Pretty much the same between both. Felt low on power compared to when the evap was disconnected from the purge valve.

 

Ok thanks for all the good data. Let me see how my purge valve works and effects fuel trims and then Ill get back and we can talk about this.


George

 

Awesome, thanks George for all your help!

 

I have some information. Family commitments kept me from finalizing the tests but here is what I know so far.

My Blazer has over 200K miles and needs some TLC so its not running tip top but I think that I am getting what we need. It idles a little rough but drives smooth.

I am enjoying this project because the functioning of this system has never been fully documented that I know of and of course the interwebs are full of BS experts. Because of this, some of my assumptions were wrong. The evap vent solenoid was commanded open the entire time. AT start up and cold the purge valve is commanded 100% closed. It does not open until warmer and I think it started at the transition to closed loop. This would make sense because the engine needs the O2 sensors to keep fuel delivery in the slot. I'll check that timing tomorrow. Once the valve was commanded open it never closed again. I ran the truck for about 30 minutes. It started commanding a ramp up in open % to somewhere around 30% and stayed there for some minutes. I will capture these exact times and levels. Eventually the open command ramped down to 15% open and never moved as it continued to idle. After the purge valve stabilized my fuel trims were around -10 which is consistent with past data for me at idle. When the purge valve was opening more the fuel trims got more negative to compensate topping out around -25. The engine always idled OK. I then commanded the purge valve to open in 10% increments and somewhere between 30 - 40% the computer started losing fuel control and the truck ran all crazy, surging and almost dying, and running really rough. Fuel trims got over -30% when this happened. When I started lowering the purge valve command level the truck smoothed out and then ran fine. The idle was no different between a purge valve command of 0% and the 15% level.

So here is what we know so far. When the truck starts cold the purge valve is closed. There is a purge valve cycle that starts around closed loop. the valve ramps open then holds for some minutes and then ramps down. It always runs OK because the O2 sensors keep the fuel delivery in good shape. The valve never closes once open. Fuel trims offset the added fuel content but there is a limit above which the truck runs like crap.

The question for your problem will be, why does your truck run like crap during the purge cycle. Is it:

Purge valve opens too much
The O2 sensors dont work well at the limits
There is faulty fuel delivery that gets aggravated by the purge cycle
There are more fuel vapors than expected for a given purge valve command


Can your scanner monitor the purge valve open % command?

I will be confirming these levels and timing as well as capturing fuel tank pressures tomorrow.

George

 

Great information and thank you for testing.

I'm thinking this might be the case. I've searched all over the internet on how to test if the charcoal canister has been flooded and can no longer do it's job. When I removed the tank line I got a whiff of gas (no liquid), but I suppose that's supposed to happen given its job.

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I don't think it can, at least I haven't found a PID that allows me to monitor that in any of my internet searches. One of the other "car scanner apps" might have that PID, but it's a crapshoot of what you get with them.

 

PM LesMyer about which cost effective BT scanner can monitor the purge valve position, he's the local expert. I will look t a few of those BT apps tomorrow to see what I can see. I went for the big gun today because I wanted to be able to command various conditions.

George

 

This morning I took a look at the behavior of the lowest mileage and best running truck in the family "fleet", a 2006 GMC Sierra, 5.3L:

Vent solenoid never closed
Purge solenoid closed at start up, still closed at closed loop
Purge solenoid opened after a few more minutes, not sure what the criteria was. ECT, time, ...?.
Ramped up to about 20% open and held for a few minutes
Trims went to about -10
Purge valve settled down to 12% open and trims went to +/-0 to +/-4
Purge valve never closed after about 30 min

Let the truck cool down for 2 hours and repeated. Everything was the same except that the purge solenoid never had an initial higher open %. Steady at 15% the whole time, trims very low.

Heading over to the Blazer in a bit to finish getting that data, hopefully with no operator recording error this time. lol.


George

 

And the final numbers on the Blazer:

The vent solenoid was always open

The purge solenoid started closed at engine startup
FTP is 0.52 inches H2O after start up

The purge solenoid opened shortly after closed loop to 12%
FTP went to 0.33
Trims went to -14 pretty quick

Purge valve slowly climbs to 19% open
FTP drops to 0.23
Trims go to -25

After a few minutes purge valve goes to 14% and stays there
FTP goes to 0.33
Trims go to -6

It would help if you can monitor purge valve status and/or fuel tank pressure to see why you go so rich that the fuel trim process cannot control fuel delivery enough to run right. You could also do an injector balance test to see if fuel delivery looks wacky.

I had always thought that the purge valve opening was an occasional process but this is not true. Ramping to 19% is occasional but the purge valve is always open , or at least for 30 minutes of idle it is. The vent solenoid being open except for testing does appear to be true.

Let me know where you are at and what you want to do from here?

George

 

I just thought of one more experiment that might help. Take the gas cap off to create an intentional leak to see if this significantly increases fuel vapors/trims because now the purge valve pulls not just through the vent solenoid but also a tank full of gas. My GMC will be good for this test because the canister is new and won’t have the potential to have been flooded. When I took this truck out of long term storage the canister was plugged and gas filling was slow so I replaced it.


George

 

I ran my GMC until the purge value was stabile at 15%. Trims were -4. I then opened the gas cap and forced open the filler thingy. Trims doubled to -8. I did not force the vent solenoid closed to simulate a blockage in the vent circuit but I have no doubt that this would cause trims to go up further. In the next couple of days I will try that on the blazer since the base trims are worse so we can see what happens.

George