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Old 01-11-2025, 05:03 PM   #1
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Engine driven air compressor/Slober issues

Is it possible that the engine driven air compressor can "leak" air pressure into the engine sump on a ISC 350 8.3 liter engine? Engine appears to have high blow by pressure in valve cover. I installed a new engine driven air compressor as the original was passing oil and the air dryer was dumping it overboard. It made a mess on rear of coach.

I have installed a new turbo but have only run the engine with coach stationary to leak check. Did place unit in drive and loaded engine a bit to 10 psi boost pressure, to see if the turbo would develop boost.

The slober tube on my coach when connected to the added oil separator system is leaking at the added return to engine - fake drip stick drain port. With the system disconnected permitting a straight overboard dump all seems ok and nothing is dumping overboard, no pressure felt by hand. I know the added slober system has an orifice in the drain back hose run, at this time I don't know if its creating issues or not - I think I'll remove it to see if it makes a difference.


Any thoughts?
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Old 01-11-2025, 08:18 PM   #2
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The engine will build pressure if you block all the vents, there is always some blowby.


The oil drain to the alternate dip stick port should not leak, if it's leaking at the block, then clean off the area and pull it out, I couldn't tell you what the seal is, but it should be relatively obvious once it's out. That should have oil leaking down, so leaking oil just means something isn't sealed, not that there's more pressure.



The extra oil return system should have more oil fume filtering material, so it may raise the crankcase pressure slightly. There may be a cleaning procedure for that filter material if you can figure out how to take it apart.
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Old 01-14-2025, 08:26 PM   #3
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Well spend a bit more time on the coach and found the "dummy" oil filler tube installed on the passenger side of the block split at formed lip where it had been bent in the past. This is where the engine breather slobber tube extension drains back into the oil sump. I brazed the dummy tube and bonded it back into the block the way it was found. Connected the slobber system back up.

I also calibrated the dipstick as after filling it with the 20 quarts [per my sump part number / Quick serve]. The dipstick showed being 4 + quarts low. I cut the dipstick holder outer tube, removed ~3/4" which is the distance between ADD and FILL. The tube is some non-metallic material and I bonded an 1 1/2" long x 1/2" ID metal bushing joining both pieces of the outer tube, once dry followed up with black shrink tubing. I then drained 2 quarts back out of the sump so the dipstick now shows as half way behind the ADD and FILL marks.

I pressure washed engine and transmission and allowed it to dry overnight. I started the unit and brought it up to temperature to check the transmission oil level. So finally no leaks from engine or transmission plus no smoke from exhaust.

Going to leave it overnight to check engine oil level cold, if all's well time to put the wardrobe back together and drive it.
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Old 01-14-2025, 08:47 PM   #4
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Dennis, I talked to my Cummins tech friend this past week. He said that at idle you will get blowby due to the rings being not fully seated. At highway speeds and RPM,you will have less blowby.

To test blowby, there is a certain size orifice (he didn't remember the size), that is used along with a modified oil filler cap and a manometer to measure pressure. He didn't remember the number but I seem to remember the number 11 but I don't remember how to convert that to pressure.
With your fixed barb, you should be good to go
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Old 01-15-2025, 09:47 AM   #5
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Don, I have this attached document which I need to read again. I also recall reading somewhere that the slobber tube added modification added ~11 inches of back pressure, need to find that document.



I was going to purchase and modify another oil filler cap but I don't its necessary. There's a good chance that the engine oil was overfilled due to the dipstick issue. I think [not always good] the turbocharger seals were just old and finally failed. On the cold side I got lucky with no oil into the after cooler, lines were spotless.
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Old 01-15-2025, 10:42 AM   #6
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Dennis, that document talks about 12 inches of water and that is not the blowby pressure in pounds
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Old 01-15-2025, 11:50 AM   #7
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A manometer reads pressure directly as water height difference, so 11 or 12" is the common measurement. That will be the most common unit given on a pressure gauge that would read that range, such as a dwyer magnahelic gauge, or other HVAC air pressure gauge. Of course you can rig up your own U tube manometer also.



The additional filter adding resistance is irrelevant to the measurement, the orifice comes off the valve cover for the fixed bleed, and the static pressure could be read anywhere else in the system. With no flow through the filter, there would be no added resistance.
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