Searching for data - Official limits on maximum crank case pressure OM617 - PeachParts Mercedes-Benz Forum

Stretch · #1 01-06-2013, 02:08 PM

G'day All,

Calling all book worms!

Has anyone ever stumbled on an official maximum allowable crankcase pressure figure for an OM617?

(Or am I going to have to calculate it?)

EDIT:-

WARNING

There are some tests considered in this thread that are not necessarily as smart as they may seem. Please read this post (post #30) before you consider performing them yourself

Searching for data - Official limits on maximum crank case pressure OM617

t walgamuth · #2 01-06-2013, 02:13 PM

Not very much.

Phillytwotank · #3 01-06-2013, 02:59 PM

i don't know the specification.

if we do some thinking we could probably figure it.

Crankcase pressure works in opposite the force crated by the combustion cycle.

max crankcase pressure would have to be < combustion pressure

Smell what im cooking?

t walgamuth · #4 01-06-2013, 03:08 PM

A whole lot less!

All you have in the crankcase is that generated by the combustion force that escapes from around the rings.

I am thinking normal is near zero with really bad blowby giving maybe three or four pounds per sq in.

Stretch · #5 01-06-2013, 03:22 PM

I think I've got some good information concerning the calculations - I'll post up the results if I can get some sense out of them (along with the source of course).

A rough estimate can also be made from the lowest allowable compression in a compression test (chapter 00-115 in the FSM) and the maximum leakage data (chapter 00-120) but I was kind of hoping that someone somewhere pops up and says "blah blah blah in WIS" or something like that!

4x4_Welder · #6 01-06-2013, 04:51 PM

I think the better measurement is volume, since it only takes a few PSI to push oil past some of the gaskets and seals. The popular method seems to be filling a trash bag from the oil filler opening, there are a few videos on Youtube of this. I would recommend you remove the vacuum line first, and plug the vacuum pump inlet, since the air it draws in is dumped into the crankcase. This will give a false result.

1project2many · #7 01-06-2013, 05:20 PM

Max possible pressure would be just below the point where crankcase pressure closes the fuel shutoff valve on the IP. So take a typical reading from the vacuum system and convert it to pressure. A reading of 18" mercury is equal about 8.4 psi.

Normal pressure based on homegrown checks with the oil cap is just enough to support the cap. My cap weighs about 12 ozs or .75 lbs and has a surface area of about 3.54 in/sq so normal crankcase pressure is .75lbs/3.54 sq in or .211 lbs/sq in.

Now if you're looking for a "high normal" I can't help. My old car was above that point when I got it and the current one isn't there yet.

I can say that my experience with other engines using thin, flat paper gaskets is that one or two psi is all it takes for those gaskets to start leaking. Gasket size, pressure applied, &etc will all play a part but a couple of psi is usually all it takes.

Diesel911 · #8 01-06-2013, 11:34 PM

I posted in one of the threads but done remember exactly what I said. I hooked a pressure gauge up where the Plug on the IP Governor Housing is and when I obstructed the Valve Cover Outlet somewhere after 3.2 psi the Engine shut down.

Can't remember the exact figure.

I am guessing that the condition of your Vacuum Shutoff Diaphragm is going to make a difference. I am thinking that an new Shutoff (with the nice new well sealed Diaphragm) might shut down earlier than an older one.

Stretch · #9 01-06-2013, 11:43 PM

Quote:

Originally Posted by 4x4_Welder

I think the better measurement is volume, since it only takes a few PSI to push oil past some of the gaskets and seals. The popular method seems to be filling a trash bag from the oil filler opening, there are a few videos on Youtube of this. I would recommend you remove the vacuum line first, and plug the vacuum pump inlet, since the air it draws in is dumped into the crankcase. This will give a false result.

I agree that blow by tends to get described as volume but for the average mechanic a pressure measurement is more likely to be made - I was hoping that there was a specification that has been dumbed down for the masses.

Stretch · #10 01-06-2013, 11:50 PM

Quote:

Originally Posted by 1project2many

Max possible pressure would be just below the point where crankcase pressure closes the fuel shutoff valve on the IP...

Welcome to the forum 1project2many.

