'86 XJ6 VDP extremely rough running and cutting out

Looks normal for idle. Which one is #1? Try again just after the engine dies, maybe 1 is wet. Clean the plugs ( say 1&2) a little and see if they both got black the same.

Test shows engine itself is in good shape, Graham - no engine worries…

The first picture shows more soot than the second; which cylinder is which - and are all the other like the first or the second?

Bear in mind that the engine has not been driven ‘properly’, and with lots of idling I would expect the others to be more like the first picture - which is ‘normal’ for cold, fat running; the second picture seems too lean/clean.

However, the heading says ‘extremely rough running and cutting out’, as was described i your original posts - has anything changed?

Again; disconnecting one plug at a time is unreliable for detecting a failing cylinder. A crude injector test; * disconnect CSI and coil positive - and crank for some seconds*. With functioning injectors; the plugs should all be wet…

However, whether this is easier than the more precise check by removing, and testing, injectors is your choice. But referring to the above reservation; all injectors should be checked…

At this stage I would say that the first picture shows a working cylinder - the second picture shows misfiring. Either not igniting, igniting at the wrong time - or is not getting fuel…

Frank
xj6 85 Sov Europe (UK/NZ)

I’ve cleaned all the plugs, I’ll see how they look after the engine runs again
Plug 1


Plug 2

Plug 3

Plug 4

Plug 5

Plug 6

Thank you

Good to know, so far the engine is still cutting out and not running on all cylinders, but I’ll perform the injector test instead of my plug method
Thank you again

Somewhere you indicated there is a small leak in the exhaust header, and before your O2 sensor. Got to think that is going to allow O2 into the exhaust. That excess O2 will be read by the sensor, and signal the ECU that the engine is lean (excess/unburnt O2 in the exhaust stream), the ECU will correct to richen the fueling, causing the engine to eventually cough and die fron now the excess fuel. Fix the hole. Temporarily, use muffler tape or muffler putty. Then see what happens.
SD Faircloth www.jaguarfuelinjectorservice.com

2 Likes

Clearly plug one is the only one that isn’t shiny. Whatever you do you need to focus on this cylinder.

Put the plug onto the cylinder head and see if it sparks when you crank. I think it does, after all it is combusting. That leaves you with the injector which is probably faulty.

Also the above but I don’t think that’s the issue - doesn’t the Lambda sensor take ages to warm up? As far as too rich goes these plugs look normal.

To my eyes, the plugs are sooty. Got to remember, the plugs/engine is only being run for very short periods during Graham’s testing…as far as we know. He’s posting almost daily so can’t be running the engine for very long. SD

And…forgot to include…If the plugs are sooty, so is the O2 sensor, which, if sooted or carboned up, will also cause the ECU to richen the fuel mix. So, you get a double wammy. Hole in the exhaust header and sooty O2 sensor. SD Faircloth

Of course, but not too sooty given the circumstances! Mine always looked like that in the same situation.

My question to you is, the o2 sensor doesn’t do anything initially but the engine runs rough from the start, doesn’t that rule it out?

Ok… You made me read the entire novel…again. Seems like this cutting out started after taking apart and refitting the air intake system. Described in the 8/12/2023 posting. This engine has a mass airflow meter or sensor. Correct ? Might go back and re-examine its operation. SD Faircloth

There’s a pretty big leak not in the exhaust header but where it connects to the stock exhaust system, well past where the catalytic converter was and oxygen sensor is
The fuel injector site is another good resource, thank you

I’ll double check to confirm the plug itself sparks, but I agree with you on the injector at fault somehow
Thank you

You’re right, the engine is being run on average 20-40 seconds throughout my testing, save for a few times a few days ago where I let it run for maybe 5-7ish minutes

I hadn’t considered that, if I have time after some more injector/plug 1 tests I’ll take a look how dirty it is
Thanks

Correct, the engine has a mass airflow meter, but I honestly have no idea how to check that its working properly
I’ve found posts explaining repairs, but I’m unsure how to check all is well with it. Either way, it’ll happen after I investigate all I can with Cylinder/Injector/Plug #1
Thanks

I know I‘m repeating myself all the time but fix the cylinder first before you go off on unlikely tangents :slightly_smiling_face:

Taking apart and refitting the intake system is clearly the area that got disturbed and that includes the wiring and injectors and also the air flow meter (AFM) but this would affect all six cylinders.

But…
Two weeks ago I removed my tach and the same time the oil pressure warning light right next to it failed.
Checked the area ten times. It was the sensor at the engine - totally unrelated :grin:

Hi, I haven’t neem a Jaguar owner for 10 years now after discovering M-B S Class cars. Just got an email re AFM and these postings.
Easily found my post of 15 years ago which may help.
AFM Repair
HTH Brian
PS I miss my XJS convertible but I don’t miss being exposed to the Australian sun when the roof was down.

With any sort of fuelling/mixture problem; the O2 sensor must be disconnected, Graham…

Any attempts of adjustments will otherwise be countered by the Lambda system - leading to confusion. The engine, properly set up will run perfectly without the Lambda system…

I doubt if the Lambda is involved - but further down the track it will cause no harm…

Frank
xj6 85 Sov Europe (UK/NZ)

Doesn’t hurt to check injectors, Graham. But be aware that each injector is not specific to each cylinder or synchronised with cylinders’ valve opening times. They all spray into the manifold simultaneously and there is an element of cross feeding…less on the end cylinders, of course…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)

Very comprehensive, Brian…

But the sweep resistance is not supposed to be smooth - the AFM is a voltage divider and the resistance will vary between max and min several times during the sweep…

Frank
xj6 85 Sov Europe (UK/NZ)