Leaking oil "down tubes" on 8v71

Seann · October 09, 2025, 06:10:03 AM

Finally got under our bus...there are 2 what look like upside down dipstick tubes on either side of the engine, one is dripping constantly while running. What's their purpose, and what's this a sign of? It's not overfilled...
Thanks,
Seann

muldoonman · October 09, 2025, 07:03:09 AM

Blow by? Rings ain't sealing good and combustion pressure being forced through worn rings down into the crankcase. Be my guess.

luvrbus · October 09, 2025, 09:21:28 AM

Those are your air box drains they leak if idled for long periods, some 8v71 have check valves that close with higher RPM and some are just a open line ,the 92 series all have the check valves you remove and clean, if you could post a photo we could tell you if have the check valves or not and installed on the right angle

Seann · October 10, 2025, 08:30:18 AM

Both tubes originate in the engine block...see photos

Seann · October 10, 2025, 08:31:28 AM

other side

luvrbus · October 10, 2025, 08:47:38 AM

Quote from: Seann on October 10, 2025, 08:30:18 AM
Both tubes originate in the engine block...see photos

That is your air box drains yours doesn't have the check valves they drain all the time till you run it hard enough to remove the excess fuel and oil build up,if it doesn't clean up after long runs you have something else going on like blower seals or broken oil rings,if you idle for long periods you will have that on a 2 stroke DD

Seann · October 10, 2025, 10:01:09 AM

OK, it'll be a while until I can give it a good run, so I'll hang some "catch cans" for now,
Thanks

Coach_and_Crown_Guy · October 14, 2025, 04:51:41 PM

One other very important thing to consider regarding engine leaks in general, and I don't consider air box drains as leaks per se, but the thing is to be extremely diligent in ONLY using straight 40wt mineral oil. NEVER use any multi-grade oils. The key thing is to make sure the oil is ONLY CF2 or CF4 (a newer cert that superceded CF2 and OK to use) less than 1% ash content rated oil. The only brands to use are DELO 100 or DELO 400 is OK, Mobil DELVAC 1240, and only when nothing else is available ROTELLO T1. All of these are straight 40 wt Mineral oil. Order of preference is DELO 100, then DELVAC 1240, DELO 400, ROTELLO T1.

Multi-grade oils will find many more paths to leak and seep out of the engine due to the blower pressurizing the block, thus causing visible leaks and seeps. The air box drains are there to prevent the accumulation of oil in the air box surrounding the cylinders. As the pistons move down they uncover the intake ports where higher pressure air from the air box is forced into the cylinders. then they move up closing the intake ports for the compression stroke. As the rings move past the ports a little oil is left behind on the port openings, this will find it's way to the bottom of the air box and start to accumulate. This is why the drains are there.

The multi-grade oils like 15-40 or synthetics will cause more oil to get trapped and escape through the intake ports. The proper DELO, DELVAC, etc. 40wt. oils do not lose past the rings anywhere near as much vapor or droplets. The other CRITICAL factor in the CF2 rating is the very low (less that 1% ash content) of these 2-stroke oils. This is to reduce the abrasive wear to the interior side of the intake ports as the piston moves up and down across the ports. If the ports get worn beyond the ability to retain the oil by the rings you will have more oil leaks and maybe even some loss of compression. The ash acts exactly as an abrasive powder and will take a destructive toll over time. So use the correct oil all the time.

If you watch while the engine is running you should see a wispy and tiny bit of smoke coming from the drain tubes due again to the air box over-pressure expelling air and any liquid or vaporous oil. This is perfectly normal. This little bit of vapor and oil droplets will get caught up in the aerodynamic air-stream and eventually find it's way to the back of the bus and coat it with an oil and un-burned fuel film which in turn will attract dust and dirt and thereby creating the iconic black coated rear end of all Detroit Diesel 2-stroke buses.

Seann · October 20, 2025, 08:21:06 AM

All good to know...haven't got to oil change yet (don't know what's in it), but will keep that info in mind when we get there. I've hung a couple of small plastic bottle "catch cans" on the drains to contain and see what drips out. Still a ways off giving it a good run...have to solve air leaks/excessive leaning issue first.
Thanks