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E85 / E86 Z4 (2003-2008)
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  #1  
Old 01-29-2015, 04:11 PM
John 682 John 682 is offline
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check engine light

Any body have the check engine light go on because of the gas cap. Mine came on had it checked out said faulty gas cap.
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Old 01-29-2015, 07:12 PM
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dc_wright dc_wright is offline
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It's not an unusual fault. All cars from the late '90s on have a built in test system for the evaporative control system. There's a small pump that pressurizes the gas tank and evap control components to check for leaks. If the pressure doesn't hold, it sets the CEL. A worn gas cap gasket can cause the leakage and is usually the first thing you try for this fault because it's quick and cheap to try.
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Old 01-29-2015, 11:40 PM
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I believed our cars have a separate light for the gas cap. I distinctly remember a gas cap light in the dash when I forgot to close it after filling up a couple of times...I'm an idiot, I know
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Old 01-30-2015, 04:12 AM
Mikey48 Mikey48 is offline
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Always make sure your gas cap is on good and tight.
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Old 01-31-2015, 06:05 AM
kalali kalali is offline
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Quote:
Originally Posted by dkl View Post
I believed our cars have a separate light for the gas cap. I distinctly remember a gas cap light in the dash ....
This ^^^^^^ . Its on the bottom edge of the gauge cluster, I believe next to the fuel gauge.

Last edited by kalali; 01-31-2015 at 06:07 AM.
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Old 01-31-2015, 11:49 AM
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dc_wright dc_wright is offline
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Quote:
Originally Posted by dkl View Post
I believed our cars have a separate light for the gas cap. I distinctly remember a gas cap light in the dash when I forgot to close it after filling up a couple of times...I'm an idiot, I know
This comes on when the gas cap isn't in place. If the gas cap isn't in place the DME knows the evap test has failed because of that and lights the check gas cap light. When it's in place and not holding pressure you get a CEL due to evap system test failure.
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Old 01-31-2015, 01:42 PM
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Originally Posted by dc_wright View Post
This comes on when the gas cap isn't in place. If the gas cap isn't in place the DME knows the evap test has failed because of that and lights the check gas cap light. When it's in place and not holding pressure you get a CEL due to evap system test failure.
How would the system know if the evap test failure was due to no gas cap versus gas cap not holding pressure? Both will and can fail the test in the same fashion. There isn't a sensor to indicate if the gas cap is installed or not.
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Old 01-31-2015, 03:32 PM
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dc_wright dc_wright is offline
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Originally Posted by dkl View Post
How would the system know if the evap test failure was due to no gas cap versus gas cap not holding pressure? Both will and can fail the test in the same fashion. There isn't a sensor to indicate if the gas cap is installed or not.
I believe if there's a gross leak you get the gas cap warning and if there's a fine leak you'll get a CEL with a P1434 fault code. Then the fun starts because there are no diagnostics below that level so anyplace in the evaporative control system can be the fault. The gas cap, the DMTL pump, or any of the hoses. The gas cap is $25-$30 and takes about 60 seconds to change out. The DMTL pump costs $75 if you buy Bosch or $130 if you buy BMW OEM. The pump is located under the center rear of the car in front of the fuel tank. If you search on fault code P1434 you'll find dozens of posts where replacing the gas cap and clearing the code has fixed the issue, including on my Z4. Which do you want to try first, the gas cap or the DMTL pump?
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Old 02-01-2015, 07:33 AM
Fastatv Fastatv is offline
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Originally Posted by John 682 View Post
Any body have the check engine light go on because of the gas cap. Mine came on had it checked out said faulty gas cap.
Interesting. I didn't know there could be a fault issued for a faulty cap. I had a gas cap warning once, but I believe it indicated something about the vent. I had overfilled the tank, After a little driving and using a little fuel, the light/indicator extinguished and has never been on since. But yours sounds like a different problem. Hopefully it will be a quick and inexpensive fix. Good luck.
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