Al, for what it's worth here's my recent experience with similar symptoms on my 97 XJR6 - stumbling leading to stalling after braking to stop/idle with dropping/fluctating voltage on console - except mine only in hot temperatures, when fans switch to parallel mode, AC on, in Drive.
Always have had some voltage drop with the fans, but stalling only recently. Occasionally got the unknown codes you had in the past - but no other specific codes.

Pulled the plugs and all 6 were black, clearly over fuelling on both banks, so new Champion plugs fitted and fired up OBD scan tool - STFTs both sitting at 25% at idle! Well, I had mechanic replace O2 sensors about a year ago (proactive maintenance I thought) so I suspected switched connectors.
Tried unplugging upstream one to identify via scan tool but unplugging one caused all the O2 sensor voltage data to disappear, so no luck identifying Bank1 Sensor1 vs Bank2 Sensor1. Took the opportunity to relocate the downstream sensor connectors from behind the engine to passenger side bulkhead.
Took a chance and swapped upstream connections and within minute STFTs were varying and O2 sensor voltages swinging at idle (and subsequently on test drives)
Cleaned all the engine bay ground points and battery+ studs and was seeing pretty much identical voltage at battery and front fuse boxes with DVM (13.5 at battery, 13.37 at fuse box, varies with temp/load), but still saw significant voltage drop when parallel fans kick in.
The good news is no longer stumbling/stalling! so I assume the overfuelling caused by swapped sensors caused the stumbling at idle (combined with fouled plugs over time) maybe dropped the idle speed to the point where the alternator couldn't keep up when the fans kicked in.
Subsequently noticed AC belt had a split so had both AC and Alternator belt replaced - no Alternator squeal before but voltage drop on parallel fans appears much smaller now :-)

Some observations...
Never got any O2 sensor related codes, even with O2 sensor(s) disconnected for several minutes
Based on wiring diagrams I was expecting Red and Blue signal wires at upstream O2 connectors but mine were Brown and Green. Found Red and Blue wires in the harness to downstream O2 sensors when relocating - cut off and taped up, seemingly by design - forgot to note actual signal wire colors on downstream connectors. Maybe I misread wiring diagrams.
Console voltmeter seems to indicate somewhat higher than measured voltage at front fusebox with DVM, and DVM higher than OBD measured voltage. For example console around 14V , DVM at front RH fuse box 13.6, OBD scantool "Input Voltage Read by scan Tool" 12.6 V. I guess I trust the DVM, cheap though it was.

 

 

Thanks for the O2 feedback. I'll check my plugs again.

Unfortunately nothing has been done with my O2 sensors since I've owned the car, and likely its entire life. So no one has switched cabling, but it certainly is possible that one or more of the O2 sensors have gotten lazy just due to 22 years age and 92k miles. So they are on my list as proactive maintenance as part of this process. Fortunately, and surprisingly, they are not expensive.

I posted my OBD data log readings including STFT a few posts back if anyone wants to look and offer any thoughts. Just look for RPM below 500 RPM for times where it died.

.

 

While changing the plugs I also "cleaned" my coils. I pulled off the rubber insulation extension to get access to the spring-loaded conductor that connects to the sparkplug.
I then cleaned/lightly sanded the end of the conductor that meets the sparkplug, gently stretched the spring end, and cleaned the plate on the coil head where the spring mates to it, and reassembled.
No idea if this contributed anything, but it seemed worth the effort while I was there.
I'm running a combination of OEM coils and some "Japan" replacements from SNGBarrett

 

Al - I threw your OBD data in to a spreadsheet and plotted Vehicle Voltage (blue) , O2 Sensors "voltage" and RPM (green) in a chart. Looks like your OBD samples every 1/10th of a second or so timeline numbers are 1/10ths of a second, so this is first 600 seconds or so (no stalls in this timeframe)

Looks like it takes a while for Voltage to come up, and some significant drops on returning to idle (560-580 rpm)

The O2 sensors staying flat initially due to open loop and then secondary air injection driving lean I assume
There should be an OBD PID for "commanded secondary air status" but I didn't see it in your data. In my data I get value 1 when air pump driven on, and value 4 when pump off

i

 

and here's a view around the stalls
Green arrow - return to idle with no stall
Red arrow - stall

Hard to say if voltage drops are cause or effect so not sure how useful this is

 

Analyzing

 

Al ,

PM me if you want to barrow a voltage regulator as the ECU is not seeing proper voltage , or adjusting to it as a reference voltage from a lower voltage ( regulator not responding correctly ).. The ECU is reading it in a different way then the instrument cluster and that difference may be accounted for as a combination of pitted relay power contacts and a somewhat floating relay . The 2 Relays for the 2 power sources to the ECU are the RH engine fuse box king relay and the large ECU controlled relay .

