1987 300d Engine Blow By Test...thoughts? - PeachParts Mercedes-Benz Forum

kelley312c · #1 01-07-2016, 12:45 AM

Hello Again All,

Tonight I decided to do quick blow by test...at operating temperature the cap does not dance, unless I squeeze the breather tube...take a look!

https://www.youtube.com/watch?v=SANw656Cbv8

Is this something I should be concerned about? A compression test was done 3 years ago and the numbers were 400,400,400,400,400,450.

vstech · #2 01-07-2016, 08:16 AM

Looks normal to me. W h y would you squeeze the hose?

moon161 · #3 01-07-2016, 10:53 AM

If it starts OK, blow by and compression are just an oil use/emission issue. Since the remedy is replacing the motor or rebuilding it, checking the blow-by or compression is more of a curiosity or used car evaluation thing.

Put another way, bad is defined by how often are you willing to top off the oil when the low level light comes on. If it is bad, you start to rule out leaks and look for motors. If it ain't bad, you got 99 problems, and that ain't one.

Mxfrank · #4 01-07-2016, 11:08 AM

The "dance test" is nonsense. On a turbodiesel car, probably on most cars, the cap will actually be held down by a slight vacuum, even at idle. If you gently lift the cap, you will be able to feel it being drawn back down. Your results are normal, and the idle sounds steady and strong. Don't look for trouble.

kelley312c · #5 01-07-2016, 11:15 AM

Thank you! I did this because I like the car and want to keep it in great shape, but don't want to put money into it if the engine is not healthy.

Oil consumption is incredibly low to non-existent. I have to top it off now, but that is due to a leaky vacuum pump gasket

Mxfrank · #6 01-07-2016, 09:22 PM

Quote:

Originally Posted by DenverW123

I would like to see evidence of this EVER happening on any engine that is working properly. Engines are designed to prevent or limit negative crankcase pressure because it would draw dust into the oil seals. VW and GM diesels for example have a crankcase depression regulator valve.
The only engines I have ever seen produce a measurable negative crankcase pressure are race engines with a high flow dry sump system, and even those are designed with a device to limit negative pressure.

If you want evidence, go outside and play with your own car. There is some negative pressure in most engines by design, including Mercedes. Try it...balance the cap as in the OP, and gently try to pluck it off. You'll find that it's held down by slight negative pressure. It's built into the design.

If you examine my photos on this page, you'll be able to see why:

Mercedes Valve Cover Breather

About halfway down, you'll find this photo of the vent valve:

http://www.coolcatcorp.com/Mercedes%20valve%20cover/IMG_1105.jpg

The hole that I've labeled "vacuum port" is a passage that's always open to the intake manifold. Thus, there is a slight negative pressure in the crankcase. If there is a lot of blowby, the diaphragm lifts and the pressure is offset by additional vacuum. If the engine is running normally and the breather is intact, the cap won't "dance".

It's not that some engines are designed this way, most are, and have been since the invention of PCV. Here's a quote from the GM's original 1965 patent for the crankcase depression regulator:

"In the usual case, the diaphragm actuated pressure regulator is arranged to maintain a depression, or less than atmospheric pressure, in the crankcase. Such a regulator is commonly referred to as a crankcase depression regulator. utilized, the crankcase fresh air inlet is commonly pro- vided with an orifice or other means of controlling the rate of fresh air inflow to the crankcase in order to provide for obtaining the desired crankcase depression with out excessive flow of gases through the engine crankcase."

https://www.google.com.ar/patents/US3263660?dq=3263660&hl=es-419&sa=X&ved=0ahUKEwjLqczU6JrKAhXFFh4KHeHmBiYQ6AEIIDAA

moon161 · #7 01-07-2016, 10:40 PM

On an NA engine the breather is connected to the intake manifold, which on average, at a particular location always have less than atmospheric pressure. This pressure difference is what draws air into the manifold.

A federal 61x also has a butterfly that develops the manifold vacuum that draws in the EGR flow.

Once the breather is plugged, the blow by stream can rattle or float the cap, fill a trashbag held over the oil cap, or with the oil cap left on, pressurize the crankcase until the shutoff valve cuts the fuel.

In my view, this is just a way to rule out a used engine before you buy. If you already drive it, your oil consumption will tell you whether or not blow by is a problem, and starting whether or not compression is a problem. It doesn't really change the rebuild/replace solution set if you're stuck w/ the problem.

barry12345 · #8 01-09-2016, 02:21 PM

If the engine is not consuming much base oil and starts and runs good. Basically all is usually well.

Some time ago I wondered why the crankcase developed more positive pressure with time as there seemed by examples like yours where everything else was well. My guess was it is old valve seals and some wear in the valve guides.

The intake valve guide clearances are not going to suck oil down them as the turbo charger keeps a positive pressure at the bottom of the valve guide. The intake manifold does not sit at a much of a negative or suction pressure even when the turbo is not providing pressure. At least compared to a gas engine.

Gas cars have a negative pressure there. That is why unlike many Mercedes gas cars changing the valve seals out gives little if any reward. The exhaust valve guide is subject to pressure at it's bottom basically as well. So one might see some exhaust gas component coming out of the filler hole as a result but not much..

One day I may think up a test but for now it really is not needed.

I am left to basically judge these engines by the current base oil consumption rate for expected remaining service in general. Plus how easily they start up cold in comparison to other known good examples if the fuel system and glow plugs are in decent shape.

Just the strange way I look at things.

BillGrissom · #9 01-09-2016, 06:05 PM

Excessive blow-by and oil consumption are different issues, but they usually go together because both are due to worn piston rings. The video says "good engine" to me, and regardless the OP would be shocked at the price of a rebuilt engine (~$8K). When the OM617 in my cars has worn rings, the crankcase tends to run pressurized, causing oil to seep out of gaskets and the PCV tubing. Indeed, they tend to leak more than worn gas engines do because the as-new blow-by is higher (higher cylinder pressures) and there is no assured vacuum to suck on the crankcase (via PCV). I don't know about the OP's OM603 diesel which I understand has a butterfly valve in the intake to build vacuum at idle. You can do the same in an OM617 by extending your air filter changes, since a used one builds more intake vacuum. That might be a poor-man's fix for oil seepage. Indeed, air filters filter better when used. Downside is it will limit your top-end power. Those with 1985 CA 300D's are forced to do so because the air filter costs $45 (I posted a Wix PN for stacking 2 cheaper filters).

babymog · #10 01-09-2016, 07:15 PM

No butterfly in the 603 intake, straight shot through the filter -> afm -> turbo -> intake.

vstech · #11 01-09-2016, 09:50 PM

Quote:

Originally Posted by babymog

No butterfly in the 603 intake, straight shot through the filter -> afm -> turbo -> intake.

True, however, there is a speed density baffle before the air filter stock on the 87 124's and 126's iirc...

babymog · #12 01-09-2016, 10:20 PM

Straight shot into the filter from the fender on the 300D/124.
Straight through on the OM603.960, not sure how the 602 and 606 are plumbed, for some reason they got fender louvers and the higher hp 603turbo did not, ...

Mxfrank · #13 01-10-2016, 06:07 PM

Straight shot is a gross oversimplification. The system consists of an inlet, an air box, a filter, a duct, possibly one or more bends, possibly a flow sensor. Each component causes frictional losses, and each bend causes dynamic losses. This resistance mans that static pressures inside the duct won't be the same as ambient atmospheric pressure. Experiment will verify this. If you use a manometer to measure in the accordion tube, you would find that static pressure at the vent junction is ALWAYS negative, usually between -.10 and -.50 psi, even if the filter is removed. It's true that you can't measure this low level of vacuum with a standard automotive gauge, which is designed to measure vacuum levels an order of magnitude higher. Nevertheless, it's a low pressure area and the breather tube is placed there to maintain crankcase vacuum. That's why the cap won't dance.

Again, the best public domain explanation is from a GM patent (3754538), While the patent applies to a shutdown device for locomotive engines, the introductory description is still applies:

"In the operation of internal combustion engines and, specifically, large diesel engines used, for example, as power plants for diesel electric locomotives and the like, it is desirable to maintain a small vacuum in the crankcase under normal operating conditions. This has the advantage of reducing the amount of vapors maintained in the crankcase so that the possibility of a crankcase explosion is minimized. In addition, it provides a pressure differential which acts at the various seals and other openings in the crankcase to reduce the possibility of oil escaping from the interior of the engine."

Which is exactly the opposite of the Denver's theory of operation. You can either take the engineering as it is or believe the myths.

babymog · #14 01-10-2016, 08:50 PM

Frank, just read above, we're talking about whether there is a throttle butterfly or other device designed to inhibit flow, not about how much friction is induced by turbulence or bends. I'm not sure how this relates locomotive engines or who asked.

vstech · #15 01-10-2016, 09:54 PM

Quote:

Originally Posted by babymog

Straight shot into the filter from the fender on the 300D/124.
Straight through on the OM603.960, not sure how the 602 and 606 are plumbed, for some reason they got fender louvers and the higher hp 603turbo did not, ...

I do not agree here...

There is a baffle under the air filter on the w124 OM603 intake.

It is most certainly NOT an unobstructed path.

The air filter sits on top of a flow meter with a flap that must open for air to enter the air filter.

It certainly is not a throttle, but it is an obstruction. It is a speed density flap that is used to calculate intake air I think... Part of the EGR system I believe.