2008 f150 fx4 4wd been running slightly rough idle. Appreciate if you have experience to comment on the obdii diagnostic test fail. Says fails Evap, then TID 81 test. Anyone seen this?

 

My guess would be to plug vacuum line to purge solenoid on firewall first to see if idle improves . The solenoid is normally closed it is commanded to open sometimes to burn off tank fumes . A leaking purge valve causes system to run rich at idle . Other test it is purge valve opened to test the seal on the system to see if it will hold a small vacuum .
The vent solenoid on the tank allows flow in during a purge to remove vapors from carbon canister . It also lets pressure out when filling tank , if you have trouble filling tank that system can be in trouble/ plugged vent system . During a vacuum test of system both solenoids are energized , the purge valve opens to vacuum , tank solenoid closes to the outside. Purge solenoids are bad about leaking thru when de energized. You can't blow thru it or put air pressure thru it for test . Very common fail ,easy to blow test it .
Your gas cap is also a weak point .Over filling tank will lead to carbon canister failure. Do not top off .
I forgot to say the tank solenoid is normally open flow and deenergized . The carbon canister absorbs the fumes ,it is located on the tank side of the tank solenoid . The other side of tank solenoid is to atmosphere.
To Test this system is best done with a diy smoke machine .

 

Don't worry about that "fail", it won't cause your symptoms.

 

Thank you both for your input. I will post when i figure it out.

 

gas tank fills fine no problem. I will check cap, and purge valve, but there are no CEL yet.

 

I am thinking the same thing, I failed the TID #81 Test says test value is 102, but range should be 250 -8000 pa. I also says that this is part of the EVAP Monitor (0.020) #3c test. I will check EVAP Purge Valve.

 

I would suggest not going down that rabbit trail unless you have an ACTUAL fault that produces a check engine light. This is simply a snapshot of certain data at a certain point in time. It does in and of itself indicate an actual problem.

Now, if you had an evap system fault code with a MIL or an evap monitor that refused to complete, then this would be relevant.

 

I hear you on not going down the rabbit hole, but I am trying to figure out a slight flutter with my truck in idle. It started getting a slight rough, (ill call flutter) idle. I think it must be related to this evap test failure. No CEL yet. Gas mileage is not too bad, 14 - 14.5 mpg. Maybe I will check the purge valve, look for evap line issues, otherwise wait and see.

 

I also noticed the EVAP Pressure was negative, but when I removed the gas cap it went positive. Is the evap press positive or vacuum? Makes me wonder if evap canister is plugged to ambient?

 

It makes me wonder why you are still going down this rabbit trail after being advised not to.

Both/either. It depends....

You can check the purge valve for leakage by disconnecting the hose from it and use your fingertip to check for vacuum while engine is running. There should be no vacuum there when the CP valve is not commanded open.