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Old 04-14-2002, 04:20 AM   #1
stanmo
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Check Engine Light - Fuel Cap??

Hi,

I know that if you don't close the fuel cap tight enough or until it clicks, it will trigger a check engine light. I was wondering how does it actually trigger the light? Is there like a sensor of some short? Or is it that the gas cap sticks out and actually touches that piece of metal when it closes? Or is it something to do with the pressure? Thank you!
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Old 04-14-2002, 05:01 AM   #2
Cecil
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Interesting... never asked myself this question...
Well try to seal it with something or someone standing there that way you'll see if it's a pressure sensor... if not... then it's a switch right?...

While we're at it - which was first the egg or the chicken?... jk
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Old 04-14-2002, 07:54 AM   #3
vernong
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vacum pressure is what makes it work,lack of pressure makes the light come on thats why the cap has to be on tight.
good luck
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Old 04-14-2002, 01:12 PM   #4
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Thanks very much!
It helped me a lot!
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Old 04-13-2011, 11:02 AM   #5
Many330i
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Old thread I know but if anyone can help me ...

Last year I had my fuel pump replaced (should have done it myself) and since, on two occassions where I have forgot to put back my gas cap, the gas cap light on the dash did not appear. I then tried it deliberately a third time and still got no 'gas cap light'. My question is what is wrong here? Is there a connection that might have been left out?

The thing is I bought the m3 fuel baffle kit that I intend to install this weekend (myself) but am worried if it will work since maybe the shop that did my fuel pump prior might have used an a/m or other generic fuel pump? Anyone who has done the fuel pump install or is familiar with the layout can shed some light? Thanks...
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Old 05-02-2011, 05:15 AM   #6
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I sometimes have problems with the Check Engine Light after i reful the car. My dealer has replaced the olicap and a lamdba. But it still turns one sometimes, and always soem time after I have refuled the car
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Old 05-02-2011, 07:15 AM   #7
shanneba
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The Fuel tank leakage test does not run every time you drive the car, at best it is every other time. It also does not run when the gas tank is full, it has to be between 1/4 and 3/4 full.

http://www.bmwmotorsports.org/BMW_docs/m54x5.pdf
from page 20:
LEAK DIAGNOSIS TEST PRECONDITIONS
The ECM only initiates a leak diagnosis test every second time the criteria are met. The criteria
is as follows:
Engine OFF with ignition switched OFF.
Engine Control Module still in active state or what is known as "follow up mode" (Main
Relay energized, control module and DME components online for extended period after
key off).
Prior to Engine/Ignition switch OFF condition, vehicle must have been driven for a minimum
of 20 minutes.
Prior to minimum 20 minute drive, the vehicle must have been OFF for a minimum of 5
hours.
Fuel Tank Capacity must be between 15 and 85% (safe approximation between 1/4 -
3/4 of a tank).

Ambient Air Temperature between -7OC & 35OC (20OF & 95OF )
Altitude < 2500m (8,202 feet).
Battery Voltage between 11.5 and 14.5 Volts
When these criteria are satisfied every second time, the ECM will start the Fuel System Leak
Diagnosis Test. The test will typically be carried out once a day i.e. once after driving to
work in the morning, when driving home in the evening, the criteria are once again met but
the test is not initiated. The following morning, the test will run again.
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