I want through 2 voltage regulators from 2 different vendors on Ebay and they both failed , so I got an X300 alternator from a donor car .

The bottem cicle on the first O2 graph is probably the sensors not being monitored during the cold / open loop mode or the sensor heaters have not be warmed up to give a reading ( normal )

 

Yes, the OBD reported Vehicle Voltage is only representative of what's going on, not exactly what ECU itself may be seeing.

The OBD value is what's measured/reported by the ELM adapter at the OBDII connector, not the ECU itself.
In my case with 14.1 V measured with DVM on OBD pin 16 (B+ from Fuse #6 in trunk) and 14.1 V on pin 9 (ignition switched B+ from fuse #10 RH Engine bay) but my ELM adapter reports 13.1 V in it's OBD stats.
So there's ELM accuracy to consider but looks like Al's ELM may be more accurate.

ECU is getting B+ from RH Engine Bay Fuse #12 through ignition switched relay and then RH Engine Bay Fuse #11 via the ECM controlled relay

Having 14.1 V on pin 9 of OBD connector with ignition should gives some confidence in the condition of the RH Engine Bay king ignition switched relay I guess? (which is same in the ECU Fuse 12 path)

 

Off the top of my head the OBD power pin 9 comes from the RH heelboard fuse box and is the same fuse to provide power to the pulling coil of the # 1 and # 2 fuel pump relay as the ECU provides a control ground For # 1 and a fuel pump control module provides a control ground for the # 2 . This fuse is different then the power fuse for the fuel pumps .

Someone had a problem with the OBD connector messed up .

 

Well it's been a few months since I've updated this thread, as we've had some other distractions and frankly I've been highly frustrated with the XJR so much that I just had to let it sit.

Once again, thanks for all the suggestions and analysis so far. I think the most recent comments about low voltage are more effect than cause based on the last year's efforts. Voltage does not seem to drop, then the car stalls, but the other way, which makes sense as RPM drops to zero.

There aren't many things left to test or replace at this point, and today I crossed another off the list. I removed the ECU and verified that it had not been exposed to water leakage from the sunroof drains, and the pins and interior of the ECU looked clean and dry. I also transferred the PROMs from my ECU to another ECU and observed no change in behavior. So I don't think this is related to a bad ECU.

...you know it's bad when you're hoping to find your ECU growing green fuzz in the connectors and full of water so you can at least chalk it up to that!

From my recollection, this only leaves the idle air control valve (IACV) from my list of suspects. I have a new IACV on the shelf, but as you may know the IACV is difficult to get to on the XJR6 and difficult to remove without breaking its terribly thin mounting bolts since they are secured into the throttle body with thread locker from the factory.

I've suspected the IACV from the start as a COI (component of interest ) since this is an idle failure issue, and keeping the car running with the throttle body butterfly closed at idle is the IACV's one job

So I guess I'll look into trying to gently work the IACV's bolts free with some heat and solvent in the next week or so.


Like many things related to troubleshooting the X300, it's really a shame that the ECU's OBDII is so primitive, as it clearly can only report on the errors it is capable of logging, and that's often not enough to figure out tricky issues beyond its limited set. For example, if this turns out to be the IACV, one would think the ECU would detect that and set a code, but alas.

...and of course, if it isn't the IACV, I'm unaware of anything else to do. And I'm hesitant to spend much more time and money on it. I'd be a shame to send a 93k mile XJR6 well maintained and in cosmetically great shape to the scrap-yard or sold for nothing, but I'm afraid that's where this is heading soon

 

Hello again Al, very pleased to see you back here. You might have been distracted by a Jaguar V12! Sorry about the XJR's troubles. It did so well for a long time and it is a great car in general. Let's give it one more good round of troubleshooting here.

Replacing the IACV is in order, since you have the part. Have you also checked or replaced the EGR valve along the way?

I'd say that the root cause of the trouble is probably something simpler than we imagine it would be. Just have to find that bugger!!

