Gas cap setting off check engine light - Chevy Colorado & GMC Canyon

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Old 02-10-2007, 09:24 PM   #1 (permalink)
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Default Gas cap setting off check engine light

Hey all;

I had my check engine light come on just prior to a planned 4 hour trip (as usual) and I took it into the stealership for a quick read. The manager tells me that an unsecured gas cap (causing tank pressure loss)is the most likely cause and he heads out to the truck, twists the cap and tells me to drive it for a couple of days to let the sensor reset. After the drive, and a couple of short trips, the light reset (went out), only to come on again a couple of days later (after another refill, and I twisted the out of it).

Could this be a faulty gas cap or a lazy manager? The truck ran on the trip without any problems, no strange noises or trouble. Has anybody else heard of a 'loose' gas cap being an issue? If so, are there better products (replacements) or there. If this is a bad gas cap, how long does it take for the light to reset itself.

Any help would be appreciated.
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Old 02-10-2007, 09:30 PM   #2 (permalink)
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The gas cap seal has probably gone bad. It's common. The emissions system pressurizes the fuel tank to make sure that raw fuel gasses are not released into the ozone. If it detects a loss of pressure in its cycle, the CEL to come on. No biggie, just get a new gas cap. Drive around with it and if it doesn't set the CEL on again, go to Autozone or similar and have them read and delete the DTC code(s).
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Old 02-11-2007, 12:53 AM   #3 (permalink)
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This man is right on it ^^^^^
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Old 02-11-2007, 01:01 AM   #4 (permalink)
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Purely FYI, the gas cap may or may not be the culprit.



EVAPORATIVE EMISSION (EVAP) 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 blockages in the evaporative emission (EVAP) system. The control module commands the EVAP vent solenoid valve ON and commands the EVAP purge solenoid valve ON, with the engine running, allowing engine 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. The control module then commands the EVAP purge solenoid valve OFF, sealing the system, and monitors the vacuum level for decay. If the control module does not detect that the predetermined vacuum level was achieved, or the vacuum decay rate is more than a calibrated level on 2 consecutive tests, DTC P0455 will set.

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 powertrain control module (PCM) sends a class 2 message to the driver information center (DIC) illuminating the Check Gas Cap message when any of the following occur:


A malfunction in the evaporative emission (EVAP) system and a large leak test fails
A malfunction in the EVAP system and a small 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.

EVAP Service Port
The EVAP service port is located in the EVAP purge pipe between the EVAP purge solenoid valve and the EVAP canister. The service port is identified by a green colored cap.
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Old 02-11-2007, 10:56 AM   #5 (permalink)
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I had to replace my cap also. Don't spend the money that the dealer wants though. I got a good one from Advanced Auto and paid a lot less. And it's very tight. The difference was amazing between the old one and the new one.
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Old 02-12-2007, 09:31 AM   #6 (permalink)
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Quote:
Originally Posted by Danny45
I had to replace my cap also. Don't spend the money that the dealer wants though. I got a good one from Advanced Auto and paid a lot less. And it's very tight. The difference was amazing between the old one and the new one.

Question though.. shouldn't this be covered under warrantee??
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Old 02-12-2007, 01:45 PM   #7 (permalink)
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I had the same thing happen to me. After the light comes on for the third time it will stay on until you take it to the dealer. i got a new cap at my local parts store for 8 bucks and took it to get the code reset, because it didnt shut off. well long story short it was a vent valve dealing with the gas tank. it was stuck open. 182$ later i was ready to roll. hope its your cap.
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Old 02-12-2007, 07:19 PM   #8 (permalink)
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Quote:
Originally Posted by dubeiram
Quote:
Originally Posted by Danny45
I had to replace my cap also. Don't spend the money that the dealer wants though. I got a good one from Advanced Auto and paid a lot less. And it's very tight. The difference was amazing between the old one and the new one.

Question though.. shouldn't this be covered under warrantee??

That would depend a lot on the dealer, and how long you've had your truck. A good dealer would eat the cost of the cap and do it. A poor one would consider it a normal wear item and won't.
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Old 04-04-2007, 04:02 AM   #9 (permalink)
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Err well I just got a P0455 code on my Jeep. I'll Try the cap and see if that solves the problem. It is the origonal and after 57k things tend to come loose.

I love this site...All i had to do was search the code # and poof here we are :D.
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Old 04-14-2007, 11:59 PM   #10 (permalink)
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I had the same thing happen to me with my S-10. The 'Check Engine' light came on and the mech at the dealership tightened my gas cap and problem was solved.
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