Let me open by saying that I KNOW I AM IGNORANT.
Okay, here goes. My 1976 XJ-S had this symptom of blowing pale smoke out of
one exhaust only, on heavy throttle + high revs, the right side, this was
worse after the car had been driven gently for a while; also the exhaust was
stuttery. It also smelt somewhat rich across all rev ranges.
Well, I reasoned that it might be because a spark plug was fouled, so that
unburnt fuel was going through the engine. To my mind this explained the
stutter and the richness and the “smoke.”
At any rate, I thought, it couldn’t hurt to pull out all the spark plugs and
This I did. (I’ll leave out the panicked moment when one of them cross
threaded. I believe that was the previous mechanic’s fault, and I think it’s
okay now - but I am going to have to have helicoils and tapping tools and
glue and a priest on hand when I change the plugs again, I think. Boy oh
Was my conjecture right or wrong? I don’t know! The smoke symptom is 100%
gone. It’s no longer embarassing to bury everyone else at the lights. On the
other hand, the plugs that came out all looked exactly as used spark plugs
should. Good clean engine, you would think.
Bad news though - the engine is not smooth, not nearly as smooth as it used
to be. Under light throttle, maintaining a given speed, there’s a noticeable
judder, a sort of pulsing in the power delivery.
Oddly though, giving a bit of pedal eliminates the symptom and the car
seems to have no less power than before.
The only other thing that springs to mind is that the new plugs, which I
gapped according to the Haynes manual at 0.025", or 0.64 mm, looked more
closely gapped than the plugs I pulled out. I don’t know if that’s relevant.
So, what cured the smoke? And what is causing the judder? Could I have
gapped the plugs too closely? Could that cause a sporadic failure to spark
at low revs?
As I said, I realise I have a lot to learn here so don’t be afraid to tell
me I am an IDIOT.
Don’t get me wrong, it’s not running like a dog or anything. It just feels
Any suggestions appreciated.