hi this is my first JAG after wanting one most of my life I LOVE it but one problem i have is when it is cold it starts and runs fine but when i go to take off it starts to miss really badly and blow black smoke after you take your foot off the accelerator it sort of comes good and doesnt do it when its warm ?? i think theres a vacuum problem too ,so could that cause problems with the cold engine ?? thanks for any advise and help

 

Lots of "could be" problems but two strong possibilities stand out in my mind.

First is the coolant temp sensor. This device controls cold engine fueling. If you have a multi-meter it can be easily checked or, at only $20 or so (USA dollars), it's isn't too painful as wild guesses go.

Next is the infamous "double fueling" issue, solved by simply soldering a capacitor into the air flow meter harness.

Here's some info
XJ 6 Series III 4.2 - Cold Start EFI Overfuelling

Cheers
DD

 

Thanks Doug for your advice I checked and cleaned the electrical connections but no different and noticed the diode fix on the 2 wires hasn't been done so will get 1 and try that thank you

 

when I bought my XJ over two decades ago, it had not been run in a long time, and it showed the same symptom and black smoke (coughing, coughing), but not everytime I drove it; At the time I had no knowledge of the Coolant Temp Sensor symptoms and much less of the capacitor fix/mod. In other words I did nothing and the symptom dissapeared by itself :-)) Lack of use?

 

On my last run (86 XJ6 4.2), engine warm and at 50 mph, suddenly thick black smoke from the exhaust and the car could not maintain speed. Then clean exhaust again, then black. I parked and the engine idled very rough and hesitant to rev when I moved the throttle.

But then I drove on and there was no problem evident most of the way. Other details:

-normally idles at 1000 hot or cold
-coolant temp sensor brand new but gauge on the dash still reads 55-70 C (120-155 F) but that can't be accurate
-it cranks forever before firing, hot or cold
-there's a leak in the exhaust well downstream of the catalytic converter
-30,000 miles from new.

I looked at the spark plugs this morning and they look like combustion has been good (normal few thousand mile Champions) except two that had black soot deposited around the face of the body of the plug (the threaded part).

Do you know what could cause intermittent black smoke and loss of power?

Edit: I'm gonna start a new thread with this post.