Hi!

I have a Jaguar XJ6, X300 -95 with AJ6 engine. The problem is, that after the engine has been running for a minute or two, it starts to go uneven. Here is a video which shows, how:


Please help me! I have been working on this car for over 500 hours. My plan was to give it to my father as a present for his sixties, but those parties already passed two months ago

I'm not super experienced with cars, but the motivation is strong! Had to study everything from a scratch.

It might be the third cylinder, because the spark in that cylinder can hardly be seen, whereas in all other 5, it is bright and strong. BUT can the cause be elsewhere? Can it be the famous crankshaft position sensor? Or the O2-sensors, since it has something to do with the engine warming up?. Something else?

I thank you all of the tips so far!


BACKGROUND:

IGNITION OF CYLINDER 3

I changed the ignition coil. No effect.

I changed the spark plugs. No effect.

I repeated previous tasks with other combinations. No effect.

I changed the clip of the wires from ECU to the coil 3. No effect.

I took the power straight from the battery (since its constant 12V). No effect.
When the power is turned on, the voltage between the coil's + and car body shows 12V, when tested with multimeter.When the engine is on, shows about 14V.

When measuring the voltage between + and - wires of the coil, it shows 0,3-0,5V. Expect the "broken" cylinder, shows 1,6V.

The wires from ECU to coil 3 are in good condition, and a 24W lamp shines bright when electricity given trough.

When the engine is running and the wires are being poked and shaked, no effect in the engine run, so no bad connection neither.

The ECU was sent to a specialist but they couldn't find anything unusual.

Spark plugs are new. Spark gap is 0,9mm as the manual recommends.

The O2 -sensors are NOT in good shape. They also give a strange, pointy curve on oscilloscope. But when detatched, the engine runs exactly the same (bad way) as if they were on.

The O2-sensors are attatched to correct connectors. Checked from the manual.

FUEL SYSTEM

The car has been unused for 6 years. The fuel tank was emptied first and the hoses as well.

The fuel filter was changed to new.

Fuel rail was detached and cleaned.

The injectors were taken out and checked with 9V battery and 3 bar compressor. They seemed to give a nice spray. Also, one liter of injection cleaner was put trough

Car has been driven about 100 km.

AIR

Air filter changed to new.

No leaks on the intake, when testing with compressed air and soap water.

Carburettor was quite black and dirty. It was totally cleaned. No effect. The automatic idle valve (or what do you call it) moves nicely and seems to react like it is supposed to.

VALVES AND CYLINDERS

The compression pressure showed 11,5-13 bar in every cylinder when running with starter motor.

The valves seem to move up and down nicely. Don't seem stuck or anything.

Oil and oil filter are new.

 

Tested air leak from the intake with a smoke machine (i know, i know. Took a risk, since it uses water based liquid) and a compressor. Throttle was fully opened. Nothing found.



air intake, leak, test, jaguar, x300, xj6

 

Also tested, if the CTS was working. Put it on a kettle and heated the water. Results:

13 celsius= 3,1 kOhm
35 celsius= 1,8kOhm
50 celsius=1,2 KOhm
70 celsius=600 Ohm

I read somewhere that it should be:

0 degrees C (32F) 5.9 kohms
10 degrees C (50F) 3.7 kohms
30 degrees C (86F) 1.7 kohms
50 degrees C (122F) 840 ohms
70 degrees C (158F) 435 ohms
90 degrees C (194F) 250 ohms

So pretty close, and working I guess? I also tested to fool the ECU, that motor was warm by connecting the pins of the CTS. No change in idle. Also tested to disconnect the CTS to fool that the engine i cold. No change. Hmmm....





coolant temperature sensor, CTS, jaguar, x300, aj16, xj6

 

First, you have made an invalid assumption, although I DO NOT think it has to do with your problem.
"The O2-sensors are attatched to correct connectors. Checked from the manual."
The connection to the sensors can be either way, and the "Lmbda Orientation" procedure, done with the dealer level software, sets the correct connection. So you cannot tell from looking if yours is right.

