Can anyone tell me what the “Check gas cap” message really means (other than the obvious)? What does the computer sense, pressure, vacuum or presences of the cap? I got the message twice recently and both times I had just turned on the air conditioner so could it be caused by temperature change? (gas expanding? fumes?). I know here in California, to prevent vapors from escaping to the atmosphere, they pull a vacuum on the gas cap during a smog check to see if it leaks. Is the XLR check simply an EPA mandate? I see in other posts replacing the cap has fixed the check cap problem however other posts indicate there might be other problems. I’d like to know how they relate to the gas cap or is the “check cap” the default computer message when nothing else fits?

The car checks fuel system pressure (since it is basically a closed loop) and determines if there is no leak (system normal), a small leak, or a large leak. Depending upon what the car "thinks", it will throw a code and possibly put a Check Fuel Cap on the DTC. The fuel pump on the XLR is variable speed and can adjust accordingly depending upon demand and/or pressure. The car is smart enough to adjust for or notice pressure changes in the gas tank from temperature changes.

The gas caps on the XLR's are notoriously bad. I've had 2 in the past 8 weeks. I get the Check Gas Cap message, then the Check Engine light illuminates shortly thereafter, since the gas cap won't tighten to specifications.

You should feel resistance when closing the cap BEFORE it "clicks". You'll know what a good one feels like when (or if) you have one.

Homegameroom is on target with his answer about feeling the resistance when STARTING to tighten the cap. Some of my clients have thought the cap is secured tightly, I have seen some of the older 04/05 caps can be pulled directly off.

Many vehicles that have had the cap replaced during the first 3 years, may have actually had a cap replaced with the same imperfections before it was actually upgraded.

www.Cadillac-XLR.com (http://www.Cadillac-XLR.com)

Thanks
Allen

Well I ordered a new gas cap this year around May I'm guessing I got the new upgraded one?

Evaporative Emission Control System Description
EVAP System Operation
The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid valve to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid valve ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve OFF, fresh air is drawn through the vent solenoid valve and the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

Large Leak Test
This tests for large leaks and restrictions to the purge path in the evaporative emission (EVAP) system. When the enabling criteria has been met, the control module commands the EVAP vent solenoid valve ON and the EVAP purge solenoid valve ON, allowing vacuum into the EVAP system. The control module monitors the fuel tank pressure (FTP) sensor voltage to verify that the system is able to reach a predetermined level of vacuum within a set amount of time.

Small Leak Test
The engine off natural vacuum (EONV) diagnostic is the small-leak detection diagnostic for the evaporative emission (EVAP) system. While previous leak detection methods were performed with the engine running, the EONV diagnostic monitors the EVAP system pressure or vacuum with the ignition OFF. Because of this, it may be normal for the control module to remain active for up to 40 minutes after the ignition is turned OFF. This is important to remember when performing a parasitic draw test on vehicles equipped with EONV.

The EONV utilizes the temperature changes in the fuel tank immediately following a drive cycle to use the naturally occurring vacuum or pressure in the fuel tank. When the vehicle is driven, the temperature rises in the tank. After the vehicle is parked, the temperature in the tank continues to rise for a period of time, then starts to drop. The EONV diagnostic relies on this temperature change and the corresponding pressure change in a sealed system, to determine if an EVAP system leak is present.

The EONV diagnostic is designed to detect leaks as small as 0.51 mm (0.020 in). The diagnostic can determine if a small leak is present based on vacuum or pressure readings in the EVAP system. When the system is sealed, a finite amount of pressure or vacuum will be observed. When a 0.51 mm (0.020 in) leak is present, often little or no pressure or vacuum is observed. If the test reports a failing value, DTC P0442 will set.

Canister Vent Restriction Test
If the evaporative emission (EVAP) vent system is restricted, fuel vapors will not be properly purged from the EVAP canister. The control module tests this by commanding the EVAP purge solenoid valve ON, commanding the EVAP vent solenoid valve OFF, and monitoring the fuel tank pressure (FTP) sensor for an increase in vacuum. If the vacuum increases more than a calibrated value, DTC P0446 will set.

Purge Solenoid Valve Leak Test
If the evaporative emission (EVAP) purge solenoid valve does not seal properly fuel vapors could enter the engine at an undesired time, causing driveability concerns. The control module tests for this by commanding the EVAP purge solenoid valve OFF and the vent solenoid valve ON, sealing the system, and monitors the fuel tank pressure (FTP) for an increase in vacuum. If the control module detects that the EVAP system vacuum increases above a calibrated value, DTC P0496 will set.

Check Gas Cap Message
The control module sends a class 2 message to the driver information center (DIC) illuminating the Check Gas Cap message when a malfunction in the evaporative emission (EVAP) system and a large leak test fails.

EVAP System Components
The evaporative emission (EVAP) system consists of the following components:

EVAP Canister
The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process.

EVAP Purge Solenoid Valve
The EVAP purge solenoid valve controls the flow of vapors from the EVAP system to the intake manifold. The purge solenoid valve opens when commanded ON by the control module. This normally closed valve is pulse width modulated (PWM) by the control module to precisely control the flow of fuel vapor to the engine. The valve will also be opened during some portions of the EVAP testing, allowing engine vacuum to enter the EVAP system.

