'88 SC Engine Slows Down When Oil Cap Off

Benesesso · 11-05-2009, 07:11 AM

Thought I'd check to see if my high milage SC engine had much ring blowby, so while it was idling I removed the oil filler cap. No smoke, but the engine slowed way down--almost stalled.

Idle returned to normal after I screwed the cap back on. Why?

Papo J · 11-05-2009, 08:18 AM

I think it's because it messes up the vacuum, the PCV is connected to that same valve cover so removing the cap creates a large vacuum leak.

BTW, that is not a good way to check for ring wear. Rings do not seal %100 of the compression so it doesn't matter how good they are you are going to get blowby. If they seal, say 96% and you have 135psi in all cylinders that still leaves you with about 6psi of blowby.

11-05-2009, 07:11 AM	#1 (permalink)
Benesesso No Skills Join Date: Mar 2006 Location: Litchfield Pk, AZ Posts: 37 Thanks: 4 Thanked 2 Times in 1 Post iTrader Rating: (0/0% )	'88 SC Engine Slows Down When Oil Cap Off Thought I'd check to see if my high milage SC engine had much ring blowby, so while it was idling I removed the oil filler cap. No smoke, but the engine slowed way down--almost stalled. Idle returned to normal after I screwed the cap back on. Why?

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11-05-2009, 08:18 AM	#2 (permalink)
Papo J 6SPD GTE Join Date: Sep 2006 Posts: 588 Thanks: 2 Thanked 50 Times in 49 Posts iTrader Rating: (0/0% )	I think it's because it messes up the vacuum, the PCV is connected to that same valve cover so removing the cap creates a large vacuum leak. BTW, that is not a good way to check for ring wear. Rings do not seal %100 of the compression so it doesn't matter how good they are you are going to get blowby. If they seal, say 96% and you have 135psi in all cylinders that still leaves you with about 6psi of blowby.