Possible Computer Problem with 2001 Cadillac DeVille- "Check Gas Cap."

Some months ago the "Check Gas Cap" message showed up. I bought a new cap, but it never went away after driving about 750 miles. By that time, the computer should have recognized the correction and the light
should have gone out *unless* either (a) the computer was faulty or (b) the new gas cap hadn't corrected the problem. I had a mechanic reset the computer (message disappeared.) I then drove the car about 250 miles, and the message did not return. Assuming everything was fine, I then went in for my annual emissions inspection and was informed that the computer " was not ready!" Since the computer was not in a "ready" state after 250 miles, I assumed there might be a computer problem. The station guy was unclear as to whether he had ever seen it take this long for the computer to return to a "ready" state, but I am suspicious, particularly since the computer had apparently not seemed to recognize the correction in the past. Can someone give me an idea of what may be going on here? I generally don't drive this car that much, but if you think this will eventually correct, I will have to drive it some more. In the meantime I have 60 days to get this all resolved as my car was rejected. Your advice please. Thanks in advnace for helping me. Frank P.S. Could this have happened because the car was not sufficiently warmed up when the test was done?
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How many times has the car been started in the 250 miles that you've driven it? The monitors require a specific number of engine cycles, besides the mileage requirement.
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-Mike-
snipped-for-privacy@alltel.net
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Thank Mike. It has been through quite a few cycles. I would bet it has gone through a dozen or so start-stop cycles. See addl comments.
On Mon, 3 Mar 2008 13:43:56 -0500, "Mike Marlow"

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frank1492 wrote:

The problem is NOT the gas cap. You probably have a small leak in the evap. system. The way the system works is thus:
When you start the vehicle and it hits a certain speed (most are between 10-15mph) the computer triggers a self test of the evaporative emissions system. The test starts by the computer closing a solenoid valve in the vapor line from the tank. Then it opens another solenoid valve on the intake manifold. The computer monitors a sensor in the tank and uses a baseline setting to measure how long it takes for a set amount of vacuum to build in the fuel tank. Then it closes that solenoid and measures how fast that level drops. If it is unable to draw the correct vacuum OR the vacuum drops to fast it will trigger the "gas cap light". It also sets a code in the computer that says "evap. system leak"
This test has a set list of items that stop the computer from running the test. If the gas tank is over 3/4 tank OR under 1/4 tank. If the ambient temperature and engine temperature are more than 15 degrees different. (Warm engine /cold day) Throttle position open more than 20%
The common problem I have found is the solenoid valve that vents the tank gets dirty and sticks. This stops the vacuum from building and sets off the light. Or the one on the engine doesn't seal properly and the vacuum drops to fast.
I have also seen rusted out vapor lines set the code as well.
A decent mechanic can hook a tester to the fuel fill neck and run a few tests and see where the problem is.
Now on the chance that the new cap actually did fix the problem you also need to perform a certain number of drive cycles to allow ALL the tests to run. If you drove it those 250 miles and only shut it off once or not at all you have a few more cycles to go. Start it and drive it 10 miles or so. Shut it off and let it cool down. (visit a friend or bar or store or mall) Start it up and drive home. Do this a couple more times and then on the day of the test take it on a LONGER drive and make sure it is fully warmed up before you take it in for the test.
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Steve W.
Near Cooperstown, New York
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Thank you Steve for your very complete reply explaining how the system works. I believe I have been through around a dozen or so start-stop cycles in two major trips. Before I have someone go after other possible sources of a leak in the evap system, I think it would make sense to pursue the possibility that it was the cap. Since the display is clear at the moment, I need to see if the warning comes back. But let's suppose I follow your advice and run the car a few more 5-10 mile cycles, and the warning doesn't come back. And then the station still tells me "it's still to early," what would you do then? Do I keep trying or have the car looked at? Remember I have 60 days. What is also a problem is that I much prefer to drive my Murano. The DeVille is supposed to be a "special occasion" car so i'd like to get this cleared up ASAP. Incidentally I called the station and asked them if they thought the car might have been insufficiently warmed up when I brought it in. (It was cold overnight, 5 minutes to the station, then 5-10 minutes waiting in the car with the engine running.) They said it wasn't likely as it was a fairly new car. Frank
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frank1492 wrote:

The warm up is just to make sure that the converter has reached operating temperature so you don't fail for excess hydrocarbons.
To find out if your OK you could do a few things. Go to your mechanic and have him plug in his scanner. It will show if all the monitor tests have run.
I believe that states that do full emissions testing will also run that scan to see if your ready to go, without actually testing.
OR go to your local parts store (or even Wal~Mart) and you can buy a scan tool that you can plug in to check it yourself. It will also allow you to read the codes that set the SEl or gas cap lights yourself and it would work on both of your vehicles. They run about 100-150 dollars for one that will give you the basics. BUT if you have one you can pull any new codes, read them and get advice here (and a few other places online as well) about what the problem likely is. Makes it nice to have a good idea what the problem is if you take it to a shop to have them repair it. It also allows you to check some items that wear over time so you could get them replaced before they cause problems. (such as seeing the voltage of the oxygen sensors or the voltage the charging system is putting out, plus a lot more). It also allows you to do some testing to pinpoint problems better.
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Steve W.
Near Cooperstown, New York
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It takes several DRIVE cycles to reset all the codes. This means the car has to cool down between cycles. Hot start and stop on a trip doesn't count. It has to be a cold to warm up cycle and driving for certain amount of time. Depending on your driving habits it could take a week or more. For an explanation do a google search on obd2 drive cycle or look at this article http://www.geocities.com/dtmcbride/home_garden/auto/check-engine.html#drive_cycle

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Frank,
You've run into one of the ugly secrets of OBD II (On Board Diagnostics Version 2) called "Monitors". The reason the PCM is showing "Not Ready" is one or more of the on-board tests the PCM needs to run to set various monitors has not been completed.
Driving the car under various operating conditions (stop and go, highway, etc.) will help this process along. Some of the monitors only set up during various operating conditions.
How did the mechanic reset the computer - by disconnecting the battery or did he use a scanner and reset the specific DTC from the gas cap? If at all possible with an OBDII car it is better to use a scanner and reset the specific DTC VS resetting the whole PCM, which also resets ALL the monitors to the "not set up" state.
Regards, Bill Bowen Sacramento, CA
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Hello Bill- Ugly is right! I am sick of this! Anyway, I will continue to try a combo of different things and keep a log, and try to be sure the engine is good and warm when I bring it in. I am going to have another chat with the mechanic *but* I do know he used a scanner. I can't be sure what keys he pressed but I believe the attempt was to simply reset the part of the system responsible for the "gas cap" message. Assuming the scanner was used properly, does that help narrow down what is going on? Here is what the print out showed when the car was initially rejected (not today when it was redone and I was told "still not ready."): I am in MA. Overall OBD- FAIL Malfunction Detected- NO OBD Connector- PASS OBD Communication- PASS Overall Readiness- NOT READY Engine Misfire- READY Fuel System- READY Comprehensive Component- READY Catalytic Converters- NOT READY Evaporative System- NOT READY Secondary Air Injection- NOT READY Oxygen Sensors- NOT READY Oxygen Sensor Heaters- NOT READY EGR System- NOT READY Thank you very much for your help. Frank
On Mon, 03 Mar 2008 13:28:46 -0800, William H. Bowen

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Frank,
All the monitors marked "Nt Ready" have not been set properly. When all of the monitors have been run and set up the "Overall Readiness" item will switch to "READY".
Just be patient.
Bill

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Individual DTC's cannot be reset with the tester. The only way to reset them is by resetting the computer. Some DTC's will be reset by the computer if the problem is intermittent but it cannot be done with the tester. As I said above the car has to go through a number of drive cycles to test the components and it could take a week or more of driving. The moral is if you are coming up on an inspection get the problems fixed early and don't reset the computer at the last minute.
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frank1492 wrote:

If the system is working properly there will not be any "whoosh"
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Steve W.
Near Cooperstown, New York
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