Hi all, completely new here so my apologies for requesting help straight away, but it is actually the main reason for signing up. For the last 6 months I have been trying to get my xj12 running again, after hibernation for about 2 years. I think I have read the entire internet, and screwed in and out every part of my jag but without any luck.

Here is what's happening, when cold engine will run (only idle) for a couple of seconds and then stall. Starting it requires me to turn the throttle half way, it takes a while but than it starts but won't rev above 1000RPM. turning the throttle any further does really make a difference. There has been one occurrence where after holding the throtle at 50%, it all of a sudden picked up and rev''d up, after that it stalled and I was back where I started from.
I have measured all temperature sensors (at the ECU) and these are all within range, both when engine cold and hot. Replaced the maps sensor, the old one did not hold vacuum very well this one does, coil-resistance is within margin. (all resistance measured at the ECU btw, to make sure it not a wiring problem).The hall sensors work ok as well, tested this by using a magnet and I can hear the injectors clicking (which btw, I've cleaned and tested). Drained the fuel, replaced the fuel filter, added new fuel. Measured the fuel pressure, perfect 2.0bars, swapped the ECU, still same behaviour. Sparkplug are rather black but completely dry, so that would indicate a rich mixture, but can that prevent the engine from running? It's not soaked? Plugs spark like there's no tomorrow...

After cleaning the auxilary air valve, and checking this was working by holding it in warm and cold water, out of curiosity I tried starting with the air-tubes (the ones that lead to the manifolds) disconnected, so sucking air completely freely. The V12 started and ran perfectly smooth at idle +/-600RPM, after a couple of minutes the started climbing up to 750/800RPM and stabilized there. So idle was fine, but the second I touch the throttle it dies. Reconnected the tubes, started, idled for maybe 1 seconds and died again, disconnected the tubes, perfect idle. I removed the air filters (left the housing open), but this resulted in nothing. It really wants the breathe completely free.

I just don't understand what the problem can be, looking at the sparkplug, and the fact that giving it a lot of extra air allows it to run (or at least idle), i tent to saying that it's running too rich, but what could cause this? I though the only factors that influence the mixture are the temp sensors and the MAP sensor and these seem to be ok? To be on the safe side I disconnected the cold start injectors but this didn't make any difference either.

Does anybody here have any clue what can be wrong, or what else I should/could check?

 

Not an expert on this engine.

Fuel is the issue. I translate air tubes to vacuum lines. So, when you remove them, you add air to the mixture and the engine gets a leaner mixture and runs better.

Why is the engine receiving too much fuel? That is the million dollar question. My uneducated guess is whatever device is sensed by temperature and aids in a cold start. It keeps on "aiding" after the engine is warm and no longer needs extra fuel..

Carl

 

Hi Carl,

thank you thinking along!
Actually it's not vacuum lines its extra tubes/pipes connected to the auxilary air valve, they are quite big (see picture) so allowing a-lot-of extra air... I did get me thinking about the cold start injectors, before I disconnected them, but only cut the electricity. Your reply got me thinking that maybe they could be leaking, so I just cut the fuel to them and tried again, but unfortunatly this didn't make any difference

btw, the engine cuts out after a couple of seconds, regards of the weather conditions, and I doubt if the enige gets warm enough fast enough to completely stall just because of the extra cold motor aid..


air pipes

 

BUGGA.

Those tubes ARE vac tubes, just damn big ones (1" Diameter).

Reading this many times, to get the coffee down, and my head around what you have done to systematically sort this beast.

The running for 1 +/- second at start before stalling is classic EFI loom shorting. This causing rampid overfueling, and the extra air, from the balance pipe connector/s, is leaning off all that fuel.

Just for giggles:

Reach down the rear of the engine, and CAREFULLY remove the rubber boot for the Throttle Switch, and ensure that the loon plug is inserted securely. They are a MONGREL for sliding out, and sitting inside that boot, so visual is impossible with the boot in place. They will start AND run with it unplugged, so I am stretching what I know does happen, but accelerating the engine with it unplugged is also possible, BUT very slowly, as the fueling metering is being sorted by that MAP Sensor. Mine ran well with that sucker unplugged, just not very quick off the mark, replugged, rocket ship returned.

Yours dies at throttle opening, SOOOOOO, look at that pug, look for "Green Growth", and broken wires, particularly wire at #12/47 terminal marking.

NEXT:

I seriously doubt this in your case, but I have had a few.
The CTS (Coolant Temp Sensor), loom plug is plugged into teh ATS (Air Temp Sensor), and vice versa. Those pigtails are the same length, and easily reach the wrong sensor if attention is wandering at all. The HE is a dead engine, the PreHE will sort of run, MAYBE.

