EVAP issue

[ssbtech]

I'm having a small EVAP issue - car will immediately die after fueling up, but then subsequent starts work fine.  I'm assuming fuel vapors are being forced out of the tank into the intake during fueling and are cleared out during the first attempt at starting it.

I see there's an EVAP solenoid and a purge valve in the system.  On the surface, it looks like the system would run just fine with only the EVAP solenoid as the vacuum on the light blue hose would draw fuel vapor through the solenoid from the green line when the solenoid is open.  

The two orange lines on the purge valve look to have equal amounts of vacuum on them.  Am I correct in assuming that under boost, the orange line from the intake manfold opens the purge valve and pressurizes the EVAP line or does it close the valve?  Or does it redirect vapors in the green EVAP line into the turbo inlet?  

I just can't quite wrap my head around what is happening and what the relationship is between the solenoid, valve what happens under boost/vacuum.  

 

Thanks!

 

 

 

[m sprank]

Going to try and simplify this as much as possible to answer your question.

Gas tank vents fuel vapor, via the vent tube, to the charcoal canister. Canister absorbs the vapor and contains until the car reaches temp and is driving.  ECM signals the charcoal purge to open, and the EVAP purge solenoid to open under vacuum. The vapors are then drawn from the charcoal canister into the intake manifold, where they are burned.

Purge control solenoid is normally closed.  Purge control solenoid has no specific directional flow (when open air can pass either direction).  The Purge Valve (round black thing) is direction specific and should have an arrow painted on the front of it (intake and evap pipe).  

So, the purge control solenoid connects to the charcoal canister, which collects fumes from the fuel in tank.  The vacuum is tee'd off to the manifold for boost/vacuum reference (via the purge valve). When the fuel tank pressure increases ECM opens the purge control solenoid to allow the fumes to be sucked into the manifold for combustion. The ECM activates the purge control valve via ground so it should be closed unless grounded.

Hope this helps. 

 

Edited March 10, 2023 by m sprank

[ssbtech]

Thanks.  I do understand the function of the solenoid in the system but I still can't quite figure out what the purge valve does.

 

I suppose I should just replace both at this age.  Odd that there's no CEL though.

[m sprank]

The purge valve to intake manifold connection is simply a reference that pulls and pushes on a diaphragm in the purge valve.  Positive pressure connects the two "side connections" of the purge valve, allowing airflow from the fuel vent to the turbo inlet pipe under boost. The purge solenoid, opens only under vacuum, allowing fuel vapors to be sucked out of the fuel vent.  The purge valve is used to vent the tank under boost conditions only and is not strictly necessary.  Removing it will cause no codes. 

You can test the purge valve operation with a small hand pump.  Dont need more than 5-10 psi. 

Is that more helpful?