Hello,

I bought my XJR almost a year ago and it has always driven well so far. But I'm now having a weird problem with my 1999 XJR
that at first looked like a failing throttle body or serious vacuum leak..

It doesn't matter whether the engine is cold or warm, how fast you drive, etc. Randomly the RPM starts acting strangely,
sometimes even resulting in the engine switching off with a message in the odometer: "Engine stalled". The engine check
light is also not on and no DTCs have been set.

Peculiar:
When the RPMs start to fluctuate (650 to just under 1000) and I put the car in Park and get out to listen, it sounds like
the engine is idling very "hard". Not fast, but hard. Louder "V8" sound (noticeably louder than normal) and a very
noticeably high air pressure from the exhaust. There are no messages, a CEL or any DTCs when this happens.

I then cleaned the connectors (which was recommended), went for a test drive and it looked like it was fixed.
Well, it was fixed... for a week or 2. Then the same problem occurred again.

Luckely, because I have a lot of parts from dismantled vehicles in the past, I had a recall replacement vacuum
throttle body lying around, so I installed it and inspected the vacuum lines. I found 1 crack, so I replaced
all rubber vacuum lines (including to and from the supercharger). This helped for a month or so, but now the same
problem has started again. It is not the battery and I replaced it.

Very occasionally on the highway I also feel it, but then it is very short and usually has no consequences for idling.
If this does have consequences, it is usually that I no longer have to accelerate in residential areas. The car
automatically drives up to 20-30 km/h (12-80 mph) from a standstill and on the highway it maintains the same
speed without touching the gas.

It's just a very inconsistent problem and I really don't know how to reproduce (or fix) it.

I also have the following issues with my XJR some of which have been present since purchasing the vehicle but may be (un)related to this issue:
1. Occasionally it will be hard to start (VERY rarely to the point of sounding like hydrolocking) and other times it will start perfectly.
Ordered new spark plugs and an overhaul set for the injectors, but unfortunately these have not arrived yet.
2. Drives fine, but when accelerating hard (after 4500-5000 rpm) it starts to surge.
Frustrating, but I don't do that that often and I have not yet been able to determine whether this is the gearbox or the engine.
3. Brake pedal switch for cruise control is defective.
It doesn't give any errors if I don't use the cruise control, so there's no rush for me.
4. O2 sensor heating element BANK 1.

 

It will be interesting to note the changes after the plugs and injector work
is completed. I would address the O2 sensor sooner than later as that has
a great deal to do with the catalytic converters going south before their time.

 

Could potentially be a throttle body issue, but something that corrected a lot of my throttle weirdness was to replace the connectors for the TPS wiring. I just snipped the wires and added new terminals and it resolved my occasional stalling and throttle hang.

 

Disconnect the maf and start the engine see if any difference

 

Carefully inspect the intake tube. Sometimes cracks form in the accordion section and it allows air to leak in thats not monitored by the MAF and can cause screwy behavior.

 

Car needs working O2 sensors to do proper RPM control.

 

Cleaning the MAF sensor can't hurt anyway, that's what I'm going to do anyway! Yesterday on the way home everything went haywire, which is a good thing because DTCs have finally been set. In addition to the engine, even the gearbox went into limp mode.

DTCs:
P0507
P1229
P1612

Because it was almost impossible to drive properly, I stopped at a gas station. I sprayed the MAF connector and both connectors regarding throttle position with WD-40 (I had nothing else with me, unfortunately), reconnected it several times to clean things up a bit and restarted the engine, which now suddenly started running normally again. The DTCs also stayed away after clearing the codes.

I had also checked the breather to see if it was partially clogged, but after blowing through it, fortunately this was not the case.

The way back home (and then the supermarket) went smoothly. However, this morning almost the same story again, in the parking lot I no longer have to accelerate because he does that himself. Also when I put it in Park and rev the engine, it goes back to idle very slowly.

