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Jim Grant's Tech Tips
'92 Ford F150, Misrouted Plug Wires
Q:
I
have a '92 5.0L F150 that has run great for years. In the last year or
two, however, it's steadily declining. I've taken it into the dealer for
a tune-up, wires, etc. Most recently, it was discovered that spark plug
wires 7 and 8 were routed next to each other -- a big no- no according
to some Ford TSB's on the subject. According to the TSB's, plug 8 will
fire when plug 7 fires. It was also discovered that the #8 cylinder only
holds 30 lbs of compression. The dealer essentially stated that the misrouting
of plug wires caused a burned valve scenario to occur. Does this make
sense to you and could you explain it to me?
-
Mike
A:
How
does one spark plug wire cause another spark plug wire to fire? Believe
it or not the condition you're referring to is a rule of electricity and
the principle of that rule is used to make the spark for the ignition
at the ignition coil. Whenever electrical current travels through a conductor/wire
a magnetic field is formed around that wire. If you place another wire
next to the first, nothing really happens until the electricity/current
stops moving thorough the first wire. When current stops moving the magnetic
field collapses. When invisible lines (called magnetic flux) of the magnetic
field collapse they will cut through the second wire. Here's where it
gets really cool. As the lines of flux collapse and cut through the second
wire, it just happens to create electricity in that wire. This is called
mutual induction. Mutual induction is applied in many aspects of or lives,
cars, computers, home appliances are just a few items that rely on this
rule of electricity. Now, I know anyone that has looked under the hood
of a vehicle would say how can this happen? After all nearly all the plug
wires run side by side, so there should be more problems. That is true,
but for the problem to occur and affect engine operation you need to add
a few more pieces to the puzzle. Ford did that by having two cylinder
firing events close to each other. To make power with an engine, air and
fuel are mixed, compressed and spark is added at a very precise time.
It just so happens in some Ford engines the next event getting ready to
happen is in the next cylinder. If the spark plug wires are routed side
by side the first cylinder receiving spark just happens to induce a voltage
that is great enough to cause a spark at the plug in the next cylinder.
The problem is, it is at the wrong time! This causes all kinds of disorder.
This condition can cause spark knock, engine misfire, false computer codes
and driveability problems. If driven under these conditions for a prolonged
time it is possible for engine damage to occur. You would know your engine
is not running right and would find it very difficult to ignore. Proper
plug wire routing is important. If you ever want to create real havoc
just run a critical computer wire along side a spark plug wire.
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