Gas cap -- causing "Check Engine" - BMW Forum - BimmerWerkz.com
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#1 (permalink) Old 08-22-2004, 04:53 PM
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I'm one of many 540i owners currently troubleshooting the infamous "Check Engine" light fault.

ghead I noticed that my gas cap didn't "click" like it's supposed to when I tightened it. So, I suspected a vapor leak was triggering the fault. (BTW, the error codes reported inproper fuel mixture in the cylinders 1-8). I went to the dealership and purchased an $18 gas cap and installed. It clicked tight, and the "Check Engine" light disappeared immediately. I also had a full tank of gas at the time.

When my gas got down below half a tank, my light came on again. So, I figured I would re-check the gas cap. When I went to open it, it came off very easily, without any suction. With my other vehicles (non-bmws), you can usually hear the suction from the tank whenever you remove the gas cap when the gas is below half or quarter of a tank. But not with my 540. I'm able to easily remove it with only my thumb and index finder.Is it normal for it to come off that easy, without feeling any suction/vacuum?

I'm the second owner of this vehicle and I noticed some touch up paint work around the gas cap area. So, I suspect she may have pulled away from the pump at some point while the gas pump hose was still inserted into the car. I wonder if that could have damaged or bent something.
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#2 (permalink) Old 08-22-2004, 05:52 PM
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I forgot to mention that whenever the light comes on and I check the fault codes, I get the same two codes:

1C - "Mixture Control, off idle, Cyl #1-4"
1D - "Mixture Control, off idle, Cyl #5-8"

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#3 (permalink) Old 08-22-2004, 05:56 PM
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interesting. are you running premium gas ?

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#4 (permalink) Old 08-22-2004, 06:45 PM
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Yes, I always always use premium -- usually 93 octane.
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#5 (permalink) Old 08-22-2004, 06:46 PM
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Yes, I always always use premium -- usually 93 octane.
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#6 (permalink) Old 08-22-2004, 07:39 PM
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i dont have any other ideas... unfortunatelly.

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#7 (permalink) Old 08-23-2004, 12:00 AM
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The real issue is you can not just replace a part for each reported code.

Normal diagnostic requires a process.

So you may want to consider clearing the codes first and recheck them to make sure they have been reset. Many codes pop up on the peak tool are temporary and they may never return. In fact you may have experienced a check engine light for a day and sometimes you don't see it on the very next.

Then you will need to be aware of the fact that one issue such as "a lean condition on one cylinder" can produce a list of reported problems. A good example is my post on 5 serious thread titled "rough idling and 8 answers". As you will see most of these codes are related and somehow somewhat they tend to reproduce themselves. I am still in the process of diagnosis. I will post the final outcome though.
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#8 (permalink) Old 08-23-2004, 03:29 AM
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loose gas cap will not cause fuel mixture faults, it will only store evap system leak faults. The suction effect can happen intermittently but i wouldnt expect it to be there all the time. Sounds like you need to troubleshoot why the mixture is off, common cause on the v8's would be vacuum leaks, PCV plate failures, or Air Flow meter failures. Test for vacuum leaks first then ngo from there. good luck,

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#9 (permalink) Old 08-23-2004, 03:37 AM
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All gas tanks should breathe otherwise your fuel pump would create a vacuum inside your gas tank - gas being pulled out has to be replaced with something, if the tanks were totally airtight you'd have a problem. However, fumes can build up and create a bit of suction due to the fact that the breather on the tank is very small, but just big enough to do the job.
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#10 (permalink) Old 08-24-2004, 02:09 AM
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Very likely the Mass Airflow sensor. When I had my "service engine soon" light,
I went around in circles and beat around the bush trying to solve the problem. Everytime something was tried, light reset...all went well for a day or two. Then damn light came back on!
Usually, the gas cap is NOT the cause, though we all want to believe it is coz' it's the most simple and cheapest fix. I ended up replacing the oil separator, throttle body AND MAF sensor.
but again, there are so many possible causes.

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#11 (permalink) Old 09-02-2004, 08:39 PM
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Well, I took it to the dealer and offered my concern about the gas cap not having a vacuum when it is removed. They explained that it was normal, and that a suction or vacumm was not necessarily needed.

Upon inspection by the dealer, it was determined that I had a ruptured vent valve that was allowing small amounts of oil into the engine causing the engine to run "rich", with a "Mixture Control, off idle cyc 1-4, 5-8" fault code.

When asked, they said this was fairly common on the V8's.

It took a couple of hours and $451.00 to fix the problem. Well, now the Check Engine light isn't staring me in the face anymore. So, I'm happy.
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#12 (permalink) Old 09-03-2004, 05:03 PM
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d3ity will become famous soon enough d3ity will become famous soon enough
Vapor lock is usually the condition that causes that for me. Especially on my 6, especially in hot weather.

Check your tank, and fuel system, make sure no hoses are clogged, especially any sort of breather hose.

Interesting solution however.

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#13 (permalink) Old 09-03-2004, 11:07 PM
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dy540i, glad you got the problem fixed. I guess that's why these cars have OBDII.

d3ity, are you talking about a plugged gas tank vent tube? Vapor lock refers to a bubble of vapor in the fuel line and that just doesn't happen on fuel injected cars.
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#14 (permalink) Old 09-07-2004, 09:11 PM
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What the...! ghead

My Service Engine Soon (Check Engine) light came back on again, after only about 50 miles of driving since the dealer replaced the vent valve at $450.

So, it's not over yet!
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#15 (permalink) Old 09-07-2004, 10:42 PM
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Well, so much for diagnostics, I guess. Are there any other codes?

YOu'd think that might be an O2 sensor, but the ODII should report that failure, too.
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