Fuel tank pressure vs. evap purge

[Marky Dissod]

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.

Back when NYC mech shops that did emissions testing had to have dynamometers, what I kept reading in GM materials provided was 'under 3/4 and over 1/4'.

 

[TJ Baker]

This information "might" be useful ??

I/M EVAP System Set Procedure — 2005 Chevrolet Suburban 3/4 Ton 4WD V8-6.0L VIN U Service Manual | Operation CHARM

Detailed repair manual for the 2005 Chevrolet Suburban 3/4 Ton 4WD V8-6.0L VIN U.

charm.li

 

[Fless]

Back when NYC mech shops that did emissions testing had to have dynamometers, what I kept reading in GM materials provided was 'under 3/4 and over 1/4'.

Take a look at what @TJ Baker posted in the prior message. 15-85%.

 

[Marky Dissod]

Take a look at what @TJ Baker posted in the prior message.
15-85%.

In other messages in previous threads I've posted as little as 'between 3/8ths & 5/8ths'; I've been aware, I so stipulate, etc.

 

[TJ Baker]

Take a look at what @TJ Baker posted in the prior message. 15-85%.

This part here surprised me....

 

[Fless]

This part here surprised me....

View attachment 484769

That surprises me, too. When my evap problem (fuel pump gasket) was corrected, mine reset and cleared the CEL/MIL within two miles on the first drive, so I'm not sure how that works exactly.

 

[TJ Baker]

When my evap problem (fuel pump gasket) was corrected, mine reset and cleared the CEL/MIL within two miles on the first drive, so I'm not sure how that works exactly.

I'm not clear on it either. Does the I/M clear as soon as the offending DTC clears? Or does the I/M update some time later?

 

[Fless]

I'm not clear on it either. Does the I/M clear as soon as the offending DTC clears? Or does the I/M update some time later?

Good question. I was too elated to check what happens to the I/M, and thankfully haven't had the need since.

 

[Marky Dissod]

If the trouble code clears, readiness tests may begin, but it's not automatically ready the second after the code(s) is (are) cleared.

 

[afpj]

That cold start temp requirement kills me in the desert, as we can have a long stretch when it will not go below 86° at night. I can go a couple of months of driving everyday but never meet that requirement to get the EVAP to be ready.

 

[LsHart]

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 open at startup.

(higher voltage is less pressure) So I would have expected voltage to do down..
(seen rnentions voltage should be 1.3-1.5V, I would have expected 1.3V since I'm at sea level..)

View attachment 484582

Defective valve or clogged line/canister or? Normal pressure spikes?
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..

Purge valves are constantly purging. They don't hold open or closed. Vent valve does. Purge valve is burping its line from the tank so to speak. Am I reading this wrong?

 

[mikez71]

If you look at the datalog, sometimes purge is open 100% (when on the throttle) and sometimes it is fully closed (mostly at startup, sometimes when stopped) and the rest of the time it is PWM some percentage. Just curious if tank pressure is supposed to be unstable during the PWM modes, at least thats what mine is showing. (post #1)

And here's another clip with throttle position..(purge and tank pressure bottom graph)
I thought purge was fully open at WOT, I was wrong. Looks like 100% mostly during light throttle

 

[Marky Dissod]

... curious if tank pressure is supposed to be unstable during the PWM modes, at least thats what mine is showing.

With returnless fuel systems, pressure is supposed to be variable (but predictable) based on fuel demanded/allowed.

 

[Fless]

With returnless fuel systems, pressure is supposed to be variable (but predictable) based on fuel demanded/allowed.

Let's not confuse fuel rail pressure with fuel tank pressure.

 

[mikez71]

With returnless fuel systems, pressure is supposed to be variable (but predictable) based on fuel demanded/allowed.

Let's not confuse fuel rail pressure with fuel tank pressure.

The levels of vacuum in the evap don't even reach 1psi if search engines can be trusted..
(AI was just telling me 1lb/inch = 12lb/ft, so who knows anymore!)
That is not to say it's 1:1. and that 1psi of vacuum will equal 1psi of fuel pressure drop..

It seems like tank vacuum would have relatively little effect on fuel pressure?.
YET, purge (yellow) lines up with LTFT increases rather neatly.. (except at WOT, the first section following the flatline is WOT)
Not a cause and effect situation, since LTFT jumps a bit earlier, but maybe the LTFT and EVAP% partly utilize the same algorithm.

It's the jagged-ness of my tank pressure readings that makes me wonder if anything is amiss.
Maybe that is just the PWM action of the purge valve and completely normal..
Maybe fuel tank pressure is only read at startup, and after shutdown for the engine off natural vacuum reading..

During closed throttle coastdown, the evap SEEMS to mirror the fuel tank pressure..
Probably coincidence and graph scaling. Closing the purge will reduce vacuum to tank.

Looking through just a handful of logs, purge can begin at ~144degF coolant temp, ramping up to 100% at 180degF ECT.
Fully warm idle can be either closed or 16%. Free rev to redline purge hits ~50% @ ~25% throttle...
Light throttle driving it's 100% open.. At WOT acceleration, it mirrors RPM's (but jagged!) up to 81% open at redline.

 

[mikez71]

Caught the PCM doing an evap test! Two tests.
Vent open/closed is labelled backwards in hptuners scanner.

First test? starts 10seconds after startup @79degF ECT.
Gas tank half full & half empty. Vent stays closed for 1min37sec.
Purge 0%.

Second test begins 5min30sec after startup, ECT @160degF.
Evap vent closes for 1min8sec. (cursor at start, peak tank voltage is end)
Here purge tested in two phases, first vent closes for 20 seconds.
Then purge valve reaches 36%, lasts for 5 seconds.
Drops to 16% for 13 seconds. Second phase reaches 64.3%, lasts for 10 seconds.
Drops to 16% again for the remaining 20seconds. Tank pressure sensor peaks at 3.85V.
Have not checked if EVAP monitor has set. Doubtful, but will check tomorrow..

Previously I have logged the evap vent closing, but did not log tank pressure.
12seconds after startup @55degF ECT.
Stayed closed for 1min36sec, purge 0%.

Second phase, the vent closed again at 4min30sec @171degF ECT.
Stayed closed for 1min17sec.
Purge opened to 40%, 50%, 16%, then 0% for 20seconds..
Then, while I happened to be at WOT, 42%, 30%, then off WOT, 20%, and finally 52%.
The vent opened when dropping below 36%.

––----–--------

Another log, vent closed from 11sec after start @61degF ECT.
Also closed for 1min36sec duration, purge 0%.

Second stage at 5min35sec @158degF ECT.
50second test.. Two bumps, 36% and 50%.

---–-–-------

And another 16seconds after start @57degF ECT.
Closed for 1min36sec duration again, purge 0%.

Second stage starts at 4min4sec @164degF ECT.
20second test, one purge at 76.5%.

–-–------------

One more 7 seconds after start @68degF ECT.
Same 1min36 duration, 0% purge.

No second stage vent closure.

 

[mountie]

“ Tighten Fuel Cap”……. I then clear the check engine light….. 95% of the time I have no issues filling the tank.
Some years ago I had vent parts replaced by a mechanic.,,,,, but I don’t know what he didn’t do.

I did replace the solenoid on the intake manifold……. seemed to reduce the issue …..

 

[mikez71]

I haven't had that message pop up. (yet!)
I am guessing that message could show if the tank cannot get to a certain vacuum level?
Just a leaky or stuck open vent, or a stuck closed purge (solenoid on intake) could cause that I imagine.

Got my new solenoids, the vent is always open, and the purge is normally closed.
I did test (the new ones) by blowing, and the purge was difficult to tell, because it's a small nipple and I would lose seal easily..
(plus being small, I expected only a little air to flow through... but I probably would have noticed airflow with it powered and open)
With the pressure bulb attached, it felt deadheaded. And when removing the hose it let out a little 'pssh', letting me know it was sealing.

I haven't filled the tank enough to know if I would have an issue filling..
Sounds like you still occasionally have a fill issue?

To be clear, I may have no evap issues whatsoever, but only noticed the jaggy fuel tank pressure readings..
(Which is probably just the purge valve pulse width modulating, but I didn't expect a little hose to alter tank pressure so easily)

 

[mountie]

I haven't had that message pop up. (yet!)
I am guessing that message could show if the tank cannot get to a certain vacuum level?
Just a leaky or stuck open vent, or a stuck closed purge (solenoid on intake) could cause that I imagine.

Got my new solenoids, the vent is always open, and the purge is normally closed.
I did test (the new ones) by blowing, and the purge was difficult to tell, because it's a small nipple and I would lose seal easily..
(plus being small, I expected only a little air to flow through... but I probably would have noticed airflow with it powered and open)
With the pressure bulb attached, it felt deadheaded. And when removing the hose it let out a little 'pssh', letting me know it was sealing.

I haven't filled the tank enough to know if I would have an issue filling..
Sounds like you still occasionally have a fill issue?

To be clear, I may have no evap issues whatsoever, but only noticed the jaggy fuel tank pressure readings..
(Which is probably just the purge valve pulse width modulating, but I didn't expect a little hose to alter tank pressure so easily)

I imagine… the whole evap system is over engineered…….. hence, the issues

 

[mikez71]

Over engineered as in over screwy and hard to replace!

Yea, one tangle of hoses.. which costs $200, looks to be a pain to replace.. And I think there maybe 2 or 3 bundles really...
At that point you're dropping the fuel tanks....
Could probably spend $2000 on all new hoses, 2 pumps and a canister..