Thanks for your input - my reason for asking the question about crankcase pressure is actually to do with your first statement. I'd like to do some calculations and then some measurements to see if this actually happens - I'm sorry to say that I'm not a believer just yet.

There are several variables that may or may not have an influence - I'm planning on doing a little science project to check them all out; and in the mean time learn a little bit about crank case breathing.

Stretch · #11 01-06-2013, 11:55 PM

Quote:

Originally Posted by Diesel911

I posted in one of the threads but done remember exactly what I said. I hooked a pressure gauge up where the Plug on the IP Governor Housing is and when I obstructed the Valve Cover Outlet somewhere after 3.2 psi the Engine shut down.

Can't remember the exact figure.

I am guessing that the condition of your Vacuum Shutoff Diaphragm is going to make a difference. I am thinking that an new Shutoff (with the nice new well sealed Diaphragm) might shut down earlier than an older one.

Now I've had to come clean(!) perhaps you can find that post? I'm sorry - and please don't take offence - I think something else might be influencing this sort of thing too...

...I'll be looking for the post as well of course - I think I might remember the one you mean 'cos I was wondering if the pressure in the IP could be measured in additon to the crankcase pressure (I think Kerry was involved in the conversation too)...

Stretch · #12 01-06-2013, 11:58 PM

Has Anyone Used an Electric Pump to Suck out Blow-by and Decrease Oil Leaks?

Diesel911 · #13 01-07-2013, 12:42 AM

Quote:

Originally Posted by Stretch

Has Anyone Used an Electric Pump to Suck out Blow-by and Decrease Oil Leaks?

Well it still does not answer your question as to what the Mercedes Service Manual would have to say.

Thanks for digging that up; it is in post #37

"At idle speed I removed the Rubber Elbow from the top of the Valve Cover and covered and started counting 1000 one; 1000 two and so on until the Engine Shut Down.
It took 10 seconds for the Engine to shut down at an approximate pressure of 3.1 psig."

The inside of the Fuel Injection Pump is exposed to the crankcase pressure by way of the area in front of the IP that the Oil drains out of and one side of the Shutoff Diaphragm is also exposed to the same pressure.

1project2many · #14 01-07-2013, 02:08 AM

Quote:

Welcome to the forum 1project2many.

Thank you.

Quote:

Thanks for your input - my reason for asking the question about crankcase pressure is actually to do with your first statement. I'd like to do some calculations and then some measurements to see if this actually happens - I'm sorry to say that I'm not a believer just yet.

Well...

A very simple test would be remove the vacuum line from the shutoff diaphragm and install a low pressure pressure gauge (similar to an old carburetor fuel pump gauge) with as short a length hose as possible directly to the shutoff diaphragm. Then, start the engine and plug the vent. Because the gauge and diaphragm are now a closed system, if there's any connection between crankcase pressure and the diaphragm you should see a pressure reading when pressure builds in the crankcase.

Stretch · #15 01-07-2013, 03:39 AM

Quote:

Originally Posted by Diesel911

Well it still does not answer your question as to what the Mercedes Service Manual would have to say.

Thanks for digging that up; it is in post #37

"At idle speed I removed the Rubber Elbow from the top of the Valve Cover and covered and started counting 1000 one; 1000 two and so on until the Engine Shut Down.
It took 10 seconds for the Engine to shut down at an approximate pressure of 3.1 psig."

The inside of the Fuel Injection Pump is exposed to the crankcase pressure by way of the area in front of the IP that the Oil drains out of and one side of the Shutoff Diaphragm is also exposed to the same pressure.

Can you clarify if this happened at idle?

I only ask because "people say" (I will provide a reference in a bit honest!) that at low engine speeds the piston rings are not as effective as at high engine speeds => so they are meant to seal better at high revs; but because the engine is pumping a greater volume of air through itself at higher engine speeds blow by is more likely to be greater at high revs. This isn't the full story though because it doesn't include the effect of engine loading which can also induce greater blow by...

...after all that (deep breath!) what was happening to and within the engine when the measurement was made must be considered as well.

Have you had problems with the engine conking out under different conditions? Or have you just noticed this at idle.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode