8th Generation Honda Civic Forum - View Single Post - blow-by or something else, bad MPG

BigT · 10-10-2008, 01:10 AM

There is a simply test for blow-by. It isn't 100%, but it can give you an idea. Remove the oil cap while the engine is running (be careful, the placement of the cap may allow oil to be flung out the opening). With the cap off place a piece of paper over the opening. There will be air movement in the crankcase due to blow-by (all engines have a little) and piston movement. But the PCV system should result in a slight net negative pressure in the crankcase. Once flush against the opening the paper should stay in place. If it blows away and doesn't lightly suck against the opening you could have a slight amount of excess blow-by.

Keep in mind the engine should be warmed up fully for this test because there will always be more blow-by when the engine is cold. Also, if the paper is held slightly away from the opening it will likely flutter around from the air movement. This is normal and may be what you are feeling with your hand (it's hard to tell what it feels like over the internet

).

Also, as for your question about codes. If the fuel mixture is basically correct and nothing is dramatically wrong an engine could have excessive blow-by without setting a code.

Good luck.

10-10-2008, 01:10 AM	#3 (permalink)
BigT Senior Member Join Date: Jun 2007 Location: Duff Beer Krusty Burger Buzz Cola Costington's Department Store Kwik-E-Mart Stupid Flanders Park Posts: 2,929	There is a simply test for blow-by. It isn't 100%, but it can give you an idea. Remove the oil cap while the engine is running (be careful, the placement of the cap may allow oil to be flung out the opening). With the cap off place a piece of paper over the opening. There will be air movement in the crankcase due to blow-by (all engines have a little) and piston movement. But the PCV system should result in a slight net negative pressure in the crankcase. Once flush against the opening the paper should stay in place. If it blows away and doesn't lightly suck against the opening you could have a slight amount of excess blow-by. Keep in mind the engine should be warmed up fully for this test because there will always be more blow-by when the engine is cold. Also, if the paper is held slightly away from the opening it will likely flutter around from the air movement. This is normal and may be what you are feeling with your hand (it's hard to tell what it feels like over the internet). Also, as for your question about codes. If the fuel mixture is basically correct and nothing is dramatically wrong an engine could have excessive blow-by without setting a code. Good luck.