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Loose Gas Cap Check Cap Comes up in the information center

silverstarrsilverstarr Member Posts: 1
edited April 2015 in Buick
I have a 2005 Buick LaSabre, V6 My information Panel Reads Loose Gas Cap, I replaced the gas cap 1st with an aftermarket cap, Had the code cleared, recently I am getting the same message so I went to the Buick Dealership and purchased an OEM Gas Cap and was told it should reset after 50 to 60 miles driven. I took my car to a repair shop explained the issue and they reset the computer, the message was cleared, after driving my car about 27 miles, I am getting the same message, loose gas cap, Check Cap, I was told the problem is with the emission system, anyone had this problem, or have any suggestions on where to start to resolve this issue?

Answers

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    imidazol97imidazol97 Member Posts: 27,202
    edited April 2015

    I have a 2005 Buick LaSabre, V6 My information Panel Reads Loose Gas Cap,

    I don't recall the various different code numbers to differential the symptoms, but the car puts up the gas cap message because it's the most likely cause. However, the gas cap didn't fix it. Be sure the rubber seal is against a clean area of the filler tube. Those can roughen from materials in the gas so clean the contact area with a rough cloth, say an old piece of terry bath towel or wash cloth. Apply vaseline. Then keep gas level between 3/4 and 1/4. Do short trip where engine never gets warm and is below say 150 when you stop. Then get back in and restart--say stopping for coffee on way to work close to home. The car only does the emissions check with several parameters between lower and upper limits. You want it to do two checks and it should reset the light turning it off if the car passes.

    If that doesn't fix it, the most likely cause is a defective vapor canister purge valve, it's on the front of the top of the engine, under the beauty cover. Beauty cover comes off by twisting on the stem of the oil filler below the cap--whole thing turns and screws out, letting the gray cover come off. Carefully check the vacuum tubing there from the purge valve to the engine connection. It may have deteriorated with heat and cracked.

    That purge valve sits in a very hot area. It's about $25 from Rockauto.com


    It is easily replaced if you're comfortable around the fittings. Just look carefully at the connectors to decide how they come off.

    If there are no leaks in tubing there and purge valve doesn't fix. Then you go to the vent valve under car under rear passenger's side. There are larger tubes from the gas tank to it which may leak. Or that valve might be bad, but not as likely as the hot, abused purge valve.\


    For the emissions test, the purge valve opens to apply vacuum to the tank to see if system is leaking air in. The vent valve is closed to seal off the tank vent through the carbon canister; it's usually open to let air in. Then the pressure sensor on fuel tank pump area measures the vacuum to see how fast it drops. Too fast, and it sets the appropriate code and after two failures, it sets the light.

    I had to seal up the leaking "U" on the rubber tube from purge valve to intake manifold over at the throttle body end. It had cracked. Later I replaced the vent valve, and that wasn't the problem. The purge valve apparently was staying open after applying the vacuum, at least partly, and upsetting the vacuum test.

    The shade tree mechanic who loves the 3800's GM has used for years, said he just replaced the purge valve first. If car has lots of miles, if valve hasn't gone bad it will go bad. And usually it fixes the problem other than check the soft rubber tubes to be sure they're not rotted.

    Sorry my answer is so long, but I went through this over a period of a year and half from first symptom.
    BTW, you can drive the car fine despite it's not having passed the test. It is not going to hurt anything from what I can tell.

    Good luck.


    2014 Malibu 2LT, 2015 Cruze 2LT,

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    thecardoc3thecardoc3 Member Posts: 5,768
    In a shop a technician can use the scan tool to command the vent valve to close and open several times and make sure that it can be heard to operate. Then he/she can start the engine and command the vent valve closed, the purge valve open pulling the system into a vacuum which can be read in the data on the scan tool. Once a certain level is reached, the purge valve can be commanded off sealing the tank and the canister and then the rate that the vacuum decays is an indication of just how big of a leak might be occurring. When the tech confirms that the system is leaking with this test, the next step is to repeat this test and isolate different sections of the system. The filler neck would be one of the first to isolate and if that stops the vacuum decay then the leak is in that direction. the tech would also isolate the vent valve and its hose. This routine is repeated until the source of the trouble is proven.

    One of the other tests that the system runs is the purge valve leaking test. By closing the purge valve, and then opening the vent valve the system will balance at atmospheric pressure. Then the vent valve is closed and the pressure sensor monitored for a change and if the system pulls into a vacuum, then it is proven that the purge valve is leaking and that sets a code P1441 in most cases.

    The "check gas cap message" is generated by the system when it see's a change in the fuel level in the tank which indicates a refueling event and that triggers an immediate large leak test to be run which based on the code setting is failing. A problem with the fuel level sending unit that isn't resulting in a trouble code could contribute to this issue.
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    imidazol97imidazol97 Member Posts: 27,202
    edited April 2015

    The "check gas cap message" is generated by the system when it see's a change in the fuel level in the tank which indicates a refueling event and that triggers an immediate large leak test to be run which based on the code setting is failing. A problem with the fuel level sending unit that isn't resulting in a trouble code could contribute to this issue.

    I'd never heard that the filling of the tank evokes a check on the emissions to see if there's a leak. I don't find that in the 03 manual for the leSabre which I assume is probably the same. The manual says it runs only if the
    tank is below 85%. So a full tank should run an emissions check until the level drops with use.

    It does require fuel level between 15% and 85% and requires engine temp roughly between
    40 deg F and 100 deg F. Also requires engine intake temp and coolant temp to be within
    9 deg F of each other. And some other requirements have to be met.

    It would be nice if I could have a Tech II or equivalent tool with which to turn those
    various devices on or off to do a logical elimination of parts. But the scanner I have can
    command the computer in the car to run the test. I have to hope it runs the test and
    eliminates or sets the code again.

    Interesting about being able to test the purge valve for less than full closure with
    the Tech II. I have one code set for blocked vent valve or something, so I condemned
    the vent valve and replaced it first. The shade tree mechanic next door replaced the
    purge valve first and sometimes said he just replaces both. Comes out a whole lot
    ahead of taking the car to the shops and paying a diagnostic fee AND then paying
    a markup price for the parts, often from an aftermarket supplier.

    I could have mentioned looking at the surfaces of the filler tube and the connectors
    for the tube to the tank. But those rarely have gone bad based on anecdotal reports
    from Bonneville and leSabre owners who are knowledgeable about doing or
    tracking their own repairs.

    For $25 and $20 for the purge and vent valves respectively, I took the replace
    parts route. My dealer charges $89 for connecting the Tech II and most
    shops in area, if they have one, charge about the same from what I can
    tell. A lot of shops don't use the computer and analyze based on experience
    and references about what parts are most often at fault, if any, for certain
    codes on certain cars. One of the code scanner companies advertises that
    their device uses the internet data in that way.

    I was able to put my car on wood blocks under the tires to raise it and
    support the car besides to work under to get at the back vent valve. If I weren't
    able to do that, I would have had my shade tree guy next door do the solenoids
    for me one at a time. Then if that didn't work, I would have taken it to a dealer.

    Neighbor has been worth his weight in gold, if not for helping, for loaning me
    high quality tools to use for my own hub replacement work, e.g..





    2014 Malibu 2LT, 2015 Cruze 2LT,

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    thecardoc3thecardoc3 Member Posts: 5,768
    edited April 2015
    What was shared here is just two facets of one system that goes way beyond what the average DIY'er ever gets to see or even has a clue about what is really going on. (FWIW There are eleven steps to GM's base evaporative monitor) You'd have to sit in a six hour class to really learn about GM's base evaporative emissions system, pre NVLD. (natural vapor leak detection) Learning NVLD would add another four hours of class room time provided you are already competent with the base system. If you really want to learn and understand evaporative emissions systems today then you would have to do even more class time for each manufacturer that you wanted to work with and that would get you a reasonable level of working knowledge for just the evaporative systems for a number of the cars on the street, but there would still be stuff you've never seen before with many others. From there we could start talking about all of the different scan tools that would allow you to really run all of the testing the way each one of those is designed to be serviced. It's safe to say that just learning all of the different evaporative systems would amount to more training (and more expense) than it takes to do most other career jobs, so don't be surprised that there are things that you never heard of before.

