p0440 99 blazer

[autojoe]

I know how to test ground side switched Solenoid and have a Smoke machine. Just curious why vacuum bled off so quickley on my 2008 Chevy trail blazer with Vent Solenoid closed and after a vacuum was obtained. Thanks Joe

[Bob's Garage]

Hi Joe,

This is general information, and may not apply specifically to your situations. Still, I offer it as food for thought.

Keep in mind, in even the tightest evap systems, vacuum will degrade or, should I say "pressure is added". This means that although vacuum cannot escape (in a tight evap system), pressure will still rise due to heat & volatility. This is especially true if the engine is running, and even more so if the car is driving.

On cars with a fuel pressure return hose, hot fuel is constantly spilled back into the tank. While driving fuel slosh creates vapor generation. Both scenerios raise tank pressure increaing loss of vacuum.

Now, assuming you have smoked the evap with vent shut, and you are still holding pressure, this doesn't always mean you can hold vacuum. This is usually due in GM products due to the fuel tank filler check valve. This little ping pong ball rises and seals the the tank tightly under pressure, so now the filler neck is isolated and not being checked. This is why I usually smoke from the front first and then from the gas cap surface after. Several aftermarket adapters do this well.

GM filler necks are always suspect due to poor plating and bad metallurgy. The rust swells enough to crack the plastic insert inside the filler neck causing P0442, P0440 in earlier, P0455, P0456 in later models. My first few filler necks led me astray until I realized they are not being checked when smoking from the front, because the little green ping pong ball in the tank isolates the filler neck so well.


Bob

[Bob's Garage]

1997 Jimmy Evap

I'll second that, real nice screen captures, Bob. Also I need to keep reminding myself that dealing in pressure/vacuum , manually applying one inch of Hg (mercury )vacuum by a hand vacuum pump is like 13 inches of H2O, so using a hand vacuum pump typically creates way more negative pressure than evap systems operate under.

As promised, here's the 1997 Jimmy after evap repairs were done. Someone went to the trouble of replacing the tank & fuel pump but botched the tank/pump o-ring actually ripped it. On top of that, they snapped the rollover/breather in the front of the tank, causing a second leak. Thankfully, Dorman markets that little valve. GM doesn't sell it seperate (they say).


Here's the Evap monitor from the 1997 Jimmy . Tank pressure volts were not available in this older PID list:

20140530-0745-1.jpg

In this second screenshot, I added more PIDs because I was curious about the "Evap Canister" PID. It was very active, but I could see no correlation with any other activities.

20140530-0746-1.jpg


I meant to investigate it further, but time ran out when the customer picked up the truck.

Crusty, what say you? You were probably at the GM dealer when these were new. What do you know about the Evap Canister PID? My best guess is that this PID is just a holdover from the non-enhanced Evap vehicles and means nothing.

My days at the GMC dealer were long before OBD2

Bob

[Crusty]

Bob, those are some good screen shots and I encourage other techs to build their own "known good" and "known bad" data captures. Every vehicle that comes in your bays is a gold mine of information, especially the ones that are working properly.

""Crusty, what say you? You were probably at the GM dealer when these were new. What do you know about the Evap Canister PID? My best guess is that this PID is just a holdover from the non-enhanced Evap vehicles and means nothing.""

As for the ON/OFF evap canister pid, personally I never paid any attention to it-!! LOL-!
That doesn't mean it's useless but I suspect you're correct about it being a hold over from previous years and from the activity I would think it may be related to the PURGE functions.

THANKS for thinking I'm so young....LOL...but I was out of the dealer network long before OBDII was implemented. I've just done A LOT (and then A LOT MORE) READING and studying to try to stay current.
One thing I have learned is, the more things change, the more they stay the same.... V = I R hasn't changed in the last hundred years-!!

[Crusty]

is there a way to check for a leak using purge and seal while monitoring the vacuum decay using the fuel tank pressure sensor.if I were to close the vent and command 30%purge at what point in the inh20 on ftp sensor do I shut off purge and can I measure vacuum decay for a leak diagnosis.I dont have the 99 blazer here right now but on my 2008 chevy tb 4.2 the vacuum decay after closing vent and commanding 20-30%purge then 0% purge the ftp sensor inh20 decreases at a steady rate.what am I doing wrong.it will kick me off at some level of vacuum.sorry if my questions are stupid.

