The GAS CAP code!!!!!!!!!!!!!!

[GMMerlin]

The dreaded GAS CAP code!!!

I see many posts here about how to fix ( the dreaded GAS CAP) EVAP codes. Today I am going to educate you as to the functions and diagnostics involved in Evaporative Emissions issues and what the codes mean.
Lets start off with the whys.
The purpose of the Evaporative Emission system is to limit the amount of Hyrdocarbons released into the atsmosphere.
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 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 ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid OFF, fresh air is drawn through the vent solenoid 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 into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.
So in a nut shell if the control module senses a leak in the system that will allow more than a prescribed amount of fuel vapors escape, the module will illuminate the yellow engine light also know as the Check Engine (CEL)or Service Engine Soon (SES) light.
Depending on the year, make and model of your vehicle, this leak could be as small as .020” or twenty thousands of an inch ( a hole equal the point of a sharpened pencil lead)

Now with any diagnosis, the first thing we what to do after verifying the concern is to check the codes. This can be done at just about any chain auto parts store or if you own a code scanner, you can do it yourself.
DO NOT CLEAR THE CODE!!!!!!!!!!!!!!!!!!!! This is your key to finding out what is going on……we will clear the code after we repair the vehicle.

EVAP codes are considered either TYPE A or TYPE B codes..which in simple terms means the control module must see 1 failure for a TYPE A or 2 or more back to back failures for a TYPE B before the SES or CEL light is turned on.
Lets take a look at the codes and what they mean
P0440 or a P0455 – This is a Large leak indication or as people call it “The GAS CAP code”
This is a TYPE B code. Usually the code is caused by the gas cap being left off or loose after a refuel. This code could also be caused by any failure of the EVAP system that will cause a large leak.
P0442 – This is a Small leak indication.
This is a TYPE A code. This indicates a leak as small as .020”.
This code needs some serious diagnosis and knowledge of the system to find and repair the leak. Usually specialized equipment will be used to identify the leak.

P0446 – This indicates there is a restriction or blockage of the Vent Solenoid.
This is a TYPE B code. Any thing that prevents the venting of the EVAP system will set this code…usually dust and debris in the Vent solenoid or a damaged vent hose will cause this.

The other EVAP codes (P0443,449 etc) are electrical codes related to various components of the EVAP system.
Electrical codes need some good diagnostic skills, knowledge of the system and some specialized equipment.

With any car repair concern, there is a logical flow to follow when trying to identify what is going on..here in the GM world, we call it Strategy Based Diagnostics or 7 simple steps to diagnose a concern.
As always, verifying the concern and then doing a thorough visual inspection is key. It’s amazing how quickly a problem can be fixed by just the results of a visual inspection.
Now getting back to EVAP concerns….once you have identified the problem, you must come up with a repair, then a verification that the vehicle is repaired properly.
With EVAP leaks, usually a large leak is caused by a loose gas cap. Reinstalling the cap and clearing the code will be a quick remedy. But remember that you must always verify your repair..this could include professional intervention…(EEK!!!!!!!!!PAYING SOMEONE TO WORK ON MY CAR…..NO WAY!) . Well yes, this may be necessary..I have seen many times where people have come in to have a EVAP code cleared and refuse diagnosis only to have the light come back on…then blame me for the light coming back on…(then post on AF how the “stealership” did not fix their car).
But this isn’t a rant, this is information on what is going on.
Repair verification could include EVAP system testing with specialized equipment to verify that there are no leaks present..I personally check every EVAP system for leaks even if I find the customer left the gas cap loose…this insures that there are no other problems with the system that would cause the customer a trip back to see me. Far too many times have I seen someone tighten a gas cap and clear a code only to have the vehicle come back a few days later with a “real” EVAP leak…masked by a loose gas cap.
If you have a SES or CEL light caused by an EVAP concern and you have the code cleared and it comes back on after a few days….get to your local repair shop ( I recommend your local dealer with factory trained technicians) and have the system tested and repaired properly.
If you want to DIY the repairs, feel free, there are plenty of resources on the internet, but just don’t jump in and replace parts…educate yourself on the system and perform some diagnosis…many times a part will not fix the concern…
Good Luck
GMM

[oi8abug]

GMMerlin vbmenu_register("postmenu_4358002", true);
I know stuff


Yes you do my friend, Yes you do.... Keep on doin the

[kenwood guy]

what "specialized equipment" do you use to help with your diagnosis of the P0442 code????

