This information pertains to the Jaguar XJ6 Ser II-III models with the fuel
injected 4.2 engine. However, in a broader sense, many of the items
mentioned apply to other fuel injected models as well.
A slightly over-rich mixture may well go unnoticed until your car fails an
emissions test or for some other reason has been checked with an exhaust gas
analyzer. However, many owners are plagued with a gross over-rich
condition which often appears suddenly and is manifested by black exhaust
smoke, fouled spark plugs, and a "chuffing" engine. This
article is intended to address grossly over-rich running problems. Here
we go...
COOLANT TEMP SENSOR: This is a common culprit. The main purpose of
this sensor is to "tell" the ECU that the engine is cold so that a
richer mixture, required with a cold motor, will be provided. In this
sense it is similar in intent to an automatic choke on a carbureted
engine. As the motor warms up the influence of this sensor is gradually
reduced and at about 180ºF or so it no longer is really part of the
mixture control picture.
However, skewed readings from a tired sensor can tell the ECU that the engine
is still cold when, in fact, it is fully warmed up. In such instances the
ECU dutifully continues to command a rich mixture which the warm engine cannot
tolerate....and then you have the black smoke and chuffing.
I might add that hesitation and poor running on a cold or cool-ish engine can
be caused by a coolant temp sensor that is "skewed hot", telling the
ECU that the engine is warmed up when, in fact, it is not. The ECU in
these cases is commanding a too-lean mixture.
The sensor can be checked with an ohm meter. Of course, as the
temperature increases, you are looking for a smooth transistion. You may
find a lazy sensor giving skewed readings or one that is just flat dead and
gives a fixed reading.
Here's more info on testing:
http://www.jag-lovers.org/xjlovers/xjfaq/testcts.htm
Here's
our local copy
Don't forget to check the connector. A disconnected temp sensor will
result in a "full rich" command. A dirty and/or loose
connection will do nearly the same.
These sensors are inexpensive ($25 or so) and easy to replace (10 minutes) so
diagnosis by substitution is not too hateful in this case. Many owners
carry a spare.
On the 4.2 engine this sensor is on the water rail in the #3 position from the
front.
FUEL PRESSURE REGULATOR: This vacuum operated device, as the name
implies, controls fuel pressure according to engine demand. It can cause
serious mixture problems regardless of engine temperature.
The fuel pump is capable of producing 80-100 psi of fuel pressure but the
engine is designed to run with much less....usually no more than 36
psi. Excessive pressure literally pushes extra fuel into the engine when
the injectors open. A fuel pressure test will verify that the regulator
is doing the job as intended.
It is also possible for the diaphram inside the regulator to rupture. In
this case raw fuel will be drawn through the vacuum hose and deposited directly
into the inlet manifold.
The regulator is easily recognzable. It is a small cannister-type device,
mounted on the fuel rail, has a vacuum tube and a fuel hose.
Going hand-in-hand with the pressure regulator is the fuel temperature
switch. These were not used on the early cars so you may not have
one. Some early cars were retro-fitted, however, as part of a "hot
start kit".
The idea of this sensor is to sense fuel temperature and, if too hot
(170ºF or so, I think), allows full fuel pressure in the rail to alleviate
vapor lock . Were this switch to fail, or for any other reason the
regulator was cut off from the vacuum supply, full fuel pressure would be
present at all times and an over-rich mixture would result. You can
eliminate this switch as a possible problem by simply bypassing it with vacuum
hose.
OXYGEN SENSOR: This device monitors oxygen content in the exhaust and
sends it's readings to the ECU to trim the fuel mixture. Information on
testing the sensor can be found here:
http://www.jag-lovers.org/xjlovers/xjfaq/o2.htm
This is
our local copy
FUEL INJECTORS: The fuel injectors are designed to spray a precisely
metered amount of fuel into the combustion chambers. With age and
mileage, however, the internal seals degrade and may result in a drip-drip-drip
of unmetered fuel into the engine. The only way to check this is to
actually remove the injectors, pressurize the fuel line, and simply see if they
drip fuel. You can check them individually or collectively, if you remove
them with the fuel rail as an assembly. The same principle applies to the
cold start injector.
COLD START CIRCUIT: Aside from the possibilty of a dripping cold start
injector this circuit is unlikely to cause a rich-running
problem. Contrary to common belief, the circuit is operational *only*
when the starter motor is engaged, as it gets voltage directly from the starter
circuit. Unless the wiring is really fouled up in a most peculiar way,
the circuit is deactivated as soon as the key is released from the
"start" position. So, the cold start system may well cause a
starting problem, but it's unlikely to create a *running* problem.
FUEL CHANGEOVER SYSTEM: If the fuel changeover malfunctions there is a
possibilty that one tank may become overfull. If this happens, excess
fuel can be drawn into the fuel vapor cannister (in front of the right front
tire) and, from there, is drawn by engine vacuum directly into the
engine. A fuel drip in the area of the front wheel is an obvious
indicator. For diagnosis, the vacuum line from the cannister to the air
inlet tract can be removed and checked for signs of raw fuel. Checking
the changeover system is covered here:
http://www.jag-lovers.org/xjlovers/xjfaq/fuelchangeover.htm
This is our
local copy
AIR FLOW METER: The "AFM" is the principal player in mixture
control. It has an air intake flap which, as it opens further and
further, mechanically controls a resistance "wiper" which, in turn,
controls injector pulse width (the amount of time the injectors remain open)
via the ECU. The AFM can be responsible for a variety of problems but is
not high on the list of likely culprits in diagnosing a rich-running
problem. For most DIY types, diagnosis by substitution is the most common
plan of action.
ELECTRONIC CONTROL UNIT: The "ECU" is the brain of the fuel
injection system. Taking information from the sensors and AFM, it
controls mixture by varying the injector pulse. These are remarkably
reliable and replacement should only be considered when other causes are ruled
out. Diagnosis of a faulty ECU is generally by substitution. If the
ECU is suspect, first carfully remove the connector, inspect the contact pins,
and re-seat the connector a couple times. This will often clear the pins
of any corrosion which may have developed.