles.

Regards

Bo

[Dave W]

As above really, the stepper is used at all times to try and maintain an even idle. It is slow to react though and for that reason certain actions, such as changing gear in an auto (from park to drive for example) and the a/c compressor activating have an additional wire to the ECU which, when the state changes, tells it to wind the stepper out from it's current position for a second to give the idle control loop a chance to catch up.

The base idle setting is important because it ensures that the stepper is working within the correct range and it also provides a minimum idle speed where the stepper algorithm can't stall the engine due to closing the stepper too far.

Too high a base idle can lead to the stepper being permanently closed when the engine is hot and that leads to problems with it sticking when it does need to open. Cleaning (and lubricating) the stepper motor is never a bad thing although generally bad stepper motors on an otherwise healthy engine tend to reveal more problems at startup than when running providing they are clean. If you are getting problems with erratic or unpredictable idle speeds when the engine is cold/first started, that tends to be a more classic symptom of a bad stepper motor.

Too low a base idle can result in the stepper idle control loop closing off the air flow too far, especially when you take your foot off the throttle, and stalling the engine as a result.

[disconc]

Dave, Bo,

your explanations here do make sense to me, and I hope that it will be indeed the problem.

For sure, when engine is hot, I have seen that the stepper is completely closed on my engine.

On another hand, to try and compensate the stalling problem, I have increased base idle, so the idle control loop might well try to close the stepper to the max... Suppose then that fully closed it sticks a little to the housing cause of gum... when the ecu asks for the piston to retract on load, it does not move (or not quickly enough) and by the time the ecu sees the rev drop more (so it asks for more air)... then it is too late !

I will surely have a look at this during the weekend : clean the stepper plunger and housing, try to reset the base idle as close to 600 rpm as possible... and see what it tells.

Crossing fingers :-)

Thanks

Erik.

[FridgeFreezer]

If you have adjusted the base idle so the stepper is as its limit, then it cannot properly control the idle as it's hitting the limit before it achieves its target RPM. Depending how the control algorithm works, it may be that it can't then back off from the limit fast enough when you load it.

[disconc]

Hi guys,

I was a bit short on time today, so I did everything in one operation... so the shame (read stupid) part of it, is I do not know which operation seems to have improved things...

Here is all things done :

-> engine fully warmed, plugged both sides of hose, stepper to plenum, and checked base idle (only dash tacho, even if not accurate, it is all I have) : just in the middle between 500 and 750. So quite close to 600, which seems great.

-> removed the stepper : the plunger was shining clean !!! damn'it. Never mind, soaked in WD40 and a little cleaning with tooth brush.

The plenum housing side where plunger rest was like covered with a black powder (like gun powder), but not sticky... Cleaned with carb cleaner and rag. Refit.

-> removed rocker cover breather vapor trap, and cleaned it

-> removed all the breather hoses, including the little Tee, up to the plenum and cleaned everything with carb cleaner. The little side of the TEE was quite filled of sooth

-> tried to clean the area were the butterfly close on the plenum

-> checked the gap between butterfly and plenum with a piece of paper. On the bottom, no gap at all, but on the 9 to 12 o'clock part of the disk, I can feel the strip of paper moving ok.

-> cleaned the MAF with carb cleaner

-> refit everything, and checked TPS voltage, ignition on and also engine idling : voltage rock stable and always returning to the same value +/- 0,03 V, but at about 0,29 V

=> ovalate holes and made it to 0,34 V, rechecked idling and on the return to idle after a little gaz : ok to 0,34 +/- 0,02 V

Went for a test drive after that, and really seems to have improved :-))

Time will tell... I will repost in a while to tell how it is going...

If it is the cleaning that has done wonders, I am a bit worried because the engine has only done barely 2000 km after having been completely overhauled and cleaned... so that is not much before re clogging up !!

Anyway, thank you all for the ideas, and I am sorry not to have taken the time to do the tests one after the other to know exactly which one was THE ONE !

Cheers

Erik.