2001 suburban LT 160k....

it started with 3 evap codes and the engine stumbling when accelerating. I would crack the fuel cap open and there would be massive suction of air flowing into the tank. If I re-tighten the cap, the engine would stumble again but if I left the cap somewhat loose, it seemed to breathe and run perfect. I went the cheapest and easiest route first and replaced the EVAP solenoid behind the fuel tank and the codes went away. It has been 3 days with no codes but the engine stumbled again with a tight fuel cap so I have it loose again.

Before replacing the solenoid... the only code that was consistent and there everyday was the P0453.... fuel tank pressure sensor? I know it's a pain to swap that out, that's why I did the solenoid 1st. But now the codes are all gone so I don't know my next step... suggestions would be very much appreciated.

Also curious... if you have the Evap solenoid not connected and in your hand, should you be able to flow air thru it or not????

 

It sounds like the EVAP vent valve is permanently closed or the EVAP canister is plugged up. Try running with the EVAP vent valve removed and fuel cap closed tight? You say "3 days with no codes" but that's after you reset them? Is the EVAP Readiness set, though? What were the codes?

 

Sounds like the purge valve on the top of intake is pulling vac on the tank always. Which is pulling gas vapor or even raw gas into the engine at all times. Pull the hose off the purge valve and see if vac is present all the time.

 

I may be mistaken, but most gas caps have one way value built into them. They let air in but do not let gases out. Have you replaced the cap?

 

No, it was supposed to be pretty tight on the vacuum side, i.e., into the fuel tank. The EVAP tests depend on the fuel tank pressure (or, vacuum, however you want to look at it) to detect leaks. The last few years it's gotten more stringent, tight both ways.