I’ve been looking for a CHEAP 12 valve truck as a spare on my little farm. I looked at one this weekend that had a lot of blow-by when I did the ol’ oil cap upside down trick so I passed on the truck. It made me curious about my own truck which I’ve been thinking about lately…. I did the same trick on my truck and there is a tiny bit of smoke/steam/gas??… but the cap doesn’t bounce around.



My question is should I be concerned? Is ANY blow-by normal / acceptable? What would you guys who have had Cummins all your lives be thinking right now?



….btw my truck is a 2007 5.9 250,000 miles.. pretty much stock with light tune and injectors. I don’t tow much but I log in some miles. Thanks y’all

 

You are never going to seal the rings so tight there will be NO blowby. There will always be a little. The longer an engine runs and wears, the more blowby happens.
Like anything there is a specification for the amount that is healthy/unhealthy... A manometer on the draft tube is used to test in inches of water column.

A tiny bit that doesn't make the cap jiggle sounds normal to me....