[miamidiesel]

Hello everybody,

I have a friend who just bought a truck that has heavy blow by in it. He got it from the most famous auction site. When we remove the oil cap there is white smoke puffing out like a choo choo train... and when you remove the oil dipstick you can see white smoke coming out of that too..... the exhaust has some smoke when the truck is cold but that kind of goes away as it warms up. The truck does run excellent but the blow by is there.... What really causes blow by??? and is there any way to stop the blow by in this truck or reduce it???

The truck is:

1999 Ford F250 XL with 126K miles on it PowerStroke Automatic.

Thanks !

 

[gregrob]

What causes blowby is worn rings allowing compression to enter the crankcase. To remedy it, you need a rebuild.

I would have a compression / cylinder pressure test done on it to see how close it is to spec, then decide.

Usually blowby will cause reduced performance, lower economy and more oil consumption, things your friend may or may not want to to live with at the prospect of a 5,000-10,000 dollar engine rebuild.

 

[444turbodiesel]

All diesels have blow by at idle. Usually more than people are used to seeing from a gas engine. The rings are designed to cope with very high cylinder pressures and for maximum durability. A softer ring (like is used in a gas engine) would seal better at low CP's and reduce blow by but would not last 300,000mi which is what is expected from a diesel. If it smokes when cold, I'd look at the glow plug system first. If even one GP is not fuctioning, that cylinder will smoke until warmed up. After that, I'd get some oil samples tested for excessive soot and carbon (from unburned fuel bypassing the rings under power) before a tear down. Watch the oil consumption, if it is more than 4qts per 5000mi you might consider a rebuild.

 

[ITowBoats]

the amount of blow by is directly related to cylinder pressure. rings have what is called diametral tension, which means they want to expand. this is what holds the against the cylinder, but the sealing force is actually caused by the pressures of combustion. when the cylinder fires, the pressure of combustion will pass down into the area behind the rings and force them out and against the oil film that they ride on. a properly fitted ring has clearence between the ring and the ring groove in the piston. this allows the ring to move up and down slightly. when the cylinder fires the combustion pressure pushes the ring down and seals it agains the bottom of the ring groove. the pressure also enters the area behind the ring to force it against the cylinder walls. if the rings get severely overheated they lose their diametral tension and don't provide the initial seal so the combustion pressure can do its job.