Question:My engine has a steady miss and gets terrible
fuel mileage. What's wrong?
Answer:A steady miss indicates one of three things: a
cylinder that isn't firing because of an ignition problem, a cylinder
that isn't firing because it isn't receiving fuel (multipoint
fuel injected engines only), or a cylinder that has lost compression.
The first step in diagnosing this kind of problem is to identify
the dead cylinder. A professional mechanic can do this quickly
by hooking the engine up to an ignition oscilloscope and displaying
an ignition raster pattern. The dead cylinder will show a firing
voltage that is significantly higher or lower than its companions
depending on the nature of the problem. He might also do a "power
balance" test and/or a compression test to find the dead
cylinder.
One way you can find a weak or dead cylinder is to momentarily
disconnect each of your engine's spark plug wires one at a time
while the engine is running. When the plug wire is removed from
the spark plug, there should be a big drop in idle speed and idle
smoothness. When you pull a wire and there's little or no change
in idle speed or quality, you've found the bad cylinder.
It makes no difference whether you remove each plug wire from
the spark plug or the distributor (or coil pack on distributorless
ignition systems). The idea is to simply disconnect each cylinder
for a moment to see if it makes any difference in the way the
engine runs. The one that makes no difference is the problem cylinder.
CAUTION: Disconnecting spark plug wires while the engine
is dangerous because you risk getting shocked. You can minimize
this danger one of several ways. One is to wear rubber gloves
and use insulated spark plug wire pliers to momentarily disconnect
each plug wire. Another is to make sure no part of your body
is touching or leaning against any metal surface on the vehicle
(the fender, hood, grille, etc.). Or, you could turn the engine
off, remove a plug wire, restart the engine, note any change in
idle, then repeat for each of the remaining spark plugs.
IGNITION DIAGNOSIS
If you disconnect the plug wire from the spark plug and hold
the end of the wire close to the plug terminal or other metal
surface, you should see a spark and/or hear a crisp snapping noise
if voltage is getting through the wire. No spark would tell you
the plug wire is bad, voltage is arcing inside the distributor
cap (remove and inspect the cap for cracks and carbon tracks --
replace if any are found) or a dead coil on a distributorless
ignition system (Note: on most distributorless ignition systems,
each coil fires two cylinders. So if both cylinders are dead,
you know for sure the coil is not working. If only one cylinder
is dead, however, it's not the coil).
If all of the plug wires seem to be sparking okay, the next
step would be to remove the spark plug in the problem cylinder.
Fouling is a common cause of ignition misfire. Examine the end
of the plug. If the electrode is covered with deposits, clean
or replace the spark plug. Also, note the type of deposits on
the plug. Thick, black, wet or oily-looking deposits would tell
you the cylinder is burning oil (probably due to worn valve guides,
rings and/or cylinder wall). If the deposits are a powdery black,
the cylinder is running too rich (probably due to a leaky injector
on a multipoint fuel injected engine). If the deposits are brown
or gray, it indicates a normal buildup. However, the plug may
be fouled because it hasn't been changed for a long time, because
it is the wrong "heat range" for your engine application
(you need a hotter plug), or because of frequent short trip stop-and-go
driving.
In any event, if the plug is fouled you should probably remove,
inspect and clean or replace all of the spark plugs.
FUEL DIAGNOSIS
If the dead cylinder is receiving spark through the plug wire
and the spark plug itself appears to be okay (not wet or fouled),
and your engine has multipoint fuel injection you may have a dead
fuel injector.
To check for this kind of problem, start the engine and place
your finger on the injector. You should feel a buzzing vibration
if the injector is working. No buzz means the injector is either
defective or it is not receiving a voltage signal through its
wiring harness. You can check for the presence of voltage with
a 12 volt test light or voltmeter. Disconnect the injector wiring
connector and attach the test light or voltmeter between the injector
and connector. If the light doesn't flash or you don't see a
voltage reading when the engine is running, it indicates a wiring
or computer problem that will require further diagnosis. If voltage
is getting through but the injector isn't working, then the injector
is defective and needs to be replaced.
Sometimes the injector will appear to be working but really
isn't. It will be receiving voltage and buzzing as normal, but
because it is clogged up with varnish deposits little or no fuel
is actually being squirted into the cylinder. If ignition and
compression are both okay in the bad cylinder, therefore, it would
tell you the injector is clogged. On-car cleaning may reopen
the clogged injector is the varnish isn't built up too thick.
But a completely clogged injector usually doesn't respond well
to this type of cleaning. It either has to be removed for off-car
cleaning (which may or may not succeed id reopening it) or be
replaced.
COMPRESSION DIAGNOSIS
If the dead cylinder is getting spark and fuel, the only thing
that's left is a compression problem. The most likely causes
here would be a leaky valve (probably an exhaust valve since they
run much hotter than intake valves and usually fail or "burn"
first), a blown head gasket (this usually involves two adjacent
cylinders, however), or a rounded or badly worn cam lobe.
A compression check will verify if the cylinder is developing
its normal compression. Little or no compression would verify
any of the above problems. A leakage test could also be used
to further diagnose and identify the nature of the problem (valves,
head gasket or cam). Air leakage through the exhaust port would
indicate a bad exhaust valve. Air leakage back through the intake
manifold would indicate a bad intake valve. Air leaking into
an adjacent cylinder would indicate a blown head gasket. Minimal
leakage would indicate a rounded cam lobe.
Leaky valves would require removing the cylinder head and
having a valve job performed. A leaky head gasket would require
removing the head and replacing the gasket (and probably resurfacing
the head to restore flatness). A cam problem would require removing
and replacing the camshaft and lifters (old lifters should never
be reused with a new cam).