You say the Cold Start Injectors are disabled from the fuel supply, GOOD, they are a PITA, and are not needed.

ONE MORE:

QUIET PLACE needed.
Ign ON, engine OFF, rotate the throttle capstan SLOWLY, and you SHOULD hear 9? clicks of the Injectors at you move from Idle to WOT (Wide Open Throttle). If you do, the throttle switch is fine, if you dont, then that throttle switch is the problem. Probably grubby inside, or teh Gold wipers are tarnished, OR, the carbon spacer strips have arced together. The attached PDF might help.

MY FAVOURITE:

Earth wires, and the PreHE has MANY on the engine, mainly raer of the "A" bank. Look around, and ensure they are intact and clean.

 

HI Grant,

thanks for your input. Although I re-did the entrie loom some years ago when I was overhauling the engine, and haven driven a lot since, I can hardly imagine this being the problem, but I will redo it if everything else fails, never thought this could result in a rich mixture?
The throtle switch and the connector are both in almost new condition, measuring (at ECU connector) it makes and brakes connection 10 times from closed to WOT, resulting in 20 "injections" with ignition on and engine of, so I guess the ECU reacts to bo up and down slope.
Air and coolant temp sensor are not switched, I checked this before and as said, before it's hibernation the enigen ran like a dream hot/cold always two starter turns, an performing like a racer.

Your last comment about the earth wiring, I really need to check this, never thought this could be a problem, but you mentioned this and since the ignition light doesn't go off I checked if the battery was charging and it doesn't seem to. But engine on and engine off I read 12.25 Volts, so I guess theres something wrong with either the dynamo, regulator or wiring. Do you know if this could result in a too rich mixture? I searched the internet and examend all schematics i have and from he theoratical point of view this shouldn't but if you say a bad earth connection could cause this problem than maybe low votage as well?

 

DAMN, someone got at the beast whilst in slumber.

The low voltage, maybe, although the 3CU ECU is purely analogue, and not as fussy as the later 6 & 16CU Digital units.

Earths are critical, as barely nothing earths via its casing. There is 1 or more dedicated earth wires for various circuits.

In the boot, where the ECU is housed behind that rear seat trim panel, is one (memory here, as its been 25+ years since I needed to get that sucker). the main engine earth straps (probably OK, since the starter works fine), then there are various earths on the radiator support panel, and I think????? down near 5A spark plug, but that may be the HE I am thinking of.

Since you have rebuilt the EFI loom, I also doubt that is an issue. The main issue with that thing is the internal shorting to earth of a bank of Injectors, thus holding them open, and overfueling the engine.

The EFI Amp, up on the radiator support panel, is another item in the puzzle, and I have NEVER had a dud, and the fact you have triggering of the Injectors on the WOT test negates it in my opinion.

Cannot think what could have gone AWOL whilst in storage to cause.

Grubby multi pin connectors, at the ECU, etc are always a go to item, but you are at these for the other tests and clearly nothing evident.

I will keep thinking, but, like you I am hitting the same wall.

BUT

Gotta luv the OLD Grey stuff between the ears.

I had a 1977 XJS turn up unannounced on a truck YEARS ago. Had done the rounds, and overfueling like crazy.

1) Vac hose, TO the MAP sensor was blocked with ???????dunno.
2) Distributor cap had slight cracks in it, very minor in any language, but still visible, just.
3) Rotor was shorting TO the distributor shaft it sat on, not much, but there was a measurable earth path there, so a fibre washer was shaped to fit INSIDE the rotor, and this sorted that. The thing still had SPARK, but that earth path was not going to help at some stage, and once that fibre washer was inserted, all sweet.
4) 2nd engine strap, cradle TO chassis was missing. Simply ran a dedicated strap engine to chassis, just near the oil filter.

 

Hi Grant,

and again thank you... I blew out the vac hose to the maps (of course disconnected it first :-) ), no problems at the distributor, attached a starter cable between battery and engine, but nothing helped.
Since IMHO the only things influencing the mixture mixture are the two temp sensors and the map sensor... if I can find another maps :-( ...

Will keep you posted!

 

Too bad... took the MAP sensor to the place I got it from, the guy connected it to a XJ12 he was servicing and that one ran perfectly without any problems both at iddle and at high revs... Just for funs I bypassed the coolant temp sensor by using a "real" 330Ohm resistor, so that would be the equivalent of +/-80 C, which would tell the ECU to lean the mixture (right?) but still nothing...

I am really losing my mind, the d-jetronic is not complicated, have measured and tested everything, everything ticks and clicks when it should but still it won't run... wish I had a second (running) xj12, so I could just swap all the component and test everything one by one.

