[modern] MAF & Other Stuff

Jerry wrote:

Am I doing something wrong? Please tell me how to open it.
Thanks, - -jerry

Jerry, from my experience you should not try to open and adjust
your MAF signal unless you have an exhaust gas analyzer
to set the proper carbon monoxide level at the exhaust manifold.

The Bosch Injection book by Probst (published by Bentley) discusses
setting the MAF idle signal in the “right ballpark” when the CO levels
stay at approximately the same value with the Oxygen sensor either
connected or disconnected. The Idle mixture adjustment is supposed
to be independent of the MAF load circuitry (ie the hot wire and its
associated circuitry). Our MAFs incorporate the idle mixture control
adjustment for convenience - it has to go somewhere. The output
should read about 1.4 volts and it is factory adjusted. It should not
need to be reset.

I have noticed several notes about trying to clean the MAF sensing
wires. I strongly recommend not doing this. The wires are about
one-tenth of human hair thickness. Under certain engine operating
conditions, the system will automatically “clean” them for you after
engine shutdown. Four seconds after ignition off, a current is applied
to heat the wire to about 1000 degrees centigrade. This will do a
better job than anything we might try, and it will not cause mechanical

Thinking about Brett’s problem - he seems to have a working O2
sensor that only puts out near-zero voltage (0.04V) at idle. If we
assume that this is due to a true excess of oxygen in the exhaust
at idle, then there might be several causes besides the MAF.

I don’t have an air pump on my 1989 model, but from what I understand
about the system:

  • it should deactivate (electrical clutch) when above 2500 rpm or above
    35 degrees C coolant temperature.
  • if it was not deactivating, it seems logical that at idle it could be adding
    enough fresh air to the exhaust to drive the actual O2 sensor value low.
    At higher speeds, the higher flow from the six cylinders would dominate
    and the O2 sensor would see a normal exhaust composition.
  • the lambda system (O2 sensor feedback) is deactivated when the air
    pump is operating. In open loop, idle can certainly be rough. At higher
    loads, even though O2 sensor output looks normal, it wouldn’t do any
    good if the system is running in open loop.
  • since this system is also temperature controlled, perhaps the ECU is
    also getting bad temperature values. Even though the engine
    should normally run pretty well in open loop, that would not be true if
    the ECU is providing Ignition advance and Injector duration signals
    that are based on the wrong temperature.
    (It bothers me that Brett reports he gets no change at idle when he
    disconnects the temperature sensor.)

Another thing to look at is the evaporative emissions control valve.
On older Bosch systems, this valve is mechanical and closed at idle
speeds (high manifold vacuum). It opens at higher engine speeds
and draws canister vapors into the intake manifold. On the Jaguar,
canister purging is controlled by a thermal/vacuum valve that should
not purge until coolant temperature is above 43 degrees C. I don’t
know what the air flow rates would be, but this could be a source of
un-metered, false air that would cause the engine to run lean at idle.

I don’t have any good test values for the coolant temperature sensor.

From experience, I would estimate that sensor resistance should be
about 2500 ohms when cold and about 200 to 400 ohms when hot.
My books, however, only list a voltage value of 2.06 volts at 30 degrees
centigrade on ECU pin LI 13-2. Since the other lead from the coolant
sensor goes to ground (ECU pin 12-6), I would expect this voltage to
rise as the coolant warms up and resistance drops, but I don’t know
what the fully warm engine voltage should be. Additionally, the sensor
can be fine, yet wiring damage could result in loss of signal. End-to-end
continuity checks might help here.

Stretching the imagination a bit further, I would consider a bad spark
plug. Despite apparent healthy spark from disconnected leads, a
bad plug might cause a misfire or no-fire on a cylinder. That cylinder
would probably appear (to the oxygen sensor) like a “giant air pump”,
putting lots of unburned oxygen into the exhaust. Removing the plugs
and cranking the engine with the plug grounded might provide a better
indication. Yellow or orange spark color is a weak spark.

Hang in there, Brett! George Kasten '89 VDP