George opined thusly:
Date: Tue, 8 Feb 2011 23:22:25 -0800
From: George Graves <[email protected]>
Subject: [alfa] GTV-6 has a miss
My GTV-6 has developed a miss. It runs fine at steady throttle at
anything below about 4000 rpm. But if the car is asked to pull an
incline, even a slight one it's starts to miss randomly -but not
badly; just noticeably. Also it will miss more consistently if you
take the revs above 4500 under hard throttle. I don't see any loose
hoses or any such thing and it has essentially new Bosch +4 plugs. Any
suggestions?
George Graves
'86 GTV-6 3.0 'S'
Under load a few things happen. One is that cylinder pressures go up.
This makes the spark plug harder to drive to breakdown, and as others
have said, can reveal ignition components which are then weaker than the
plug gap. I had a 164 dist. cap get dirty enough inside to fail from
arcing, although when the engine and cap were cold they'd work if you
kept the RPM up and the manifold pressure low. Lug it down and let the
cylinder pressures rise, or let it get warm and increase the
conductivity of the dirt, and it would not run. I've also seen problems
at the coil tap give your symptoms, and because the coil supplies all
sparks, the failure would wander around the various cylinders very
irregularly. Your problem sounds to me like this, although a failing
plug wire or a cracked or tracked distributor cap could do it, too. Or a
rotor irregularity could also be the culprit. I'd suggest carefully
cleaning the coil tap (which fixed one of my examples above), and
possibly swapping out the dist. cap/rotor and finally the plug wires.
You can create higher cylinder pressures by running on fewer cylinders.
You wouldn't want to burn your cat, but if you were to pull a few
injector wires, you might be able to create high enough cylinder
pressures in garage conditions to reveal weaknesses in the ignition
components. I haven't tried this....it just occurs to me that it should
be possible as a diagnostic test. It wouldn't run raw fuel through the
cat like pulling plug wires alone would do.
Michael
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