Ok, ok, I know we have hammered the octane question to death.
BUT, the problem I face is not preignition 'ping' but dieseling
upon shutdown. a.k.a. 'Run-on'.
The 9:1 compression ratio engine in my Jag produces enough
pressure to light the flash the 92 octane fuel.R Ideas?
In the Olde Dayes, carmakers used to recommend "decoking" a
cylinder head after a certain amount of use. The problem is
most likely that deposits of carbon left over from combustion
are retaining heat in the combustion chamber (it's also possible
that they are reducing combustion chamber volume to the point
that the car will preignite, but that's not borne out by your
compression numbers). It was worse in the days of vegetable
oils and poor fuel quality, and in fact decoking was sometimes
recommended as a regular maintenance operation, every few
thousand miles, in the Thirties. It's the natural result of
combustion residue, particularly in an engine that is running
a little rich, because the extra gasoline doesn't burn completely
and causes deposits in the combustion chamber. In some cases
these deposits will actually glow like embers, enough to cause
run-on after you shut the engine off.
Traditionally, to decoke (or decarbonize) your car's cylinder
head, you'd remove it, use a rotary wire brush to scrub off
the carbon deposits from the cylinder face, preferably resurface
the head, and install it with a new gasket. Since your Jag has
an aluminum head, don't use a wire brush, and I don't know what
is recommended. See your shop manual for their recommended
techniques for decoking or decarbonizing the head.
There are also various products that go into the gas tank with
varying degrees of effectiveness in cleaning combustion deposits.
Some of this is no doubt snake oil, but some of it might be good.
Ask over the cubicle walls and see what they tell you. :-)
Also, make sure your car isn't tuned too rich, as this makes the
situation worse on both ends: that is, a rich mixture is more
likely to ignite if there are hot spots in the deposits, and it
is more likely to cause deposits in the first place.
I'd try:
- Running a tankful of something said to clean deposits from
valves and combustion chambers
- Replace the plugs, retune the carbs
- Give the car a good "Italian Tune-up" (rev it to redline at
full throttle a couple of times to blow out the cobwebs; getting
onto a clear freeway is a good place to do this more or less
legally)
- If this still doesn't work, decoke the cylinder head in the
approved manner
There's also a somewhat suspect technique that I've read but never
tried, so get out your salt shaker for this one. Supposedly, if you
squirt a thin stream of water into the carb while the engine is
running, the resultant steam and vapor will clean deposits off the
inside of the combustion chamber. (I've heard it described as pouring
the water down the carb for domestic V8s, so I'd guess that using an
adjustable sprayer will do the trick on SUs.) As the story goes, when
you do this, you'll get huge black clouds out the tailpipe for a
few minutes, then things will run cleanly again for months or years.
(Has anyone on this list done this trick? Does it really work?)
Richard O. Lindsay / / / / / / / / / __
Senior Research Scientist / / /-/ /_ -/ /-/ /_
Amoco Production Research / /_/ / / /_/ __/ o /_/ __/
Why does this remind me of the cobbler's children going barefoot? :-)
--
"Some people put their hands in the dishes the moment they have sat
down. Wolves do that." -- Desiderius Erasmus
Scott Fisher/sfisher@wsl.pa.dec.com/DEC Western Software Labs/Palo Alto, CA
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