AJ6 misfire

[Jag190902]

Can anyone help with a misfire problem on my 3.6 xjs?

It starts from cold fine and runs well with full power until warm then starts to misfire. If I use heavy throttle it gets worse, then finally cuts out. If the car is left for a while, it runs fine then the problem re-appears. I recently drained the fuel ank and cleaned it as there was a large amount of sediment in it and this appeared to solve the problem - until now. HT leads have all been replaced so I would be grateful for any advice.

Thanks

[almcl]

When it cuts out, what's the spark like? Big, fat and blue or weak and wimpy?
Carry a spare spark plug with double the normal gap and try that when it happens next.

If the spark is good, a fuel pressure reading at the rail might be the next check.

[dhdove]

When you drained the fuel tank did you remove and clean the swirl tank underneath the battery that the main tank feeds into?
If you've got sediment in the main tank you won't believe what's in the swirl tank, the fine nylon gauze filter it contains will give exactly the symptoms you describe.

TonyS

[Jag190902]

Thanks for the tip on the spark plug - I'll try it today and see what happens as I have a spare. When I drained the main tank I also did the swirl tank and as you rightly suggest there was a tremendous amount of sediment in there, so much so that I had to replace the filter at the bottom of the pick-up tube! My next thought was to have the injectors cleaned in case any crud has found it's way in there and/or replace the fuel pressure regulator. Any thoughts on this?

[poprivet]

If you have the later 3.6 (with cylindrical air filter) it may be worth changing the Crank Position Sensor. One that is failing can give all sorts of intermittent faults when hot, once cooled down everything seems normal again for a while.

The plastic filter in the swirl tank protects the pump, the main fuel filter protects the injectors. As you have changed the plastic filter you will have the later improved one, the early small ones could get pushed up the tube too far restricting the end of the pipe.

If you add a signature with the model and year it helps others to give advice, I hope you find the problem soon.

[Jag190902]

Thanks for the advice Pop - my car is the later model as you suggested. I tried the spark plug as below today but got no joy so I think the next option is to check the fuel pressure. As a precaution I'm going to change the coolant temperature sensor, ignition amplifier and crank position sensor (as you suggest). After all, she is almost 24 years old and with only 63K miles on the clock lack of regular use may be a contributing factor to my woes.


1988 XJ-S 3.6 Coupe

[piman]

Hello Jag190902,

forgive me if I'm teaching you to suck eggs but don't change the three items you mention in one go. Change components one at a time otherwise you won't know what was faulty; cheapest first?
Not being familiar with new cars, do these sensors have any indicators on them to show when they switch as most industrial sensors do?

Alec

[Jag190902]

Hi

Tried changing the temp sensor which didn't work, then tried the ignition amplifier which I thought had cured it but the problem soon came back again. Took the plunge and bought a crank position sensor which I hope will cure it. Incidently, I got this from Jaguar - a genuine part for less than a pattern part from sng Barrett after the club discount was applied and no p&p either! It just shows that it does pay to shop around. I'll post an update when I've fitted the sensor with how things are running.

Thanks for all your replies so far

Iain

[Jag190902]

Update on the misfire:

Swapped the ignition module back for the original tonight and noticed that the screws were not fully tight - I assumed they just held it in place. So, cleaned up the aluminium back plate and retaining bolts and applied a thin smear of copper grease. Did the same for the mounting screws and put a thin layer of light grease between the module and the back plate and put the whole thing back together. Result, almost eliminated but still not quite right.

Then changed the crank position sensor for the new one purchased from Jaguar. It's a real git to remove with limited access and the retaining nut was very tight though soon gave way with a little persuasion. It was immediately obvious when compared with the new one that it had seen better days so I fitted the new one remembering to replace all the tie wraps securing the wiring. Result, sweet as a nut.

The car was a little quicker to start, and is now incredibly smooth - like a new engine. Much better than it has been in the 7 years I've owned it. Haven't had chance to drive it yet as it's very wet tonight but can't wait for a nice dry day

Finally, thanks to all for your advice and words of wisdom - It's saved me a fortune in garage billa and possible expensive mistakes!

[dhdove]

RESULT!



Congrats and we'll done!

"Finally, thanks to all for your advice and words of wisdom - It's saved me a fortune in garage billa and possible expensive mistakes!"

That's really great, please do write to the Club mag to say how fantastic the XJS BB is !!!

TonyS