I left mine off today and was surprised when it didn't set a code or turn on the mil I thought it was supposed to do that.Could it be that my evap system hasn't completed in two months.Got this idea from the 2005 Toyota's they have a vacuum pump that fills the gas tank with vacuum and the evap runs its self test.Because of it and it runs when the car is off as well as when its cruising low rpm's automatically with out any drive cycle.So I crafted a gas cap with a 7/16 copper pipe going through the cap and it is air tight so all I have to do is energize the evap vent or disconnect it all together and cap off the connection at the canister were it hooks up just long enough to run a 0.5 vacuum into the gas tank to see if it will trick the evap into completing and setting codes or just staying complete we will see how the test works because the drive cycle sure don't. For me anyways.

 

Looking at it laying under the car feet towards the engine there are three connects the lower left is the tank I believe the 5/16 or 3/8 connect in the middle is the purge valve that leaves the upper right to be the vent valve and if that's the case that would be the what is it 11/32? or so the one I want to cap off long enough to fill the gas tank with 0.5 vacuum to test to see if it will turn on the purge valve and kick start the evap system.That is how it works in 2005 Toyota's.And I made a special gas cap with a 7/16 copper pipe sealed and going through it to use to supply the vacuum to the gas tank.

 

This one is for you and all those blazer lovers out there.As you know since doing my lower intake manifold gaskets and the battery and alternator the week end after Thanksgiving I haven't been able to get the evap cycle to reset or complete I have done 15 or more drive cycles with every thing off and with wipers htr, radio and four way flashers on because of weather just to let the drivers know that I was going to be slowing to 20 mph then accelerating to 55.mph 5 minutes then doing it again just as the drive cycle is printed on line under gm drive cycle.But still not able to get the evap to complete.So finally in desperation I contacted the shop supervisor at a local gm dealership.I told him of my issue and he said call back on Monday and he would let me talk to his best drive cycle tech.So I called back on Monday talked to Randy and here is what he had to say.Since I pulled the battery and the alternator with a confirmed or pending code that I didn't know about the evap cycle was stuck wouldn't run its complete cycle or set a code because it hadn't completed its cycle test and until it did it wouldn't set a code or complete its cycle it was stuck.He recommended his tech could upgrade my computer so it would recognize the issue and set the code then they would repair it for me.My thought was why don't I just test and replace any evap censor that is defective then the computer could recognize the problem and set a code and I myself would repair it and he said yes that would work too.there wouldn't be any reason to upgrade the computer if I replaced the bad censor and there is quite a few evap test for the DIY er on line to help out.

 

I think I found all, (8) of your threads that relate to the EVAP issue on your vehicle. I have merged them together to keep it all in one "pile" for less confusion. In the future, if you have EVAP questions about your vehicle, please post them in this thread, rather than start a new one Thanks


The dealer told you, (in more or less words) the same thing we have been telling you all along: The EVAP monitor must run, and complete, all of its tests before it will pass or fail the EVAP system. If there is a problem with a sensor while the monitor is running, the PCM will detect it, illuminate the check engine light, and set a DTC in memory. If the monitor "gets stuck", it's not the fault of the PCM. It means that not all of the steps have been duplicated. The PCM will continue, indefinitely, "watching" for the missing or incomplete steps. When they have all been duplicated, the monitor will complete its tests.

 

Sorry for starting another thread but thought when you hit page 3 that was it. So like I was saying I think you told me this once before but it's not me or my attempts at the drive cycle I have got that down pat still don't know if when doing the drive cycle every thing has to be off the only thing I am clear on is don't touch the brake during the drive cycle coast to 20 mph.But talking to the tech he said it didn't make any difference how many times I tried the drive cycle it wasn't going to work because something in the evap system wasn't working and that's why he offered to re write the program in the computer so it would recognize the problem even though the evap system was stuck.But I told him that I would test every thing in the evap system and he said that would work as well.Once the sensor or valve that isn't working is replaced the evap will run its test and complete so today I started with the evap purge canister tested it with a 9 volt battery and the valve clicked clearly and hooked it up to my vacuum pump and tested it for leaks none so that leaves my best guess the evap vent valve might be stuck or plunged either way it may be the problem although it could be the tank pressure valve or the canister maybe even a wire shorted to ground and last of all maybe a harness issue and last but hopefully not a computer problem itself only time and testing will tell but thank you for your suggestions and help with the issue.Oh yes hope this down the road can help another DIY er in there attempts to repair it their self.

 

Captain yesterday I pulled and tested the evap purge valve and it checked out as good.Today I tested the evap vent canister first back probed and tested for power key off had 12.4 and with the key on had 11.6 so thought valve was working so I took it out and hooked it to power to energize it and the solenoid clicked every time power was touched. Hooked up my vacuum pump pumped it up to 10 hg still energized and it wouldn't hold the vacuum tested the vacuum source and connection all good.So could this be the fault for my evap not pressurizing intern not completing the evap test.?

 

Just so we're all on the same page here with terminology


This is the EVAP Vent Solenoid:







This is the EVAP Purge Solenoid:





This is the EVAP canister:



The purge solenoid, and the vent solenoid, should both have battery voltage
only when the ignition is in the "RUN" position. The PCM applies PWM ground to
open the purge solenoid, and full ground to close the vent solenoid.


Vacuum in the EVAP system is measured in inches of water, not
inches of mercury. ~1" of mercury = 13.61" of water.
Make sure you use the correct gage, or you can easily destroy things
by applying too much vacuum to them. The diagnostic flow charts tell you
which gage to use for each test.

 

Ok Captain tested the purge valve on the intake it tested out fine held vacuum until energized then the vacuum drooped held my finger on the manifold port while adding 5hg of vacume Had 12.4 at the harness with ignition on..Then tested the vent solenoid at the harness had 12.4 with key on.Took it off the mount to jumper it to the battery and the solenoid clicked each time I touched the alligator clip to the battery supplied 5hg vacuum to the vent solenoid while energized and it wouldn't hold the vacuum and that is what it is supposed to do isn't it when the pcm commands it on it stops the flow of air that is continues when not energized.?Did I get it rite hope so would like to see the evap complete its test cycle so I can move on to another project.? What is PWM ground as compared to full ground.P?S thank you again for your help and answers I really do appreciate it.

 

From post #67:
"The purge solenoid, and the vent solenoid, should both have battery voltage
only when the ignition is in the "RUN" position. The PCM applies PWM ground to
open the purge solenoid, and full ground to close the vent solenoid."

Are you not reading my posts, or are you not understanding them?

 

Ok Captain yes I do read your post some times there complicated but I can always ask what you mean.If I don't under stand it by the way yes I do know what the different parts are sorry purge valve vent valve canister that I tested it showed to have a small leak the pcm didn't pick up on it.I tested the new one before installation and when energized it held 7hg of vacuum until I disconnected it.With the old one disconnected while I went to the parts store the pcm set a hard code. I bought a new vent canister tested it the same as old one it checked out as good put it in erased the pending and current code the same thing happened it reset the IM monitors so took another drive cycle on freeway it reset all but the evap monitor.So back to patience to see if its operating correctly or not what bugs me is the pcm didn't pick up on the leak in the vent valve but when I unplugged it and went to the parts store it noticed rite away.P?S Captain you can erase any or all of my post that carry on and on.It's fine with me unless there is something there that would be help full
to another Blazer owner I am just glad to get honest help from someone that knows how it works and will share it with a DIY er.Thank you very much.Oh by the way Pulse width Moderation is that how I was able to get the purge valve to engage with only a 9 volt battery I don't fully understand it but the purge did activate with the 9 volt battery when you said key on 12 volts PWM does that mean somehow the pcm reduces the voltage to 5 volts when the pcm supply's ground but the rest of the time key on it reads 12 volts and if you was to touch it with a light probe with the alligator clip connected to ground wouldn't the 12 volts pcm side drop to 0 or a real low reading.So tell me Captain am I learning something here or am I way out of my league.