 

There was a specific loctite softener that someone mentioned last month Ir the position with the can't recall

The IAC valve does not have a followup position sensor to compa with the command like the EGR valve /

They could have put a current sensor on each of 4 legs of the step motor as tte inside he wires originare

2 example while you were away :

spinning of the harmonic balancer beyond the initial squealing can account for your voltage dropping then recovering some how ( an assuming your SC engine HB is the same based on the ignition pulse timing not evenly spaced around the 720 degree rotation relationship )

But you may see a relationship at the time the ECU shifts the timing degrees on purpose which may upset your HB at that point to recover later in your graph . There is a specific PID on your ELM 327 for timing shifts

timple belt tension / belt length ( 735 mm off the top of my head ) that I think you have already addressed but may need to revisit

Sleepy editing so bear with me

 

The car has a relatively new EGR, which was replaced in 2015 with the R493 recall. That of course doesn't guarantee that it is OK. I've tried unplugging the EGR to see if that makes any difference, and it doesn't seem to. But I'm not sure if that is a good test. Would the OBD on this car set a code if the EGR were misbehaving?

What would be the symptoms on this car if the EGR were bad?

I hate to just buy an EGR if that's not it.

.

 

Thanks, yeah I recall that there were no sensors for any IACV confirmation/malfunction detection, which is too bad.

I ordered some special solvent that is supposed to dissolve thread locker, and had to order it from a chemical supply company. It even came in a brown glass bottle like the "good old days" so you know it is good stuff. I don't recall what it is offhand, but I also bought some special bottles with needle-like droppers so hopefully I can get the solvent right on the bolts.

That's interesting commentary on the harmonic balancer. I don't think mine has slipped, as I have the silver paint line check on it, but there are some low RPM groan/rattle from up there when the car first starts sometimes and when it stumbles. So I may look into that.. I have to pull the HB on the XJ12 too anyway.

 

Hi Al,

I haven't tried any threadlocker solvents but from what I've read, the most successful method of use is to soak the parts in the solvent for some period of time, possibly days. Simply applying drops of solvent around the head of the screws may have little effect, but I'll be very curious to hear if you have good luck. Also, I've read that these solvents can damage plastic parts and possibly rubber seals, so take care not to get any on the TPS or stepper motor electrical connectors, for example.

I've had success using a soldering iron or gun on the heads of the screws, and Bob Gauff (motorcarman) says he uses a small butane torch to heat the throttle body in the area where the screw threads are located to help soften the threadlocker and reduce the risk of the screws snapping during removal. The throttle body serves as an effective heat sink, quickly cooling the threadlocker, so once the screws will turn, back them out a little, carefully apply heat again, back the screws out more, and repeat until they're all the way out.

Cheers,

Don

 

Yes the OBD SHOULD throw a code if the EGR is misbehaving. But we all know that the OBD is "lenient" in these cars. If it was dirty and gunked up, it could cause random drop-outs and perhaps stalling. But yours should be clean since it is so new. The purpose of the EGR recall was to cycle the pin so that dirt and gunk will not build up so easily. You can hear it do its thing soon after you shut down. Your EGR has a gasket underneath it right? It should. I'd say don't buy another EGR now. You've got the upgrade (including the correct 9355 PROMS) and that system SHOULD NOT be the cause of your troubles.

 

A long overdue update: Towed the car to a new mechanic we've started working with, and had him get the IACV out, and although I'm not counting any chicks yet, he did say that the IACV he removed looked properly buggered up and stuck in the extended (closed/choked) position along with looking misaligned. So that's actually good news that we've found SOMETHING WRONG finally.

Additionally once I had his new and fresh perspective listening to the symptoms and general scenario, he said it sounded like a classic IACV issue. Even the scenario of coming to a stop exacerbating the issue made sense to him due to the brakes pulling vacuum along with an already insufficient airflow with the IACV probably not opening properly, choking off air and causing it to stall.

So I think he'll have it all back together by the end of the week, and we'll see if that was it. Keep your fingers crossed!

 

Hey Al,
I am working through a similar set of symptoms. I notice on my car, when the engine cuts out and recovers the air injection pump cycles. This suggests that despite not going through a ignition key cycle, the ECU is encountering the equivalent. Have you experienced similar behaviour?
best of luck,
Erma

 

I can't say for certain, but I don't recall this scenario with the air pump when my XJR dies and is restarted.


My XJR is still at the mechanic, so no update on results of changing the IACV yet. The original cheap IACV I had will not fit on the XJR due to the clocking of the connector and then interference with the connector/harness and surrounding hard bits and throttle linkage. I forget which other European car the cheap IACV I had was from, but it was recommended here as an inexpensive alternative. It may have worked on an XJ6, but not the XJR. So we had a slight delay while I ordered the correct OEM replacement from Jagbits and waited for it to arrive.

The mechanic was hoping to be finished late last week, but I told him it wasn't a priority, so maybe we'll hear something early this week.