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Loseing oil out my crankcase vent tube

7K views 5 replies 3 participants last post by  GAmes 
#1 ·
Ok, so I have a 98 1/2 12 valve 3500 extra cab 235,000 and runs strong, it starts very easy and stops as soon as I shut the key off. It still has factory pump and injectors, I have ground the fuel plate but put it back in the stock position, ( plenty of power ) it also has a K&N air filter and a 4" exhaust. About a month ago I was headed out on a trip with some horses, 4 and the trailer so 8K to 9k I thought since I had plenty of time I would just go slow so I kept it in 4th gear and went about 60mph at approx 2300 rpm I do not remember exactly because I was just taking it easy, when I got to the nearest town 50 miles away when I stopped for a stop sign I lost oil pressure.Two years ago my pressure sending unit was bad and if my rpms dropped to fast my needle would drop then come right back up so at first I thought oh this again, I proceeded to the next stop and it did it again, as soon as I gave it a little throttle my pressure would come back just like before, but I thought just in case I better check my oil! I was 7 qts down in fifty miles!! The bottom of my truck from the vent tube back was drenched with oil. I went to napa and bought a case and headed home slow. I checked it 3 times on the way home and not a drop low, I now have about 1000 miles on the truck since it happened both towing and empty and it has only used a little bit. The only thing I did differently was drive that fifty miles in fourth gear at higher rpm`s than normal. What could cause my crankcase to build so much pressure to cause this? This engine has to have good compression to start, run and pull the way it does. I read somewhere it could be the vacuum pump if it has a leak but the cruise works and so do all my control valves for the heat and air. Tailpipe has normal suet but no oily residue at all. Any and all advise would be greatly appreciated. Oh and I am still getting approx 20mpg empty.
 
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#2 ·
Will the stupid myth that a vacuum leak will cause blowby ever die?:doh:

Excessive blowby is caused by ring wear. K&N filters will accelerate the wear because they don't filter very well. First thing is get a good filter. If you still have the stock box it, along with a stock paper filter, is way better than a K&N. Second, pay for a complete oil analysis, not one of the quickies you find at truck stops. Blackstone is one, another is Fleetguard Oil Sampling Kit-Geno's Garage Sample your current oil and see for yourself the amount of silicates that are present. You will also find out what type and how much wear metals are in the oil. Also pay for a quality oil filter like Fleetguard or Donaldson on your next, and subsequent, oil changes.

As for your current deal, just changing the brand of oil will sometimes cure the unexpected loss of oil out the vent tube, which I suspect has happened. No, there isn't any logical reason and one brand of oil isn't any better than another.

As an FYI, a '98 1/2 Cummins is a 24 valve, you have a '98.

Take the time to read this. http://www.cumminsforum.com/forum/94-98-powertrain/287781-beginners-thread-read-first.html
 
#3 ·
The Vac. pump overflows into the crank case, if the leak was big enough IE broken tube, it would seemingly cause what looks like excessive blow-by. I've seen it first hand, why do you think this is a myth?
 
#4 ·
Seemingly is the key word there. I suppose, if blowby were just below the thresh hold of excessive, a vacuum leak could push it over the edge. "The straw that broke the camel's back" sort of thing. However, the leak would not be the cause. Follow me so far? Take a few minute to measure the diameter of the intake hole where the big hose attaches to the vacuum pump. You will find it is just a tad less than 5/16th (.3125) of an inch. For now, I'll call it a full 5/16th. Remember the formula for finding the area of a circle? pi multiplied by (r squared) which works out to .0767 sq inch. The tube that comes out of the tappet cover has an inside diameter (IIRC) of 5/8th (.625) inch. That works out to .3066 sq inch, four times as large. It is impossible to suck enough air into a hole to pressurize a hole that is four times as large. Got it?
 
#5 ·
Haha, I never said anything about pressurized, just that a visible amount more blow-by comes out of the breather tube. Without going into a mathematical nightmare. Before and after the leak was fixed, no doubt a very slight flow difference. This is on a truck with a very small amount of blow-by, if it were more I wouldn't have been able to tell I'm sure. That's all I'm saying.
 
#6 ·
I guess you don't get it, even though I used 8th grade algebra. I could never imagine that middle schoolers go through "a mathematical nightmare" The air coming out of the breather tube is always pressurized when the engine is running, i.e. the pressure inside the crankcase is greater than atmospheric pressure. When it is excessive, it isn't due to anything other than worn components. A vacuum pump cannot be the source of excessive pressure, but it MIGHT add to an existing condition. If it is only a small vacuum line the ratio of hose diameter to breather diameter is probably around 30 times different, not just four. I'd do the math, but why waste my time? Bill, if you are reading this, I think you are correct.
 
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