View Full Version : Crankcase Depression Regulator

09-18-2011, 12:34
Is it necessary to run a CRANKCASE DEPRESSION REGULATOR on a 6.5 or can you just "road draft" like a 5.9 Cummins ? My motor with just 25k on it puts alot of oil into my turbo and intercooler on its way back into the motor to get "reburned".


09-19-2011, 06:52
The CDR valve is part of the overall dsign and is set up to keep a slight negative pressure in the crankcase.
This negative pressure keeps oil from blowing by the gaskets and seals and generally making a mess.

The reason you are seeing the sloppy mess coming through is due to excessive blowby.

Unless you are planning on doing an overhaul and replacing the rings, there is little that can be done to stop the blowby.

You can add an oil trap to catch the excess oil and return it to the crankcase.

The 6.5 should not see a really sloppy turbo air inlet.
I have seen some nearly new rigs doing it, but its not a "NORMAL THING"

A 6.5 in top condition should have a nearly dry turbo air inlet.\

A road draft tube is just a bandaid to a problem that needs fixing.

The oil seperator idea is basically a bandaid too, but it will at least keep the negative crankcase pressure.

Good luck


09-19-2011, 12:28
Well I guess these motors all come with bad rings because being a dealer I have owned about 20 trucks equipped with these motors and I have not seen one that does not put excesive oil back into the turbo and intake system. It's just now I have one with a spearco intercooler with 25000 miles on it and the intercooler is acting as the "catch can". I'll be reel upset if I by a new AMG P400 and it does the same thing after 25000 miles of use.

09-19-2011, 14:47
I have seen a few "new ones" from the factory that slobber all over, but they should not.

I have built many 6.5 engines and the turbos all stay very dry, and these all have the stock CDR valves.

Had a 93 GMC that from the factory, slobbered oil terribly.

Its just a case of, the rings did not seat well.

Not good having the aftercooler as an oil collector for sure. Eventually the engine is going to suck that oil in a manner that could allow a runaway.


09-20-2011, 14:24
So the big deal and or question is how to get the rings to seat properly? I am installing a $8000.00 P400 and I don't want to have this problem! Imagine this post title! Now imagine it being yours!

09-20-2011, 16:06
Don't know what to tell ya, I bought a $40,000.00 Burb a few years back and the sucker slobbered oil and I could not keep a PMD alive.

If the cylinders are honed right there should be very minimal blow by, even on a zero hour engine.
The last 6.5 I rebuilt had zero visible blow by from the time it lit off the first time.

I would not worry too much with a new P400 though.


09-20-2011, 16:45
Don't baby it any longer than it takes to flush the engine of all assembly lubes and debris. Drive it easy for 500 miles, changing the oil at 100, 250 and 500 miles. Then, cut loose with it. Generally, a Diesel engine needs cylinder pressure and heat to seat the rings fully. When you hit the 500 mile mark, hook up a load and burn the paint off the headers. Don't worry about your mileage during this time. The more fuel going through it, the better.

Dittos on the CDR. If you omit it, you'll likely have to stock up on main seals, and replace them often. It is much better to have a slight vacuum on the crankcase than a little pressure. A vapor condenser would be a good idea to keep the intake clean. Or, just put a drain at the low point of your CAC. The oil in the intake never hurt anything. It is ugly, though.

09-21-2011, 06:56
When do you think it's ok to switch to amsoil synthetic? I guess after running it for about 20K then do a crankcase pressure test. What about an aftermarket vacuum pump and some kind of dump container that would have to be emptied maybe @ oil change intervals? I know it all depends on how much vacuum pressure and then again how much blowby, but atleast this system would keep the oil out of the intake system. Any thoughts appreciated.

09-21-2011, 07:17
Make your switch to synthetic anytime after break-in. If there's any reason to not do this, then it isn't really broken-in, is it?

I suggest staying with the CDR system. More plumbing and more parts (especially electrical) is just more to fail. If you are set against allowing any oil into the intake, install an oil vapor condenser (off-the-shelf, or make your own). These either automatically drain back to the crankcase, or have a catch-can that needs periodic dumping (nothing wrong with either).

09-21-2011, 09:43
I am unfamiliar with the oil vapor condensor but if I could buy one or make one to keep oil out of the intake system and put it in a catch can then I'm all for it. I understand that you don't use synthetic oil untill an engine is broken in and the rings have seated but I would like to know how to tell when this has occured on a particular engine. I would imagine if you used synthetic oil from the get go on a new engine the rings would never seat or it would take alot of use to get them to seat. I don't know. I do know that I have a new GM 6.5 that I think the rings have not seated in 25k miles of use.