This leads me to believe that, as Quadcammer also said, it is the wiring or connectors. When I got home I looked at the wires and they look normal and undamaged. I'll de-pin them this weekend and check further.

 

Spraying the MAF with WD40 is the wrong thing to do. The MAF wire will heat up and burn carbon deposits on it making it worse. Always use MAF cleaner.
have you tried taking out the AITS? Air Intake Temperature Sensor. It’s located on the side of the TB. Take plug off, unscrew it and you’ll probably find it’s covered in black deposits. If it is, she will think it’s still cold and tell the ECU to provide more fuel. The overfuelling will cause stalling, lumpiness and higher revs, restricted performance etc. we cured this by cleaning it up. Here is before and after…

 

That's a great tip, Julia700!

For clarification, the Jaguar service documentation usually refers to that sensor as the Intake Air Temperature Sensor (IATS), in case anyone needs to search the schematics or Workshop Manual.

Cheers,

Don

 

Hi Julia700, thanks for the tip regarding the IATS. I'll take a look at that tonight. What did you use to clean it?

I replaced the O2 sensor. Unfortunately the problem has not been solved. Today I did a test drive with another MAF but still the same problem. Sometimes I get a transmission failsafe message and sometimes I don't. It's running / driving very rough.

My cleaned chrome exhaust tips had turned completely black after a drive of 2 kilometers / 1.2 miles.
By the way, I did not spray the MAF sensor itself with WD-40, but only the pins in the connector.

So the IATS is my priority now.

 

 

I would be very suspicious of unmetered air leak(s) beyond the MAF. Given that the engine operates as an effective suction pump, physics demands that air will be drawn in at any unintended leak point, and especially at non-metallic junctions. If such occurs. the MAF>ECM may seek all sorts of weird compensations, including fuel richness . . . and that is what you are seeing at the exhaust tips.

+1 on the O2 sensor, but it may not be the culprit, if you find unmetered intake leaks as above.

As to the BPPS, are the rear and high level brake lights and park-lock solenoid all working? If not, check rear boot/trunk fuses. Any excuse to avoid working on the BPPS.

Finally, I would disregard trans 'limp mode' faults as least priority . . . most often symptom rather than cause in your precise situation of a sick and unreliable fuel/air mixture.

Cheers,

 

Hi,

Small update. As Julia700 recommended, I disassembled the IATS (it wasn't really dirty), but cleaned it anyway. Unfortunately had no effect.

Then I started looking at the MAF sensor again, and I discovered (thanks to another post here on this forum) that I had a MAF for a 3.2/4.0 *NA* engine. The MAF that I installed afterwards simply had the same part number that came with the car when I bought it from the previous owner, but I did not check whether this was the correct one. I have now installed an XJR specific MAF sensor. Unfortunately this made no difference, but it was still a step further.

I have now found something else and that is that the O2 on the left (bank 1) is reading almost nothing..

The STFTs and the LTFTs are all over the place (I took a screenshot of my live data tool while driving).
These results are with the MAF disconnected (otherwise the engine will not run good at all).

Short term fuel trim BANK 1: 19.5%
Long term fuel trim BANK 1: 11.7%

Short term fuel trim BANK 2: -7.0%
Long term fuel trim BANK 2: -15.6%


But what stands out most:
- Bank 2 (with the mostly clean exhaust tip) quickly reads varying voltages between a minimum of 0.030v and a maximum of 0.935v.
- Bank 1 (with the pitch black exhaust tip and sometimes soot in the parking lot) reads almost nothing, despite the fairly high corrections for bank 1. minimum 0.000v and maximum 0.040v.

After some changing of the plugs, I believe that the O2 sensor is broken, or at least transmits incorrect values.

It's not a clogged catalytic converter, I replaced it with a good one from a donor (XJR) engine.

Unfortunately I don't have any working lambda sensors, so I ordered one.

 

Since bank 2 is running rich, I’d pull the plugs out of that bank and see if one spark plug is especially blacker than the other 3 or are they all identical. I’m looking for possibly an injector hanging open.