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Jim Grant's Tech Tips
91
Nissan SE-V6 Pickup Truck, Non-Removable Spark Plugs
Q:
I have a '91 Nissan SE-V6 pickup truck. I had the 120,000 mile inspection/service
performed. The schedule calls for spark plug replacement. When I went
into pick up the truck they told me: "We could not remove the rear
spark plug without use of excessive force and risk breaking the plug as
that would cause an expensive head removal to remove and replace the plug.
The plug was firing all right so we left it in and you can decide what
to do later. However we are sure the plug is not cross threaded."
The same service department had installed those spark plugs during the
72,316 mile service. Which brings these questions; 1. Since a spark plug
is continuous metal, how could it break, leaving part of it in the cylinder
head? 2. Can they, in fact, determine that it was not cross threaded?
3. Since they installed the plug would they not be liable for any damage
or repair necessary to remove and replace the plug? 4. What course of
action do you suggest?
A: A
spark plug does have a continuous metal housing but, it becomes very thin
at certain points. Forcing a frozen spark plug is almost a guarantee of
snapping it off in the cylinder head. The length of the threads on the
spark plugs for your vehicle is nearly 3/4 of an inch long. If the spark
plug was cross threaded during installation it "never" could
be fully installed. This condition would be visible at the spark plug
and, once removed, there would be indications as well. The problem you
are experiencing is not uncommon to engines with aluminum heads, it happens.
All it takes is a small burr or piece of the aluminum threads to break,
causing the spark plug to bind as it's being removed (which is not the
fault of the installer). Sooner or later you'll have to find out what
happened, the spark plug will have to be removed. With some luck and an
experienced technician the spark plug can be removed without causing costly
damage or the need to remove a cylinder head.
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