You can activate the system, pull a vacuum (inH2O going down) and watch the Fuel Tank Pressure (Volts) go UP when both are available as I posted in the screen shot from my Solus-PRO.
It's pretty hard to see of find a vacuum leak unless you used an ultrasonic sound detector, but you can verify WHAT it is doing.
There are two things to be aware of.....first there is a predetermined maximum vacuum limit the ECM will allow you to pull down before it aborts in order to protect the system from caving in and collapsing. The actual amount varies by design and the manufacturers don't tell us but it's typically between 9 and 15 inH2O.
The second thing is, (NOTE what Bob says) the vacuum will bleed back up to atmospheric pressure for a variety of reasons, the ECM is monitoring how long the vacuum stays above a specified value once it reaches its' maximum vacuum capability, and again this varies by Year/Make/Model.

All the more reason to save the data from "known good" vehicles for comparison.
Once you have studied 10 or 12 GM vehicles of different years, study 6 or 8 Chrysler vehicles and you'll learn that they are virtually the same as the "Euro" makes (Daimler owned Chrysler for how long-??). Next study some Ford products, and then study some Asian makes......
Every single one of them is doing the SAME THING. They apply a vacuum or a pressure, monitor how long it takes to achieve a specified amount of vacuum or pressure, and then they monitor the decay rate of the vacuum or pressure back towards atmospheric pressure, and the engineers have already determined the acceptable rates of signal change depending on things such as ambient temperature, fuel tank level (more or less area of vapour in the tank), fuel volatility, fuel slosh, etc.

The BASIC laws of physics don't change just because a vehicle was designed in Europe, North America, or Asia.
Pascal's Law deals with hydraulic pressures.
Ohm's Law deals with electrical.
Ideal Gas Law deals with vapours.

The only differences in EVAP systems is how the vapours are tested and how the laws of physics are applied.

In this picture a friend of mine wanted to add an auxiliary fuel tank to his Dodge (VW evap system) pickup truck. This changed the amount of vapour area the EVAP system would measure. We designed the add-on and then tested the evap system in a very high tech way-!! ROFLMAO. IT WORKED-!!

[Bob's Garage]

2008 Tahoe Eonv

I know how to test ground side switched Solenoid and have a Smoke machine. Just curious why vacuum bled off so quickley on my 2008 Chevy trail blazer with Vent Solenoid closed and after a vacuum was obtained. Thanks Joe

Here's some screenshots of a similar GM product, a 2008 Tahoe EONV evap system.


Purge and seal. I pulled 10.5 inches H2O vacuum in the tank (about the max before the tests boots you out), and as tight as it is, it still shows vacuum decay:

20140501-1458-1.jpg


Key off data EONV finishing the very small leak test, note that as long as the vent was shut, pressure built in the tank:

20140502-1844-1.jpg

[Crusty]

June 29-2013
EVAP self test of EONV cycle.
7 to 11 including abcdef

This is a complete EONV self test cycle.
In movie 007.scm the engine is still running after 1-hr 37-min, then shut down and the key removed.
The ECM stays awake while it self tests the EONV where it looks for a pressure build, then a cool down to a vacuum.

View these movies using ShopStream Connect

007.scm
008.scm
009.scm
00A.scm
00B.scm
00C.scm

following movies in next post.....

[Crusty]

Here's the follow on of the 25 minutes of EONV self testing.

View these movies in order

00D.scm
00E.scm
00F.scm
010.scm
011.scm

The vehicle is a 2009 (Canadian) Chev Uplander. It was cold started, driven for over an hour then shut down. The ECM stayed awake while it set the evap system to open to allow the pressure to stabalize at atmospheric pressure, then it closed and watched as the tank pressure increased. Once it was satisfied there was no pressure leak it then self tested for a cool down into a vacuum.
Once it was satisfied there was no vacuum leak the ECM finally shut down completely.
This data in these two posts is pretty typical of GM EONV (Engine Off Natural Vacuum) testing from about 2006 model year to at least 2011 model year.
There will be variables according to each year and model but this is what the ECM is looking for.

[Bob's Garage]

One More GM EONV Example

2008 Tahoe complete evap monitor including EONV:

20140603-2125-1.jpg

One of the advantages of a Verus, getting the whole thing in one capture, 14,000 frames, 45 minutes:

20140603-2150-1.jpg


Movie of same event:

2008 Tahoe evap monitor EONV.zip

Thanks for your interest.

Bob