Hi All,

A strange problem has started on my XJS -1989 V12 with Marelli ignition.
Car starts fine when cold.
After driving approx 10 - 15minutes the car develops a very slight misfire/cough (approx every 20 seconds or so the rpm dips down, but irregular), then not long after the engine dies completely and cannot be restarted for 1-2hrs.
This is a sudden engine cutoff, like the ignition has been switched off. no stuttering, or almost running, or running on 6 of 12 cylinders or similar.
When it first happened I thought I had run out of fuel, but no change after topping up with 30litres. I can hear the fuel pump priming etc, and the fuel hoses harden up so I know there's fuel pressure.

Listening to the engine turning over, I'm fairly sure none of the injectors are firing. I was listening for the ticking whilst turning over & when it did start up I could suddenly hear them.
There's not a hint of life from the engine when it won't start - it just turns over completely smoothly.
I haven't done anything recently that I think could have caused it, the car completed a 200 mile journey recently and was faultless.

I've been able to repeat this fault over 2 separate days now. When I did get it to start today after it had died but was still moderately warm, I left it idling in the garage and sure enough the engine coughed, then died after 5 minutes or so.

Does anyone have any ideas?

So far I have wiggled lots of engine bay wiring, tried bridging the air & water temp sensors on the Right hand side, and also tried a 2nd resistor pack (silver box that sits behind Right hand headlight) as I had a spare one & heard they were prone to failing. No change

 

This happened to me several years ago. It turned out to be a failed crankshaft position sensor (CPS) - 1990 Marelli V12. Replacement of the CPS solved the problem.

 

Hi, which sensor did you need to replace?

I've just checked the resistance of both of mine & they are both around 0.7k ohm. I think they are just a coil of wire around a magnet so would expect one to read differently to the other if one had failed somehow?
I don't really want to go down the 'throw parts at it' avenue but reading around I see these do fail, aren't too expensive and look fairly easy to access so might just replace both & see what happens.

Tried to start it again today and it's running very strangely now even from cold. Never experienced it before. It sounds like it's starting on 4-5/12 cylinders, and won't rev off idle (it almost feels like it will stall if you blip the throttle enough to disengage the idle switch), it did seem to clear after 30seconds or so & would then rev up but still felt quite rough.

I checked resistance of both the temperature sensors for coolant and air & they near enough matched & seem in spec.

 

Asdrew
I am not sure that the resistance measurement is a fault-finding test. I think these gizmos just give up after a bit, probably because the magnet demagnetises.
As none of the usual Marelli specialists have responded yet, this is the suggestion of someone with no Marellli actual experience. If the sensors have never been renewed, then doing so is a sensible bit of maintenance in any event. If the injectors are not firing, I seem to remember reading on here that the rear flywheel sensor tells the system the engine is turning and hence to activate the injectors; but I could well be wrong
Having said all that, a sudden stop could be the ignition amplifiers, have you tested for spark as soon as the engine cuts out?

 

Well I have updates. I don't usually move this fast but I bought 2 crankshaft position sensors from eBay yesterday on the basis they were quite cheap, and they arrived this morning, & I've just fitted them!

One note on the job of fitting them - it isn't fun or easy.
The front sensor loom is kept in place with 2 metal P clips, one of which is behind the main radiator water hose and is basically inaccessible. I couldn't get the cable back into it.
The rear sensor uses a plastic clip thing to clip the sensor wire to what I think is the small metal tube that goes up the block for the oil supply to the oil pressure sensor etc etc. Again you can't see anything and everything you touch is disgustingly oily - I had to just yank it out in the end and cable tie the replacement back as best I could to keep away from the left hand side exhaust manifold.
Rear sensor also has a little heat shield around the mounting, which is clearly designed for a sensor with a right angled cable outlet, but mine were both straight. I'm a bit uncomfortable bending cables like this to fit but it seemed ok in the end
The female connectors in the engine bay also both cracked due to age...

Anyway the car did the rough starting thing but did start, eventually on all 12, and I warmed it up to operating temperature on the drive and it does seem to be running ok.
I left it running for 15 minutes or so and no cutting out, and engine seems nice and smooth when revved up so I'm calling that a successful crank sensor replacement job at the very least.

Just have to get the courage to actually drive it somewhere & see if it breaks down now.

I'm thinking/hoping the rough running/difficulty starting could be a hang-up from all the attempts at starting - spark plug soot deposits or similar? & this isn't the first step in a lengthy diagnosis/deeper issue. I'm a bit worried about the fact it almost stalls & doesn't rev at all when first started.

 

Just take your cell phone with you, and enjoy the trip. Hoprfully you've got it fixed. Electronics are great when they work. LOL! Good Luck.

Jack

 

I think i've sorted it!

I found a break in the wire to the coolant temp sensor on LH engine bank - on the basis the car did run fine I possibly broke it/finished it off when removing the pigtail to test the sensor/jump the connector. Turns out this sensor is essential for Marelli to run 'properly' even though it's not listed as a Primary Input in the workshop manual. Both crank sensors are listed as primary inputs.
The manual is not specific but I'm interpreting the result of the lack of primary inputs as the symptom I had - no fuel injection at all.

I also found a break in the wire to the intake air temp sensor. I think this has been broken for a long time. Although it's part of the same cable loom as the Marelli sensor, It connects to the engine ECU in the rear (not the Marelli ECU in the left hand footwell) & from what I can find out is used for a slight fuel trim.
So less essential, but maybe explains why the exhaust has always been a bit 'smelly' as I guess without it connected the car would have always run slightly richer than optimal?

Both of these breaks were in the stiff rubber section immediately after the connector, approx 3cm from the connector, where Jaguar used some sort of amber resin stuff to hold everything in place that had gone rock hard and caused the cable insulation to split (which has also gone hard), and corrosion in the wire resulted, with lots of green dust.
I definitely won't be pressure washing this engine bay anytime soon!

 

Good find. The CTS is a primary sensor for the Lucas ECU to run the injectors. I suggest cutting back the loom to where the wires are still flexible and connecting new silicone wires to new sensor connectors. I did this a few years ago having had a similar near miss.
Still a good piece of prevention to replace the crank position sensors though.

 

Yeah, just to clarify i'm sure I had an issue with one of the crank sensors heating up & giving the total no-start.
Then in process of looking into that I broke the coolant temp sensor wire so even once i'd fixed the crank sensors, it did sort of start but still very poorly, either very lean or very rich on 4-5 of 12 cylinders.

Meanwhile the air temp wire had been sitting there broken.

 

An ounce of Prevention, is worth a pound of Cure. Glad to hear you got it sorted, and the extra repairs you did will simply keep you off the side of the road. All is Good.

Jack