[GMMerlin]

Originally Posted by kenwood guy

what "specialized equipment" do you use to help with your diagnosis of the P0442 code????

I use an EEST (Evaporative Emissions System Tester ) which has a flow meter and can interject smoke into the system to identify leak sources

[kenwood guy]

Originally Posted by GMMerlin

I use an EEST (Evaporative Emissions System Tester ) which has a flow meter and can interject smoke into the system to identify leak sources

yea I have that code on my truck now...... is there any possable way to fix this without such equipment maybe even make shift equipment of some sort???

[GMMerlin]

The problem with the P0442 code is it can be set by a leak as small as .020 on your truck.
If you find a better way let us know

[whalerboy]

Your the best Merlin!!

I, like many unfourtunate souls, have the dreaded "gas cap code" issue.
For me, it's a P0440, which from your post, tells me I have a gross leak, which makes me happy...it should be easier to find that way......

At this time, I'd like to point out that I suffer from ASEphobia, that is an irrational fear of somebody else working on my truck. I often sit in the waiting room, reading that issue of sports illustrated from 1986 (the one with Icky Woods on the cover), and wondering what the hell is going on in there.

Several bad experiances with professional mechanics in my youth have damaged me for life!!! (a few long stories I won't bore you with)

My question is this, what can the average guy do to seek out the gross leak? I've gone through the whole system, and checked for obvious cracks, loose hoses, and such, but have obviously found nothing. (already put a new gas cap on).

My sending unit is currently on the fritz. My gas gauge goes back and fourth often like an old analog multimeter, could that have something to do with it?

can a bad air filter can cause a P0440 (internet myth?)

Thank you O' wise master of automobile (yeah, I'm sucking up...but I need help!!)

[GMMerlin]

Your the best Merlin!!

I, like many unfourtunate souls, have the dreaded "gas cap code" issue.
For me, it's a P0440, which from your post, tells me I have a gross leak, which makes me happy...it should be easier to find that way......

At this time, I'd like to point out that I suffer from ASEphobia, that is an irrational fear of somebody else working on my truck. I often sit in the waiting room, reading that issue of sports illustrated from 1986 (the one with Icky Woods on the cover), and wondering what the hell is going on in there.

Several bad experiances with professional mechanics in my youth have damaged me for life!!! (a few long stories I won't bore you with)

You don’t suffer from ASEphobia, you just took your vehicle to a mechanic instead of a technician

My question is this, what can the average guy do to seek out the gross leak? I've gone through the whole system, and checked for obvious cracks, loose hoses, and such, but have obviously found nothing. (already put a new gas cap on).

There could be many causes of the leak, I would look at the tank seal for the sender, vent solenoid to see if it is closing (may be operating electrically, but not mechanically)
Do you have any gas leaks around the tank area?
Without specialized equipment such as the EEST I use, finding leaks on the system is difficult…no impossible, just difficult.
Most of the time it is knowing where to look and what you are looking for.

My sending unit is currently on the fritz. My gas gauge goes back and fourth often like an old analog multimeter, could that have something to do with it?

The PCM does use the level sender for tank volume so it knows when to run the test, it uses the FTPS to determine if there is a leak.

can a bad air filter can cause a P0440 (internet myth?)

NO! That’s what I love about the net…someone will post something that someone else will believe…who knows, I could be a 12 year old hiding behind a computer, giving car advice

Thank you O' wise master of automobile (yeah, I'm sucking up...but I need help!!)

No sucking up needed…usually cash does the trick, usually in large unmarked bills..I’m partial to Ben Franklin myself

[GOINGLIVESTANDBY]

Does anyone know the best way to fix a p0446 code? its the evap code circuit malfunction code.

[GMMerlin]

Originally Posted by GOINGLIVESTANDBY

Does anyone know the best way to fix a p0446 code? its the evap code circuit malfunction code.