    Part of what you wrote above, especially about the fuel level limits actually refers to the requirements to run the evaporative monitor. The monitor enable criteria, which overlaps individual code enable criteria make sure that the tests are run under as controlled of conditions as can be achieved so that the engineers can get repeatable results. The loose cap test isn't part of the monitor so it doesn't need to meet all of the same criteria, but it does rely on previous results from when the monitor ran the last few times for the software to make a decision. Essentially the system see's the refueling event so it commands the large leak test to run, and in some cases they also run the medium leak test. Then the software compares this result to previous test results. If there were no leaks detected previously, but there is now a detectable leak the logic is that the cap might be loose. If there were previous results of a leak and the large leak test fails, then the system re-codes for a large leak and suspends the rest of the testing.

    This leads to a very important point, which is usually made by starting with a question. "What is a trouble code?"

    The answer is a trouble code is the identification number of a test that failed. Once you understand that, then you learn that when faced with a trouble code it is the responsibility of the person getting ready to do diagnostics to know (or find out) exactly how the computer ran the test that failed and then he/she needs to test the system or circuit themselves the exact same way that the computer did and they will solve the situation efficiently and the first time, without tossing any unnecessary parts.

    Lets take a look at P0440, which is often listed as Evaporative System Failure, or A Large Leak or The Weak Vacuum Test. This is the test that runs after a refueling event. This a straight forward test where the system commands the vent valve closed and the purge valve open. The system pulse width modulates the purge valve and depending on how fast the vacuum does or does not build the PCM will vary that command. If the vacuum is building slowly or maybe not at all then a longer pulse-width command should result in a stronger vacuum pull. The PCM monitors the fuel tank pressure sender and looks for the vacuum generated to reach -8" in water. If it cannot pull that much vacuum, the test fails. If it does pull that much vacuum, the weak vacuum test (large leak)passes and the system would then move onto the next test if the monitor is running.

    Something typically missed by those who have never really studied how this all works is the progressive nature of OBDII testing. Certain tests have to run and pass in order to allow other tests to run and it is all happening in a prescribed order. Techs always have to remember that when certain tests fail, they block other related tests from running. That's why its important to retest the system in the shop after a repair is made in order to make sure that another code that was blocked from running doesn't turn around and cause the MIL to come right back on.



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    imidazol97imidazol97 Member Posts: 27,202
    edited April 2015

    Part of what you wrote above, especially about the fuel level limits actually refers to the requirements to run the evaporative monitor. The monitor enable criteria, which overlaps individual code enable criteria make sure that the tests are run under as controlled of conditions as can be achieved so that the engineers can get repeatable results. The loose cap test isn't part of the monitor so it doesn't need to meet all of the same criteria, but it does rely on previous results from when the monitor ran the last few times for the software to make a decision. Essentially the system see's the refueling event so it commands the large leak test to run, and in some cases they also run the medium leak test. Then the software compares this result to previous test results. If there were no leaks detected previously, but there is now a detectable leak the logic is that the cap might be loose.

    I have reread the FSM description of the P0440 and other tests along with the summary of the operation of the emissions system in my 2003 manual. This car is 12 years old so it may not have the current level of sophistication of the tests like you are describing. If I have time I'll look at the 2014 FSM for the Malibu and see if it shows more intricate testing like you describe.

    Something typically missed by those who have never really studied how this all works is the progressive nature of OBDII testing. Certain tests have to run and pass in order to allow other tests to run and it is all happening in a prescribed order.

    The FSM lists the codes which cannot be already set in order for each code to be tested.

    The summary of the emissions in FSM leSabre 2003 states that the test message about the
    gas cap is turned on for a "malfunction in the EVAP system and a large leak fail" and for
    "malfunction in the EVAP system and a small leak fail"
    So I'm thinking the system on my 2003 is a level below running the test when it's filled up.
    The 2005 leSabre may be different.

    Having all the fancy equipment to test would be nice. I'll bet 99% of the independent garages here don't have that for most cars.



    2014 Malibu 2LT, 2015 Cruze 2LT,

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    thecardoc3thecardoc3 Member Posts: 5,768
    It definitely has the ability to run the large leak test after a refueling event to see if the cap is loose. BTW my 2002 Explorer had the gas cap light and the testing strategy was exactly the same, but the end result was different. The Explorer would set a pending P0457. It then took another trip to set a pending P0455, and then a third to mature that into a hard code and turn the check engine light.