I notice the exhaust manifold has been repaired, and likely fairly recently because the paint is still in good condition. Is it tight and not leaking?

At this stage, you need to get a way to measure the Short and Long Term Fuel Trims.

 

It's also an AJ16 vice an AJ6, but I reckon has nothing to do with your problem. Sounds like you've conducted a fairly extensive investigation of the #3 coil.....but have you tried rotating the coils from cyl to cyl to see if the problem migrates?

Start with a new coil on #1, if no change, move it to #2, and so on until the new coil is on #6 and still no change?

 

It sounds like problems begin when it enters closed loop? Does that make sense?

If that's the case, I wonder if it could be the O2 sensors of the MAF sensor.

 

Have a look at this magazine article- BUT, you will make little progress until you have a way to chart the trims

 

Arkkitehti stated that it does the same with the O2S disconnected. So not them causing the misfire. In the video it sounds very irregular, not just all rich or lean but one cylinder not firing occasionally. Cyl 3 ignition is suspect as described but could still be firing ok and the misfire could be any one of the cylinders. I would swap each coil one at a time with a known good one.

 

The OP has gone silent! I would like to hear more about how he determined "the spark in that cylinder can hardly be seen" with his new coil. If the other coils are the same in that location, I would have a look at the pins on the ecu connector and generally test the resistance of the coil to ecu wiring.

 

When I have had a bad coil, I also get the code for it - like are you getting a P0303 - Misfire in cyl 3 ?

 

There have been several instances reported where a failing coil resulted in a sporadic misfire, like when up to operating temp., but no code until complete failure.

 

Hey, thanks for the replies, i appreciate it! And apologies for not been writing for a while. There was one other problem with ABS, that I had to fix immediatelly. However...

Oh? Is there any way to check it? I read somewhere that an error code should occure if attatched wrong. Is it true?

You are right! They both had cracks, and got welded. I cannot feel any leaking gas with my finger tips. Thats all i know.

Should i get my car to a professional to do this? Is it to check the lambda signals, if they are right or what?

NEWS: I have been driving a hundred kilometres now. It seems to have all the power when cold, but after a short drive, it gets uneven. Not only the idle, but the whole....thing.....engine.

 

Yes, I have tried quite many coils there but still the same cylinder was misfiring.

 

Thank you for your post! Well, I checked the MAF sensor, and the two components seemed clean. I also checked the voltage of the sensor, and got 1.4V when idling and it got higher when giving more throttle. Seemed quite close to what it should be. But what does a closed loop mean?

 

You are right! But when the engine runs longer, the misfiring is more constant. And when the #3 coil is detached, there is no change in the run. So there is definitely something wrong with #3. But could also be a combination of problems!

 

I took a coil in my hand, when idling, and grounded the spark plug to nearest plain metal. There was an inch long "lightnigs" in the other 5 spark plugs and just a tiny spark (size of a flea) in #3. And yes, the other coils were the same in that location. Perhaps I should take a photo of the sparks? The pins of the ECU connectors seemed ok. I also tried to shake them hard when idling, but no change. Is there need to check the resistance? I tested with a light bulb and got a bright light

 

Unfortunately no codes....

 

Today I drove 10 km, with the O2 sensors de-attached. No change. Still misfiring. Does that tell anything?

 

Have you checked your injectors? When I first got my XJ6, I was getting random misfires. I went through coils, plugs and finally switched out injectors and solved the misfire issue.
I wasn't sure if it was starting after it entered closed loop though.

Might just be something to check out...

 

It seems like your spark test is valid, so I guess I would go there first! The spark works by giving +12 volts to each coil and the ECU grounding the other lead for current flow. Go ahead and disconnect both ECU plugs and look carefully for corrosion.

A guy with your skills and interest should definitely look into getting a OBD scanner that can trace parameters, (Although I do not believe it will solve this one!).