Our 320i runs fine but I have suspected for a while that the mixture is
a bit rich. The exhaust smells quite strongly of fuel (noticable with
the windows down when stopped), even at the end of a long run. Also the
consumption is quite high, I estimate around 20mpg (imperial) on a
mostly motorway journey, and the OBC agrees. It should be nearer 30 I
think. I've considered worn injectors or the fuel pressure regulator as
the suspects; are they possibilities? Anything else to consider? The
car's done 130k miles, and is a 1990 non-cat UK model.
As a side note, the car has the full OBC with the average consumption
function. How does it arrive at its reading, is it based on actual fuel
flow or just a function of engine load against distance? If it's the
latter then pressumably it would be safe to assume it's an ideal figure
rather than a measurement?
My humble guess is that ou have a failed O2 Sensor, and it's feeding more
gas than is needed because it always thinks the engine is cold.
When the O2 Sensor is cold, it feeds more gas, kinda like the choke circuit
in the olden days of carburators. As the O2 Sensor heats up, it sends a
Heated Up Now signal to the computer, and the computer then sets about to
lean the mixture. My guess is that your O2 Sensor has failed in a mode where
it refuses to send the Heated Up Now signal, hense the mixture remains rich
long after it should have leaned out.
Your car is OBD I compliant, this means you need a BMW technician to pull
codes to see if the O@ Sensor is reporting that it is slow to heat up. The
newer ('96 and later) cars are OBD II, and they give a code for this that
you can easily download yourself. Not much point in discussing OBD II with
you, you are stuck with OBD I -- as am I.
There is a way to pull codes on your car by holding your tongue just the
right angle and switching the ignition ON and mashing the gas pedal just
right in a very specific amount of time.
The most likely culprit is the coolant temperature sensor that is feeding
the computer. Another possibility is the coolant system thermostat. If the
engine is always cold or if the computer thinks that the engine is cold it
will feed too much fuel. A failed oxygen sensor can result in a somewhat
richer mixture but not to this extent. The oxygen sensor just does not have
the authority to enrich the mixture by this amount. On one occasion, my
friendly local mechanic destoyed the thread in my 318i exaust manifold while
he was attempting to replace my oxygen sensor. We had to put a plug in it.
I drove the car for a month without an oxygen sensor until I could find a
good time to replace the manifold. The effect on my gas mileage was very
minor - only 2 or 3 mpg not the 10 mpg that you are experiencing.
The OBC uses the length of the fuel injector pulse to determine how much
fuel is being injected and compute the mileage displayed. The fact that it
agrees with your poor mileage should rule out worn injectors and faulty fuel
pressure regulator because they would enrich the mixture without affecting
the injector pulse length.
I think the coolant temperature sender only operates the guage on the
instrument cluster. There is a thermoswitch next to it in the
thermostat housing which i think controls the cold start procedure in
the computer, which i think I verified as working a couple of months
ago, but will check again. As John noted, the car doesn't have an O2
sensor. Thanks for the OBC info, useful to know.
Further to the above, I have checked the readings from the thermo
switch before and after a long run. It read 65ohms when hot, and open
circuit when cold. I think this is the correct behaviour, can anyone
confirm? Just cleaned the connecter contacts for good measure.
I'm not familiar with your exact model of car Mark because I'm in the US
with a 318i but I do believe that the fuel injection computer has to have an
engine coolant temperature input in order to function properly. My 318i has
three engine temp transducers; one for the instrument cluster, the thermo
time switch to run the cold start valve, and one for the L-jetronic fuel
injection controller. I think your engine is similar. Here's a link to a
diagram of the three temperature sensors on the 320i thermostat housing.
another link for the L-Jetronic
I have a book on the Bosch controller and it lists these specs for the
7000-12000 ohms at 14degrees F (-10C)
2000- 3000 ohms at 68 degrees F (20C)
700-1000 ohms at 122 degrees F (50C)
270-400 ohms at 176 degrees F (80C)
Hi, My car doesn't have the sensor labelled 12 in your diagram, looks
like that was discontinued in 1984. I just have the coolant temp sensor
(for the guage) and the temperature switch for the cold start, which I
think is working correctly. Thanks.
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