EVAP Vent Solenoid Valve
The EVAP vent solenoid valve controls fresh airflow into the EVAP canister. The valve is normally open. The control module commands the valve ON, closing the valve during some EVAP tests, allowing the system to be tested for leaks.

Fuel Tank Pressure Sensor
The fuel tank pressure (FTP) sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The control module provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the control module that can vary between 0.1-4.9 volts. A high FTP sensor voltage indicates a low fuel tank pressure or vacuum. A low FTP sensor voltage indicates a high fuel tank pressure.

Bruce :D

Well I ordered a new gas cap this year around May I'm guessing I got the new upgraded one?

I had mine replaced in June this year and got a bad one. The new one I got last week (Thanks, Bruce, and Lockhart Cadillac!) seems to be great!:yesnod

Once again I am amazed with the wealth of information one can get from this forum. Bruce, thank you very much for the explanation of the EVAP system. That was just what I was hoping to find out. I can now see how important a well sealing gas cap is and I won’t think twice about cranking down on it after a fill up. Now for a couple of follow up questions: Do you know if the Large Leak Test is a continuously cycled test while driving…initiated at some fixed interval after startup…or by some other parameter change? I’m trying to understand why the check gas cap message occurred well over an hour into my drive and coincided with the air conditioner being turned on. Could it be just a coincidence? Also has anyone had their gas cap fail a smog test? Since there seems to be a question as to whether a replacement cap might also be faulty, would it make sense to have the cap checked prior to replacement? Would you recommend taking the car into the dealer to check the DTC code just to make sure the gas cap message isn’t a sign of some other pending failure?
I’ve only had the car for a little less than two months (CPO 2006) and I’m thinking of going in to meet my local Cadillac service rep. If the forum recommends that I have it checked out, I won’t feel too foolish if it turns out I just didn’t tighten the cap sufficiently.

Thanks again,
Jerry

Once again I am amazed with the wealth of information one can get from this forum. Bruce, thank you very much for the explanation of the EVAP system. That was just what I was hoping to find out. I can now see how important a well sealing gas cap is and I won’t think twice about cranking down on it after a fill up. Now for a couple of follow up questions: Do you know if the Large Leak Test is a continuously cycled test while driving…initiated at some fixed interval after startup…or by some other parameter change? I’m trying to understand why the check gas cap message occurred well over an hour into my drive and coincided with the air conditioner being turned on. Could it be just a coincidence? Also has anyone had their gas cap fail a smog test? Since there seems to be a question as to whether a replacement cap might also be faulty, would it make sense to have the cap checked prior to replacement? Would you recommend taking the car into the dealer to check the DTC code just to make sure the gas cap message isn’t a sign of some other pending failure?
I’ve only had the car for a little less than two months (CPO 2006) and I’m thinking of going in to meet my local Cadillac service rep. If the forum recommends that I have it checked out, I won’t feel too foolish if it turns out I just didn’t tighten the cap sufficiently.

Thanks again,
Jerry

Take her in and have her checked out. Might need updated cap....I believe all caps in GM stock are now updated...

Regards,

Elfred

This happend to me today I got the "Check Gas Cap" message in the DIC and then the "Check Engine" Light, my gas cap was loose. I reinstalled the gas cap correctly and it is secure. How do I reset the "Check Engine" light, will the car automatically reset itself or should I take it into the dealer to have it checked? Any help would be greatly appreciated and thanks in advance for your help.

This happend to me today I got the "Check Gas Cap" message in the DIC and then the "Check Engine" Light, my gas cap was loose. I reinstalled the gas cap correctly and it is secure. How do I reset the "Check Engine" light, will the car automatically reset itself or should I take it into the dealer to have it checked? Any help would be greatly appreciated and thanks in advance for your help.

This is a very common issue year round but becomes the #1 item consumers come to dealers when it starts to get cold out. Not always, but most of the time it is from an individual filling their gas tank with the engine still running. If the sensor checks the pressure in the fuel tank and the fuel cap is either not on, bad cap or just not tightened a code can be sent to the computer, which in turn will turn on the light.

If the light is on, and you have "NO" problem such as you had a loose cap or you filled up with the engine running, your engine light will go out within 24 starts of the vehicle, as it resets the system. If the light doesn't go out, there is another issue causing it to stay on.

If you have a 04/05 XLR we had some issues with loose non sealing caps. Talk to your dealer to have them look to see if it was previously replaced.

www.cadillac-xlr.com (http://www.cadillac-xlr.com)

Thanks
Allen

The Check Gas Cap message has come on a couple more times even after I made sure I cranked down on the cap after each fill up. Then the Check Engine light came on also so I thought I’d better take it to the dealer to make sure it wasn’t some other problem. When I got to the dealer the first thing the mechanic told me was the cap was loose:mad. They replaced the cap and the EVAP system checked out ok. Now with the new cap I know the old one was bad:yesnod. The old cap would open with less than an eighth of a turn with absolutely no resistance. The new one has a spring-loaded quarter-turn before it engages with the base then requires a little more force to snap it open.