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.
P.S. Could this have happened because the car was not sufficiently
warmed up when the test was done?
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
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.
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.
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.
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
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.
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.
On Mon, 03 Mar 2008 13:28:46 -0800, William H. Bowen
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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