[quote][P0446 – This indicates there is a restriction or blockage of the Vent Solenoid.
This is a TYPE B code. Any thing that prevents the venting of the EVAP system will set this code…usually dust and debris in the Vent solenoid or a damaged vent hose will cause this.
/QUOTE]

GMM

[White Lightening]

Greetings,

If I use the gas cap that came with the truck - everything is fine - no error codes. However I purchased a locking gas cap by the industry standard (Stant). It seems to fit/work perfectly - except about half the time - about half the tank to empty - I'll get a "loose gas cap" error on the DIC. Then regardless of what I do to tighten it more - later on (maybe an hour, a day or three days later) - I'll get the yellow "check engine light" going on. If I put the original cap back on - then the error light quits.

But it doesn't always do this - only maybe 50% of the time. The Stant cap only goes so tight then has a "clutch" effect so it can't be tightened more. This did this since brand new - and it hasn't changed over the last 2 years. Also - I notice I get better gas mileage with the locking cap on compared to the regular cap. As an example - in highway driving I get 23 with the locking cap - and maybe 20 or 21 at the most - with the factory cap.

Input and knowledge is desired

[oilman2]

I have a 2005 Avalanche. I forgot to put the gas cap on for my last tank. I got a computer message a few times, but it was raining so I was not inclined to go outside and tighten the cap. I then got the "check engine" light.

What I did notice, however, was the better gas milage. This is like the post before. I am very compulsive to reset my gas milage checker with every tank. That is the mode I leave it on all the time. With the current tank now down to 1/4 full, I have been getting 22.0mpg, which is the record best I have ever gotten. With regular use (i.e., gas cap on), I would have been getting about 18.3 mpg.

The question....is there any real harm in leaving the gas cap loose since I obviously am getting considerably better milage? I live in one of the big towns where all gasoline is diluted with 10% ethanol (that is another gas milage story....the day that started, milage went down from about 20.0 to 18.3).

I drive mostly lots of easy miles on the freeway...very little stop and go.

Car is still under warranty so I am not too concerned about getting code reset.

Thanks!

[GMMerlin]

Originally Posted by oilman2

I have a 2005 Avalanche. I forgot to put the gas cap on for my last tank. I got a computer message a few times, but it was raining so I was not inclined to go outside and tighten the cap. I then got the "check engine" light.

What I did notice, however, was the better gas milage. This is like the post before. I am very compulsive to reset my gas milage checker with every tank. That is the mode I leave it on all the time. With the current tank now down to 1/4 full, I have been getting 22.0mpg, which is the record best I have ever gotten. With regular use (i.e., gas cap on), I would have been getting about 18.3 mpg.

The question....is there any real harm in leaving the gas cap loose since I obviously am getting considerably better milage? I live in one of the big towns where all gasoline is diluted with 10% ethanol (that is another gas milage story....the day that started, milage went down from about 20.0 to 18.3).

I drive mostly lots of easy miles on the freeway...very little stop and go.

Car is still under warranty so I am not too concerned about getting code reset.

Thanks!

All gasoline sold in the US is 10% ethenol

[oilman2]

Originally Posted by GMMerlin

All gasoline sold in the US is 10% ethenol

Not true.....10% ethanol is used only in major metropolitan areas that do not attain certain air quality attainment goals. For example, in Texas, I believe this is only in the Houston and Ft. Worth/Dallas metropolitan areas. In the rest of the state, there is no ethanol dilution of gasoline.

As stated before, the day ethanol showed up at the pumps last spring (to replace the much better burning MTBE which was eliminated because the Dems in Congress would not allow the oil companies limited liability on spills - although MTBE was previously mandated by Congress in the first place), my car gas milage dropped about 6%.

[GMMerlin]

Originally Posted by oilman2

Not true.....10% ethanol is used only in major metropolitan areas that do not attain certain air quality attainment goals. For example, in Texas, I believe this is only in the Houston and Ft. Worth/Dallas metropolitan areas. In the rest of the state, there is no ethanol dilution of gasoline.

As stated before, the day ethanol showed up at the pumps last spring (to replace the much better burning MTBE which was eliminated because the Dems in Congress would not allow the oil companies limited liability on spills - although MTBE was previously mandated by Congress in the first place), my car gas milage dropped about 6%.

Dude, the last energy bill from congress banned the use of MTBE in the U.S.
Gasoline needed a replacement oxidation agent so the fuel refineries are using 10% ethanol in place of MTBE.