Stump the OM617 Stars !!

[rmmagow]

I own two 83 300's, a D and a CD. Both have the same OM617 turbo charged engine, one has 187,000 the other about 207,000. I was checking some vacuum lines and generally looking around under the hood. For whatever reason, I performed a sort of test with a very odd outcome and I think something is wrong with one of the motors. Here it is:
I took off the breather tube that leads from the valve cover to the air cleaner. I took it off at the air-cleaner end. On one engine while at idle I firmly plugged the end of the tube with my thumb. Pressure built up quickly and the engine very promptly stalled.
On the other engine I did the same. Again pressure built up quickly and quite strong BUT the engine never stalled or even indicated anything was going on.
As a further test, I disconnected the overboost switch on each engine, but the results were exactly the same.
Both engines have all vacuum lines and their EGR valves attached.
The engine that stalled has a little more smoke than the other but not excessive. Both cars are used regularly and perform pretty well, both use the same Sunoco diesel fuel, both have new air cleaners and fuel filters. Neither car needs engine oil added between changes (3~4K).
What is going on here?? Obviously something's not working right but I don't know which is OK. Any clues? Can some of you run a similar "test".
Thanks for your input.

[Brian Carlton]

There is quite a bit of discussion in the archives regarding this test. It can be somewhat of a dangerous test because you are pressurizing the crankcase to a level that will stall the engine. Nobody is quite sure what the oil pan can take with regard to pressure, but, the pressure caused by this test is not insignificant.

On the engine that will not stall, the answer can be as simple as a small leak somewhere (oil filler cap? or valve cover?) that will prevent too much pressure building up in the crankcase. Any source that will allow venting of the case will do it.

If you are still bent on figuring it out (which I do not recommend), you can search around the running engine for an air leak. The blowby air in the crankcase must be getting out somehow, otherwise the pressure must build until the engine stalls.

Additionally, please note that if this engine is a very good engine, there will be very little blowby. In such a case, a very slight leak will offset the blowby and the engine will run forever with your thumb over the hose.

[rmmagow]

So, does this means the stalling is GOOD, a well sealed engine performing as it should? I'm not curious enough to continue on with "test" unless there's some significant benefit to be gained by making it right. The engine that did not stall (but still built up pressure) is in a car that has some vacuum deficiences, could this be related? The engine that stalled definitely smokes more than the other one, but it still runs very strong. It is the engine w/207K on it.

[Brian Carlton]

Everything else being equal, the more time it takes to stall, the less the blowby, the healthier the engine.

On the one that will not stall, my original suggestion is that it has less blowby than the one that does stall. It also has a leak somewhere that allows the built up pressure (created by your thumb) to get out of the crankcase before the engine can stall.

Vacuum related deficiencies should not affect the outcome of this test on either engine.

I wouldn't worry about too much pressure building up in the oil pan. If there is too much pressure, it would vent thorugh the oil dipstick tube. Surely the pressure required to push up the oil dipstick is much less than the pressure it would take to blow the oil pan gasket.

Perhaps this has already happened and is the source of the air leak?

[Brian Carlton]

The dipstick!!! That's a damn good possibility!

However, its not the pressure to lift the dipstick that I am concerned about. You might develop 3-4 psi in the crankcase with your thumb. This is more than enough to lift the dipstick. However, you must now multiply 4 psi by the total number of square inches inside the oil pan to determine the total force that is trying to force the pan off the bottom of the block. There are probably 100 square inches inside that pan and 4 psi would create 400 lbs. of force trying to blow the pan apart. This might be perfectly acceptable to the pan, but, nobody really knows for sure, except those that have successfully done the test.

[R Leo]

The bottom end of the dipstick is below the surface of the oil in the crankcase. If enough crankcase pressure built up to force the dipstick out of it's tube, the exiting dipstick would be immediately followed by a column of engine lube oil.

FWIW, there's a tiny hole in the inboard side at the top of the dipstick tube that allows the pressure in the tube to equalize when you insert the dipstick. This ensures that you always get a correct oil level reading on the dipstick, not one that is artificially low because air presure in the dipstick tube temporarily forced the oil column downwards.

[rmmagow]

Guess I'm still baffled. The one that stalls is the engine with more blowby as evidenced by seeing smoke come out of the tube and it will smoke some when heavily accelerated. It doesn't comsume much oil between changes but some. It is a good running car getting 25 to 28 MPG.
The other, which doesn't stall, built up a LOT of pressure when I did this test. The flat tube from the top of the VC to the air cleaner housing was nearly round. That motor doesn't smoke hardly at all except with hand operated rev the hell out of the engine type fooling around. It too is a good runner at close to 30 MPG.
So what gives? shouldn't the second motor stall? If the pressure was leaking I should not have been able to build up so much I wouldn't think.
I'm more curious than anything since the one that does stall is actually the one I was planning to keep, but if the engine's on it's way out, I might just keep the other.
Very confusing. Wish I didn't do it :-)

[oldnavy]

Originally Posted by R Leo

The bottom end of the dipstick is below the surface of the oil in the crankcase. If enough crankcase pressure built up to force the dipstick out of it's tube, the exiting dipstick would be immediately followed by a column of engine lube oil.

FWIW, there's a tiny hole in the inboard side at the top of the dipstick tube that allows the pressure in the tube to equalize when you insert the dipstick. This ensures that you always get a correct oil level reading on the dipstick, no one that is artificially low because air presure in the dipstick tube temporarily forced the oil column downwards.

Friend of mine blocked his CCV off in his car back in the 70's and the dip stick dented the hood and sprayed oil all over the hood and engine compartment. LMAO

Usually the amount of oil in the airfilter is a good
indication of the amount of blow by.

Louis.

[Brian Carlton]

Originally Posted by rmmagow

The other, which doesn't stall, built up a LOT of pressure when I did this test. The flat tube from the top of the VC to the air cleaner housing was nearly round.........So what gives? shouldn't the second motor stall? If the pressure was leaking I should not have been able to build up so much I wouldn't think.

I wish you did not do it either. But, we need to figure it out, now. So, a couple of issues:

1) If the pressure builds up to a point (and you mentioned that it is quite a high point) and the engine does not stall, this leads me to believe that there is a leak that is allowing just enough blowby to escape to prevent the engine from stalling. When you finger is not on the pipe, the amout of air escaping is minimal. When you put your finger over the pipe, the pressure starts to rise and the amount of leakage also starts to climb. Eventually, there is equilibrium where the amount of leakage equals the level of blowby and the engine continues to run with elevated crankcase pressure until you remove your finger from the hose.

If you must follow this up, get someone to help you find the leak. Plug the hose and search for the air leak. It will be a fairly substantial leak at this point because all of the blowby must be going through it. Check the valve cover, the oil filler cap, the oil pan gasket, and any other potential area where the blowby air can escape. I am not positive about this, but, it may be possible for the air to escape via either the front or rear crankshaft seals. If it is the rear seal, you will not be able to confirm this. As Randy has stated, it is unlikely to be the dipstick, but, the dipstick oil can be forced out of the tube from the pressure in the crankcase, so be careful.

If you can find and correct the leak, you can compare the two engines. If you cannot find it, you cannot use such a test as a valid comparison.

[soydrivermatt]

When I was reading old threads about this, the theory was that the pressure isn't stalling the engine, but affecting the fuel rack in the I.P. killing the fuel flow as it's designed to do if the engine is over-pressurizing. Any ideas?

[kerry]

My theory is that the pressure in the crankcase gets into the IP and the vacuum shut off valve runs in reverse and becomes a pressure shut off valve, shutting down the engine. Nothing to worry about in my opinion.