Yesterday my 96 xjs started to run rough. I removed the ignition coils and checked the resistance of each coil as follows:
Cyl 1 : 0 ohms
Cyl 2 : 0 ohms
Cyl 3: .5 ohms
Cyl 4: 1.1 ohms
Cyl 5: 0 ohms
Cyl 6: 0 ohms.
All plugs look good. The resistance values for the coils is .75 ohms. My question is are the coils for cylinders 3 and 4 within spec or are the others bad? I tested the OEM coils that were on the car and their resistance and four of the six were.8 ohms and two of the remaining three is .9 ohms and one was 1.0 ohms. Listened to feul injectors. they are clicking consistently at the same rate. This started yesterday. Not sure where to start. Suggestions welcome.

 

Can you describe the rough idle? Does it do it right after startup, when the engine is cold? Or does it happen after the engine warms up a bit?

Assuming no codes?

There are a ton of threads, many by me, regarding the chasing down of rough idle for this particular MY and engine.

 

No codes. It occurs after start up and continues.
The rough idle has a puttering sound from the exhaust. Engine does shake not hard but is noticeable through the steering wheel.

 

Do you have any means of diagnosing? OBD2 reader?

Condition of spark plugs? (highly possible)
Coils? (Highly possible)
Oxygen Sensors? (Less likely because when engine is cold, they’re not being read)
MAF? (Same as O2)
Throttle Body? (Highly possible)
Coolant Temp Sensor? (Same as O2)

Those are the usual suspects. They all have their own ways of diagnosing.

Easiest is to take the throttle body off and clean it out. That’s free.
You can short the Coolant Temp Sensor to check that one.
Spark plugs are cheap and easy.
Coils are expensive and easy.
MAF is expensive and easy.
O2 is expensive and somewhat easy.

 

No check engine light. OBD2 readet will not be able to read dtc w/o CEL on. The AJ16 engine has gained a dubious reputation for not storing codes. Thats a alot of areas to look at. Engine idle speed is at 600rpm. What's scary about this car is runs so quiet that when at a stop light you think that it will stall. I will start with coils since one has more resistance allowed.

 

I’d start with the throttle body. If you’ve never cleaned it, it is likely caked in grease. I’d say that’s the most likely because we’ve all had it happen with these cars.

You need to disconnect the hoses, including a small coolant hose underneath the throttle body. Have a golf tee handy and you can shove it in the hose without losing much fluid and avoid the mess. Then unplug the sensors, and unbolt the four bolts holding that throttle body to the intake manifold. There are no gaskets to worry about.

Make sure you have carb cleaner and rags, and try to clean it upside down so that the TPS underneath the throttle body does not have these fluids collect into it.

The put it back the way you took it apart. For a first timer, probably 2 hours, maybe 3.

I think the bolts on the throttle body are 13mm. Everything else is a flathead screwdriver, for hose clamps, I believe.

 

I'm a little confused are the coils 0R (ohms) or 0.8R?

0.9 and 1.0R I think will be tolerance and should be ok your meter leads will be 0.3-0.5R.

 

Resistance values for the coils are .75 ohms. Cylinders 1 2 5 and 6 read 0.0 ohms. Cylinder 3 reads 0.5 ohms. Cylinder 4 reads 1.1 ohms.

 

Are you measuring the coils disconnected from the car completely. If you are then I would suspect 1,2,5 and 6 coils are bad.

Test the coil by removing the spark plug and placing the electrode on the manifold and crank the engine, is there spark?

 

Thank you all for all the information to make this a good adventure in Jaguar owning.


So I thought I would share something that is probably embarrassing but I could see it go un-noticed.
Short Story: Cyl 6 and Cyl 5 coil wires are really easy to mixup.


1995 Jaguar XJS 4.0 6 cylinder AJ16
24K miles.
Was changing spark plugs, because fuel consumption was sub par / could feel it wasnt purring exactly right.
NKG copper -- tried .025 and .03x gaps -- rough idle, returned them.
The Champions I had in there were actually not bad, but they were gapped by last owner at.035.

Put the original champions in -- rough idle.
Changed them to .025 gap, and cleaned everything.
-- rough idle.

Stood there and realized that #5 wire and #6 wire to coils are same length, and #5 (so it turns out) actually came out of the harness behind the #6 wire.

So that's it. not only was this solved by switching #5 and #6 wires, but at .025 gap... .. well its just right, and gunning it, it was a whole new beast.


GL,
David