    "I'll bet 99% of the independent garages here don't have that for most cars."

    So why don't they? Better yet if they haven't been in the habit of tooling up to that level and getting the kind of training that would have them able to explain the systems with as much detail as they really need to be able to, what is going to happen to them when the 2015's and 2016's are eight year old cars? (not to mention the 2010's and newer).

    Earlier you mentioned your shade tree guy, he's running a shop so call him one even if he is running it in an area that isn't zoned for a business, or paying taxes on the sales and income, or insured, or, or..... He's no different than a lot of the shops that are just going through the motions hoping to hang on long enough to make a living until they retire as the technology in the cars continues to overwhelm them. Why else would he need to just change both parts and hope that he actually fixed the car? A lot of shop managers are finally learning that competing with your neighbors shade tree shop has hurt them in regards to dealing with the newer cars. By not charging for diagnostics, they also didn't pay the people to perform diagnostics and that's how a lot of the shade tree guys came about. Many used to be techs who left the trade for other careers but still like working on cars and making money for doing so. But most of them are stuck exactly where they were at when they left the trade. They haven't been to training classes, they haven't spent the money to have the right tools and that leaves them trying to pick just the lowest hanging fruit along with a good number of shops who's business plans are evaporating right before their eye's.

    But what about the 1% ? The guys/gals who bought their TechII's, and DRB3's, Techstreams, IDS, HDS, etc? The ones that went to training and still do on a regular basis? Maybe if there wasn't so much of an attempt to claim that they were taking the wrong approach there might be more people who would have followed their lead.

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    imidazol97imidazol97 Member Posts: 27,202
    Because you know a lot more about intricacies of the emission systems, you might have experience with my Cobalt's P0411 code on 2.2 engine on 2008 sedan with 50K miles. Coldest weather in OSU Columbus and after sitting it turned on the check engine light. Then it would go off after couple of days with occasional start to drive to girl friend's apartment. Did this again another time. Got to dealer and of course light went off as they tried to check it--light was on when it got there to scheduled appointment. They did replace the secondary air pump motor because it was noisier than it should be. Car is under GMPP extended warranty.

    After week and half light came back on, in Columbus, and then went off. The light had been on and off a total of 5 times. Seems related to our 0 to 10 deg drawn out cold snaps. The valve contains a pressure sensor. Do you have an opinion about whether it's the valve with the built-in pressure sensor acting up? On internet someone even suggested a cracked head or exhaust manifold causing the problem due to injecting cold air at cold start.

    Dealer kept it 2 nights when he let it sit out in the cold to try to get some part to malefunction when he started it.

    2014 Malibu 2LT, 2015 Cruze 2LT,

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    thecardoc3thecardoc3 Member Posts: 5,768
    Check valve issues are common and in the cold like that ice formation can make the valve stick preventing airflow into the exhaust through the check valve and that causes the pressure inside the AIR system to be too high compared to barometric pressure. One of the toughest failures to figure out is a partial restriction of the catalytic convertor. The increased back pressure looks just like the check valve sticking and that makes it also fail the system test. One of the failures that wouldn't be obvious again start with the check valve leaking, that lets exhaust gasses pass into the line between the check valve and the motor assembly where the water vapor condenses and then freezes blocking flow.

    Keeping the car overnight and using the scan tool to take a snapshot of the test running is the right approach. The test runs in three stages for approximately 20 seconds in each phase. During phase 1, both the AIR pump and the solenoid valve are activated. Normal secondary air function occurs. Expected system pressure is 8-10 kPa above BARO. During phase 2, only the AIR pump is activated. The solenoid valve is closed. Pressure sensor performance and solenoid valve deactivation are tested. Expected system pressure is approximately 15-22 kPa above BARO. During phase 3, neither the AIR pump nor the solenoid valve is activated. AIR pump deactivation is tested. Expected system pressure equals BARO.

    Once it is proven which portion of the test is failing, then pinpoint testing is performed to prove why it failed. One note, a noisy air pump is associated to a failure of the control valve because of the corrosive nature of the exhaust gasses and the condensing water vapor that condenses in the feed pipe.
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