 

Hell.

I've reread this again, and understand you drove it into storage, and it ran spot on, then you take it out, and nothing but grief.

MAP is OK.
I am not sure on the OHMS thing for the temp sensor, I usually use a paper clip and bridge the plug, AND the ATS as well. The ATS is only a trimmer, but if I am looking for a "bug" I eliminate those 2 very early in process.
You say it sparks "like no tomorrow", and I understand the terminology, but, is it a Fat Blue spark, or a Yellow/Orange spark, AND is the spark AT the spark plug??

Still thinking and looking at wiring schematics to refresh my brain on this simple system.

 

Do not take grounds so lightly. years ago, I revisited my grounds and improved the lay out. Wing wall pot from battery - included.

Much travail over a no crank issue. Included a replacement starter!!

Last ditch, son removed that wing wall ground. Cleaned it up replaced it. whirr, purr !!!!

So, now, I have a fresh starter and a backup healthy starter!!! Oh, and a car that fires right up.

Carl..

 

Just to be sure, I also bypassed both temperature sensors with a paperclip, no change. Also checked all ground connections I could find, all 0.06 Ohms to the negative battery terminal. (which is the same reading when I connect the ends from my ohm meter directly, maybe it needs to be calibrated or something).

Does anybody know if a none-working alternator could lead to rich mixture?

 

MMMMMM.

I doubt the alternator, or lack of, has anything to do with it.

Clutching at straws:

The 2 FPR units, purely mechanical, so no vac hose to draw fuel as the HE has, IF, the diaphragrm ruptures. If yours have a crack, raw fuel would by-pass the internals, and excess fuel would be available at the Injectors.

I have NOT had any fail that way, all mine have jammed solid, so NO fuel to the system.

My thinking, the HE units will fracture, and fuel by-pass the innards, and since they have a vac hose for "variable"
pressure, that small hose sucks raw fuel into the Inlet manifold.

The pumps are good for 130psi dead ended, so plenty of reserve from them for a "leaking" FPR or 2.

SOMETHING VERY ODD, BUT I HAVE HAD 3 XJS WITH THIS.

The fuel RETURN, from the rail to the tank. IF that is restricted, rich fueling WILL happen.

The XJS cars all had damaged return pipes under the car, dumb tyre fitters not using the factory jacking points.

BUT, you have 2 tanks, and a valve arrangement to return unused fuel to the supply tank, AND, if that is somehow NOT opening the return valve/s (very common after an extended lay up), the fuel CANNOT get back to any tank.

SIMPLE

Locate the fuel return hose, LH side lower rear of engine bay, where it plugs onto a steel pipe sticking up, under where a RHD battery would be, NO idea what the LHD did, probably the brake booster?. Extend that hose over the guard, into a container, start the beast, and see what changes.

 

I'm at work at the moment, so can't check but... the return is probably ok based on:
- replacing all plumbing a couple of years ago
- having a faulty valve when I tried to run it after its hibernation, all fuel got returned to the same (LH) tank causing some serious spillage from an overflow pipe near the LH rear wheel
- the fuel pressure at the injection rails, it's at 2 bar (29PSI), if the return was blocked, the pressure would rise untill the pumps limit (right?)

as a side note, I also tried 2 new fuel pressure regulators, that was before I had a pressure meter... the pressure is 2 bar at avarage, when measuring it the needle goes up and down like crazy, between 2.1 and 1.9 bar. I blamed this on the pressure meter. In my quest I have reduced and increased the pressure from 1.8 to 2.2 but no notable difference.. at least not as far a a running engine..

I just don't know anymore, think I will try to find another ECU hopefully one that is confirmed working (I tried 2 that a guy had on his shelve, but no clue if these were ok)...

I know this is asking a lot, but if anyone with a s2 xj12 in the vicinity of Moerdijk/Breda (NL) is reading along and not to afraid to detach their ECU for 5 minutes please let me know!

 

ok... tried two different ECUs... no luck... no idea what to check now... it's sooo frustrating that the engine runs great at idle with the tubes disconnected, but on runs a couple of seconds with the tubes connect (probably until engine is warm enough to need a leaner mixture...)

 

Bugga.

I have reread the whole saga.

With that balance pipe open to atmosphere, I would expect a 2000RPM uncontrolled running engine, at least. My reasoning is based on the piston slots inside the AAV, that when cold and exposed are a SMALL aperture compared to what you have with open 1" pipes, and that AAV gives approx 1200RPM cold.

Somehow the Intake air is grossly restricted.

