Fuel tank pressure vs. evap purge

[mikez71]

So I have no codes for evap, but evap monitor has not set.
I notice my fuel tank pressure is not smooth if purge valve is not either fully open or fully closed.
Tank pressure (yellow) fluctuates about .45V if purge valve (green) is not fully open/closed.

Starting FTP voltage is 1.50V.
One log showed 1.56V after evap vent opened at startup.
HPTuners has the vent position backwards..

(read that voltage should be 1.3-1.5V)

Normal pressure spikes? Or defective valve or clogged line/canister or?
I was thinking the PWM purge might have a restriction in the line.
Purge valves are cheap, so I ordered one. (Along with new gas cap and vent valve for now)

Not saying this is the reason monitor has not set, just looking for clues.

If anyone can log theirs (maybe @strutaeng ), or can explain what I should be seeing,
it would be much appreciated!
Have also considered a small leak could be messing with the cooldown fuel tank pressure reading..

 

[strutaeng]

Sure, I can log mine. Was planning on that anyways. Although not sure how useful my log will because I had a code for small leak something related I never could find the leak...had a code...

Question: How do you get those EVAP parameters to show on the graph?

My log from yesterday shows this? You can add anything to show on the graph?

 

[mikez71]

Thanks! Yes you can add any parameter, first you need to add the parameter to the list on the left window. I think its under fuel or fuel tank. Then you right click on the graph and go settings and click the +graph icon and select the parameter again from the list.

 

[strutaeng]

Thanks! I will try it tonight or tomorrow when I get a chance. These things are good to know. I've been wanting to learn more HP Tuners anyways...

 

[Fless]

Not sure if either of the videos in the link below will help, but give them a look-see:

CEL came on yesterday, 2006 ODB trouble codes

Okay that helps. I think if I have the radio on while engine is running... Then turn key to off everything should go off except the radio. If I open and close the door the radio will go off. ? Right. Yes.

www.tahoeyukonforum.com

 

[mikez71]

Thanks Fless! Both videos are very good! Watching South Main's second video with the smoke machine now!

My vacuum level only reaches 2.6V, where the Mike@Wells video reached 3V.
Although mine was driving, (less vacuum than idle?) and different tank size etc...
-BUT- my vacuum level also seems to drop much faster, even with the purge valve slightly open..
Hopefully a small leak in vent valve or gas cap since I've ordered those already..
(I do hear my solenoids click, but maybe the vent isn't fully sealed)

 

[strutaeng]

You can check the purge valves with a handheld vacuum pump. I can't remember how much inches of mercury (?) they need to held steady. And simply apply a 12 VDC current to the terminals to open the valve to test operation. I've tested a few of those like that in the past. Sometimes, they actually fail that test brand-new, out of the box!

 

[Fless]

You can check the purge valves with a handheld vacuum pump. I can't remember how much inches of mercury (?) they need to held steady. And simply apply a 12 VDC current to the terminals to open the valve to test operation. I've tested a few of those like that in the past. Sometimes, they actually fail that test brand-new, out of the box!

You can also just blow through either valve with lung air to test.

With the valves out of the vehicle circuit, the purge valve should open with 12vdc applied (polarity doesn't matter) but be fully closed with none applied. The vent valve is the opposite -- open with no voltage, but closed with 12vdc applied.

 

[mikez71]

I figure when I get my new parts I will test and compare.. Plus I'm too lazy to crawl under there twice!

Although looking at graphs, I can assume the purge valve opens, and it affects tank pressure.
I just don't know yet if the line is restricted or if it has a tiny leak..

Vent valve, the one log I have shows a miniscule change when vent opened at startup..
(1.50V to 1.56) Opposite what I would expect.. (tiny increase in vacuum with vent open?)
The logger says open or closed, not solenoid on or off.. Possible they have it backwards?

Parts should arrive next week!

 

[strutaeng]

You can also just blow through either valve with lung air to test.

With the valves out of the vehicle circuit, the purge valve should open with 12vdc applied (polarity doesn't matter) but be fully closed with none applied. The vent valve is the opposite -- open with no voltage, but closed with 12vdc applied.

Well, I guess that's easier than my method! LOL

I was curious so I had to look it up:

Google says the GM purge solenoids should hold 15-20 in. of mercury (inHg.) vacuum which is equivalent to 7.36-9.82 psi.

Google also says the human lungs can produce 2.1 to 2.8 psi of pressure (outward), 1.4 to 2.1 psi of negative pressure or suction...

I'm guessing this is a sea level? I didn't look into that...

 

[mikez71]

I feel like when I blow on a tire pressure gauge, the needle doesn't move at all!
And I don't feel like sucking on an evap port myself.. (even disconnected)
I've gotten old.. and weak!

I have a hand bulb that came with my smoke tester to check pressure leak..
Vacuum leak.. eh.. I'll pressurize the other side and call it good enough.. (My mityvac bit the dust, and I hated bleeding brakes with it..)

 

[strutaeng]

I feel like when I blow on a tire pressure gauge, the needle doesn't move at all!
And I don't feel like sucking on an evap port myself.. (even disconnected)
I've gotten old.. and weak!

I have a hand bulb that came with my smoke tester to check pressure leak..
Vacuum leak.. eh.. I'll pressurize the other side and call it good enough.. (My mityvac bit the dust, and I hated bleeding brakes with it..)

Are the GMT900 purge valves different?

I've swapped these between my other trucks (same generation) to do comparisons, since they are so easy to remove.

Yeah, my handheld vacuum pump is like that one. TBH, that's the only time I've found a use for it...SMH

 

[mikez71]

I haven't even looked at my GMT900 purge, but the 800 looks easy forsure.
The vent, I think I need to lay down under truck.. not sure if I need drop spare tire or not..

I would think a leak in the purge, might just not suck as ******* the tank.. Shouldn't cause pressure unevenness?
Maybe a slow reacting purge with the PWM causes the jumpiness in tank?
Tank pressure is steady with purge completely open or closed, but anywhere in between the pressure seems jumpy..

And still, that might not be the reason my evap isn't setting..

 

[mikez71]

..suck as 'strongly on' the tank?

Also noticed Tech2 has an EVAP test.. have not select it yet to see what it says/does..

 

[strutaeng]

The vent valve on the GMT800 2500 is at the very back, underneath and near the license plate portion of the bumper. That's where the auxiliary tank is, and I think the vent is connected to it? But maybe mounted the bumper? I can't remember now.

What was the "*******" word?

 

[mikez71]

Instead of 'strongly on', hard! and on!

 

[strutaeng]

...censored word...SMH...

 

[Scottydoggs]

if we're waiting for the evap to become "ready" that can literally take weeks, hundreds of miles and a few tanks of gas. pcm needs to see fuel level changes and many drive cycles to finally be ready. its always the dead last thing to become ready.

 

[mikez71]

The vent valve on the GMT800 2500 is at the very back, underneath and near the license plate portion of the bumper. That's where the auxiliary tank is, and I think the vent is connected to it?

Peeked under there, you are correct!
Should be as easy as possible.. Will datalog and see if any changes occur when I get it..

if we're waiting for the evap to become "ready" that can literally take weeks, hundreds of miles and a few tanks of gas. pcm needs to see fuel level changes and many drive cycles to finally be ready. its always the dead last thing to become ready.

Darnit! Tanks of gas will be like $400 for this thing! And I just don't drive very far usually..

 

[Fless]

Well, if you want the evap to be ready and to run, avoid filling the tank. Evap won't run if the tank is lower than 1/8th or higher than 7/8 full. Half a tank is safe.