09-21-2011, 10:21
If an engine is burned in properly, the oil type doesn't matter (as long as it is the correct API rating). I suggest waiting for purely economic reasons. No sense using expensive oil when you're just going to dump it in a few hundred miles. Many high performance vehicles (almost all high dollar sports cars, including Corvette) get synthetic as a first fill from the factory. The very sloppy builds of old (prior to the 90's) may have seen more benefit, but that's due to the tolerance improvements in modern machining and metallurgy.

It's been a while since we visited the subject, but Summit and Jegs both offered options for crankcase vapor separators/condensers.

The greatest problem I see with using a vacuum pump for crankcase vacuum is it would need to be a significantly large pump. The crankcase is a large volume, and the conditions will change a lot between speed, load and atmospheric conditions. In the end, the OEM system is, by far, the simplest and most effective method. As I've said, the oil that gets into the intake is nothing more than an inconvenience. Often, simplicity is its own virtue, and carries a value. To me, the cost and complexity of a sophisticated vapor system does not offset the inconvenience of the inert mess in the intake.

09-21-2011, 13:11
I checked my records and I have to make some corrections to my original thread numbers, the motor was replaced @ 63000 miles under warranty, I used Mobile 20w 50 to 70000 miles, then switched back to Amsoil synthetic, it now has 120000 miles on it. so the motor has 57000 miles on it and used none synthetic for 7000 mile break in period. So I think I should have put more miles on it before going back to Amsoil synthetic. My question is how long and what oil should be used to properly "break in" and seat the rings on a 6.5 (or any other diesel engine) before using synthetic oil? I welcome everyones opinions and/or experiences.

09-21-2011, 14:42
Use the cheapest oil you can find that doesn't smell like rotten eggs (recycled). It only has to be rated CG-4 or later. None of the oils with the correct rating will give up during a break-in period. Save some cash. Particulate suspension qualities are more important than lubricity during break-in. I also suggest using a grade at the bottom of the range for this (5w-30). It will minimize the amount of bypass, and give the filter more opportunity for catching the junk.

Unless you know exactly what the bearing clearance is, any 20w-50 oil is probably too heavy. You should be using 15w-40 (dino) or [5-15]w-40 (synthetic) for normal service. you shouldn't go higher or lower unless you have specific need. Doing so can lead to shorter engine life, and excessive blow-by.

09-22-2011, 17:14
Far more important than the oil is the original finish in the cylinders.

Factory molly rings like a smooth finish.

The stock rings are a molly filled top ring, a plain iron second ring and a chrome oil ring.

Having all the ring gaps where they are supposed to be is very important too.

Top ring at 12 oclock
second ring at 6 oclock
oil ring at 11 oclock and expander at 5 oclock

If you have an engine that fails to seat the rings, an old trick is Bonami scouring powder, fed into the intake.

A couple of small handfuls fed to the thing at 1500 RPM and then take it out and do 0-60 wide loose and then 60-0 at closed throttle on compression. Do this Ten times, take the thing home and change the oil.

If this wont do the trick. :eek: AHHHHHHHHH fall back 30 and punt.

Again, oil use is more a factor of the original machine work than the oil.


09-22-2011, 18:25
Ahha Missy, I new there was an old trick out there! Thanks for sharing, I just hope I don't have to do this too my new P400.

12-08-2014, 10:05
should the pcv or cdr valve have a metallic rattle? test good or bad?
Is there a test for these? Can these be soaked in solvent lubricated and re-used? Thanks!

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12-08-2014, 11:01
Of course, you can clean a CDR valve with any solvent that is safe for automotive rubber or neoprene components. If someone can hear a metallic rattle of a CDR valve over the din of a running diesel engine, they have better hearing than I.

The CDR is a really simple device. Unless the internal valve is rusted or stuck shut, or if the rubber diaphragm is ruptured, it has to be OK.

The CDR's purpose is to maintain a slight positive pressure inside the crankcase, which helps the various oil seals do their job. We produced an article years ago showing how to make a simple water manometer to measure the pressure at idle and at 2000 rpm by plugging a length of clear vinyl tubing into the dipstick tube, and taping the tubing onto a board. It's easy to see a slightly positive pressure this way. It's not rocket science.

12-08-2014, 21:17
Ha what rubber?! Mine appeared to have a nonexistant rubber quality.. I reached in with my pinky to try and close the valve via the spring but all i saw was the spring creeeping into the end of the spiral center like it was not connected to anything anymore..I put some wd-40 in there and swooshed around and after some investigation I found some shreds of rubber so I believe membrane completely gone..Prolly the original one...I tried blowing and both ways it was free as a bird, no auto parts stores have them. Only the inter web.

12-09-2014, 17:04
My rubber was non- existent...Moved the spring with my pinky and all I got was a look at the spring crossing the center of the pcv... put some wd40 in there and saw a shard or two of rubber floating around completely gone! Unless the gas is minus ethanol I would not use it..

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01-06-2015, 12:16
OK, so I dug out my photos, and here is a parts breakdown showing the "rubber" diaphragm I mentioned...