Oil cap suction?

[bdowne01]

Hey guys, new owner of a slightly used Sonic LT 1.8 and a new poster here.

The car runs great aside from a slightly rough idle (while in Drive) and an occasional idle stumble, say sitting at a red light. I'm kind of an old-school V8 muscle-car guy, so I've been popping the hood and peeking around with the car running (but let me be clear: I have no idea what I'm doing with a modern engine )

When I remove the oil cap, there is a very noticeable amount of suction going on--enough to resist removal of the cap, and the car begins to idle terrible. Replace the oil cap and all is well. On my old V8's that would almost lead me to believe I have a valve seal issue!

I do understand a bit of suction is normal for PCV--but enough to causing bad idle seems suspect to me.

Anyone else with a 1.8 care to try and see if theirs behaves similar?

 

[Blue hedgehog]

I don't have the 1.8 but that is normal on a electronic fuel injected engine. You are letting air into the engine without it passing through the mass air flow sensor. My 99 Tahoe 5.7 csfi would almost stall if you did that

 

[bdowne01]

Thanks!

 

[klb1.8]

This guy is old school did he mention that lol i did the same to mine its normal nothing to do with valve seals plus the sonics have a huge one youd notice any leak..
My idle is only rough because of the mounts yours is most likely normal

 

[RS Turbo]

The actual True reason this happens is because it has a closed PVC system and Not because it has Electronic Fuel Injection.

 

[ChrisJ]

The actual True reason this happens is because it has a closed PVC system and Not because it has Electronic Fuel Injection.

RS,

How does the 'closed' system function?
As in, older style you had your PCV valve often on one valve cover, and a fresh air inlet on the other valve cover so you had good ventilation but didn't cause actual suction in the crank case.

I'm guessing the modern systems do not use a PCV valve but rather something more complicated, and do not have a fresh air inlet?

 

[RS Turbo]

Cant explain in words you would need to see the Cam cover cut out and the Baffle and there is a drain in the head that goes directly for that system into the Crank case below the oil level.