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3.
Exhaust system blockage
A trend that
we probably would have never recognized as a problem, were it not
for the shear volume of our service calls, relates to the pesky
business of clogged exhaust systems.
In the latest manifestation of this problem, a customer was reporting
on a serious power loss, even in neutral. The engine had become
more and more sluggish over a two year period, and by the time the
customer called us, the engine would not accelerate past 1500 RPM,
even in neutral. Whenever the throttle was advanced much past idle,
a "gasping" sound came out of the mouth of the carburetor, along
with small droplets of fuel.
After much "let's try this" troubleshooting and repeated cleaning
of the carburetor (all to no avail), we finally convinced the customer
to remove the hot section of the exhaust from the back of the exhaust
manifold. It only took several seconds of running to determine that
the engine ran perfectly OK with the exhaust removed.
Further investigation revealed that the one and a quarter inch exhaust
pipe had become at least 80% restricted with rusty scale in the
immediate area of the water entry point, just ahead of the water
lift muffler. The blockage was also noted in the immediate area
where engine cooling water is introduced into the hot section, which
leads us to conclude that minerals and other crud coming in with
raw water have a tendency to build up serious encrustation in this
area. Only the area directly above the piece of pipe used as a hose
barb was open for exhaust to pass through. The fact that the water
nipple comes so deep into the pipe doesn't help this particular
situation.
A valuable diagnostic technique grew out of this latest episode.
Whenever any one of the four spark plugs was removed, the engine
ran and even accelerated somewhat better. Clearly, by the time a
restriction grows to this extent, the exhaust system is only capable
of handling a three cylinder engine. As soon as the forth cylinder
is added, the engine bogs down due to being unable to discharge
its exhaust.
Blockages of this magnitude don't develop overnight. There is clearly
a time period of several years of buildup, before the acute symptoms
described above will begin to manifest. During this time, the engine
will show early signs of the blockage, such as losing power, fouling
plugs, and/or experiencing sticking valves.
Unfortunately, these symptoms can also be caused by more routine
things, like low compression, poor ignition, rich fuel mixture,
etc., so I am at some risk of initiating a bunch of exhaust system
work, fixing things that aren't really broke. However, in cases
where compression is good (85 psi or better), and an engine is otherwise
being well maintained, it could well be suffering the early stages
of exhaust system blockage if it is still manifesting one or more
of the above symptoms.
In those cases, removing the hot section of your exhaust system
for inspection and replacement gets my personal vote for a most
important winter project, particularly if it has not been removed
for 30 or more years and is showing signs of age. - Updated: November
4, 2003