If you have not done so, remove both air fiter covers, look inside the snorket/trumpet. Something may had taken up residence in 2 years. Then check the butterflies, and ensure they are OK at the 0.002" gap. They probably are, but worth a look.

I am NOT a fan of starting a PreHE without air fiters in place, as they tend to belch flames, and burn paint, and scare the crap of humans, BUT, I suggest giving it a try.

This engine is starving for air, proven by the running with the balance pipe opened.It is getting fuel, the D Jetronic is not smart enough to know anything about air flow, but without AIR it is too rich.

NEXT, and its a LONG shot.

Maybe the exhaust/s are clogged, again, restricting breathing, but as I said a looooong shot.

I repeat, with an open balance pipe 2000RPM would be a minimum I would expect, and you are NOT seing that.

One of mine split the elbow hose under the RH throttle body, the one that leads to the balance pipe and the AAV, and I had 1200RPM, and that was only a split, NOT a 100% open hose.

 

Hi Grant,

thanks again for thinking along, I really appreciate it!

"I am NOT a fan of starting a PreHE without air fiters in place, as they tend to belch flames, and burn paint, and scare the crap of humans, BUT, I suggest giving it a try." :-D ...
Did that already, completely open... covers removed, filters gone...

"and ensure they are OK at the 0.002" gap"
I will check it just to be sure, but it seems it needs much more than the small gap.. seeing that the 1 inch pipe open results in running at (a stable) 750RPM...

"This engine is starving for air, proven by the running with the balance pipe opened."
... this is my main problem... I'm quite sure the problem is the air/fuel mixture, but I don't know if its lacking air or having too much fuel. Since it seems that both banks suffer the same problem I tend to think it has to be caused by something both banks have in common. So this is why I initially was convinced it would be the map sensor or ECU. But as I wrote, this worked perfectly on the other car.
From what I understood the bigger the vacuum the longer the injectors would open up, so I connected the map to only one bank, figuring this would half vacuum which would result in shorter open time and therefore leaner mixture... but no.. exact same behavior. So hence trying to blame the ECU.. .but no... so only thing remaining (if it's fuel rather than air) would be the amplifier, but according to everybody this never fails, and since it does open the injectors... maybe the earth is too well connected, grounding it too much, and opening the injectors too wide... this is what the amplifier does right? it pulls the one of the injector leads to ground? so at ignition on, engine not running, there would be 12V between wire and earth for both wires, and when the engine is running one of the two wires gets earthed and hence there is a +12V difference between the two, triggering the injector? => please say that I'm wrong, so I have another thing to focus on....

 

DAMN.

More coffee needed, for us not the car.

The Injectors have a 12v Ign power supply to one wire, and the ECU/AMP/ETC trigger the EARTH circuit for the Injectors via the 2nd wire.

This is why the most comon issue on the V12 is the EFI loom itself. As it degrades, and the wires "short" to earth, which basically keeps the Injectors, some, or a group, OPEN, and the ovefueling is the result. The added air via the open balance pipes is enabling the engine to burn that fuel, but the idle revs with those pipes off is waaaay too low, so it "sounds" like there is much more fuel getting in than 1st thought.

Your loom is new, so ticked off.

If a spare Amp is available it would be worth a shot, but, yes, I have never seen a dud Amp.

I am trying to keep the focus on the fact it ran 100% when it was parked up, now it dont, so what has happened??????

 

Think tonight I will disconnect all injectors and measure resistance between connector and earth... maybe my loom has been eaten by rats somewhere, or my repairs weren't well enough.... Could one or two (or 3 or 4) failing injectors flood the entire engine? this would imply the fuel is sucked into the manifold in reverse, against the airflow, and then be distributed over all cylinders, flooding them all? is this possible? (I'm not much of an hydro/aero dynamics expert...)

To make it more easy on myself, maybe I can listen to the injectors.. when I turn the throttle I hear injectors clicking, should I be hearing all injectors, or just one bank, or just one group of three (as dictated by the amplifier)? Do you have any hints how to best listen to them, without me having a stethoscope?

 

1) The fuel will not get back up the ram tubes, no matter what. This whole engine rich is usually from the 2 cold start Injectors, hence isolating them very early in the diagnosis isalways mentioned.

2) I use my hand, and FEEL the Injector/s, one at a time, and thats always worked. I disable the fuel pump for this, or you WILL flood the engine seriously.

When opening the throttle all 12 should click, and sound like one single CLICK, and it would beyond us mere humans to determine with our ear, which, if any are not clicking.

You do know the banking layout of the Injectors????

 

"You do know the banking layout of the Injectors????"
I know it from online schematics and could measure mine.... why the four question marks?