Help me understand some things about the infamous engine diagnostic...

What exactly is the gas escaping from the top of the crankcase? It isn't all from air escaping past the rings...it looks like the vacuum pump exhausts into the crankcase as well.

What is the white smoke? It smells like oil...is it...or could it be unburnt diesel?

My engine has more than normal white smoke escaping from the top of the crankcase, but passes the 'tea-kettle' oil cap test just fine, even with the plastic oil cap. Engine runs just fine. Does this mean something else?

Thanks,
dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

IIRC it's oil vapors and combustion chamber gasses that make it past the rings during the compression stroke.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Some blow-by is normal even on fresh re-built engines. As they wear, combustion gases do escape and increases the level on the gases.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section

The Vacuum Pump, sucks from the Vacuum System and Discharges into the Crankcase.......soooooo.......if you have a large Vacuum leak, anywhere in the car.....the Atmosphere that enters at the leak will exit out of the Valve Cover vent pipe to the Oil Separator located within the Air Cleaner.......

More Vacuum Leaks, More volume from Vent......

SB

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG

That makes a lot of sense.

If an engine hasnt been run for a while, once it is run for a while could blow by decrease at the rings get used more and maybe some of the crap is burned off?

How can the oily blow by be reduced other than a ring job? Is there anything that could be done?

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Here is a trick that may reduce some of the oil in the blow-by off the valve cover vent. Sometimes the oil return galleys from under the valve cover on older engines get clogged up like arteries to and from your heart. This prevents the oil from returning back down to the oil sump fast enough and the valve cover gets flooded out. The extra oil gets splashed up into the oil seperator in the cover and goes out with the Blow-By.

An old gear head told me to use a guitar string (low E with the Brass wire wrapped around it) like a long pipe cleaner and hone out the crud/tartar from the return oil galleys. You can run the guitar string all the way through to the sump in some holes. Then he said to run the engine to flush all the crud you knock lose to the pan and then change the oil. It works great if thats your problem....and can reduce smokey exhaust and oil consumption.
Hope that helps!
Cheers

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FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K

Fred,

Thanks for the suggestion. I will try this at some point.

Its very interesting to hear about these 'other causes' of blow-by.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

THANK YOU! this explains my easily starting motor with tons of air coming from the oil filler cap!

Folks, besides my Diesel Chief Engineer License, I am also a Steam C/E......and when working around steam you must learn a bit about Vacuum.....

When evacuating something like a condenser, or in this case the Car's Vacuum System (Engine Controls, Brake Assist, AC Climate control, Door Locks, etc.)....if the system is tight you will have a large volume of atmosphere pumped from the system, gradually diminishing until your maximum vacuum is attained, probably about 25-26 in/hg......if you are pumping from a good, tight system there should be very little atmosphere to be removed after this point.....However if you have leaks, whatever the volume of atmosphere that the leak or leaks will allow, will continue to be pumped (evacuated) from the system....

I don't know if this was a design consideration at MB, (the discharge of the vacuum pump to the crankcase) but.....on larger Diesels it's a good idea to maintain a very slight positive pressure within the crankcase....this helps to eliminate some of the contributing factors that will allow a crankcase explosion to occur.....large Diesels also have explosion doors that will rapidly relieve internal pressures in an attempt to minimize structural failure and shrapnel....I have experienced 1 (one) crankcase explosion and an associated fire....and I never want to see another......

SB

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG

Unless you block the breather tube port on the valve cover, I don't see how the "tea-kettle" test signifies much and even then what is "too much". With it open, you would have some serious venting if the oil cap dances. The only test with accuracy here is a compression check.

Every diesel, car/truck, that I have seen vents a little grey smoke when you remove the oil cap on a warm engine. I believe this is due to the fact that oil is injected to the underside of the pistons for cooling. With this in mind, operating temp., or more importantly oil temp., would be the biggest factor on how much "smoking" is seen. Check your oil cooler after a long drive and see if it is warm.

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Sam

84 300SD 350K+ miles ( Blue Belle )

SD Blue, that is a great explanation and a great test. May be synthetic oil would reduce the smoking some since it resists higher temperatures.

Shorebilly, I assume you talk about ship diesels, What causes an explosion and why would it be any different if there is a slight over or under pressure. I would think that with under pressure the crankcase get vented and a possible explosive mixture would be burnt of because it is sucked into the combustion chambers. Overpressure would press the possible explosive mixture to the outside?

Thanks Johan

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85 MB 300TD with om616 4 spd
83 MB 300TD
83 MB 240D scrapped
82 MB 240D scrapped
97 Ford E350 diesel 4x4, WVO
94 Ford E350 4x4 diesel blown head..parts car
85 cherokee diesel 2.1, 300.000, WVO
85 cherokee diesel 2.1
85 lebaron convertible, to be electric
85 lada niva 4x4 with 1.9 peugeot diesel,
72 citroen DS 5spd, LPG
77 Jaguar XJS, LPG
73 corvette 350 auto
88 MB Gelande 240 GD military
98 Ford E350 mini bus to be new 4x4 camper

A lot of these older engines seem to have some scary blow by yet they still run pretty well. I almost wonder if its a 617/616 crankcase vent design thing.

A tight engine should have slight suction, IE put a sheet of paper over the oil fill hole and it will slightly suck it down. For a demonstration try it on any 603/6 or modern MB gas engine.


I have seen a ton of 617's that do the tea kettle thing, and they all seem to run good. I have yet to see a 603/6 that does that. I almost wonder if its the crankcase vent system? I'm to lazy to look it up, but I wonder what the difference is. Anyone with a FSM that cares to look?

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

This never seemed to be a huge issue on pre -1970 Clean Air Act internal combustion engines that were loosely vented to the atmosphere. Air moved in and out freely neutrallizing crankcase pressure through vented oil caps, gas caps and even vented tappit covers on some exotics.
But even on that now antiique struff...as the tolerences decreased with age you had increasing amounts of blow-by and oil being forced by perfectly good engine seals no matter how well you maintained and paid attention to everything. The engines still held up pretty well inspite of occassional open street class runs down at the local drag strip on hot Thursdays during the summers.

Internal combustion engines are messy!......"Energy connot be distroyed...only transformed into a smoke plume and oil slick" I think that was Einstine mixed with a little BiG Daddy Roth and Ghoulardi!

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FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K

How much oil is lost through blow-by? Mine seems to get worse when it gets hot, actually releasing visible smoke from under the air cleaner (return tube from oil separator doesn't seal). I'd like to vent it into the exhaust behind the turbo instead..

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Ich liebe meine Autos!

1991 Mercedes-Benz 560SEL | Megasquirt MS3-Pro | 722.6 transmission w/ AMG paddles | Feind Motorsports Sway Bar | Stinger VIP Radar | AntiLaser Priority | PLX Wideband O2 | 150A Alternator | Cat Delete
1981 Mercedes-Benz 300SD | Blown engine, rebuilding someday...
1981 Mercedes-Benz 300SD | Rear ended, retired in garage.
2009 Yamaha AR230HO | Das Boot

Excessive speeding? It ain't excessive till I redline!

A crankcase explosion occurs within the engine's crankcase......usually caused by a very hot bearing, heating the lube above it's flash point.....this ignites all of the oil vapors within the engine.....and makes a very loud noise!!!! Occasionally, fuel oil mixed with the lube oil will either cause, and/or exacerbate the explosive mix.....

As the theory (safety/fire prevention) goes .....if you maintain a slight positive pressure (about the pressure that natural gas is in your stove) then....in theory the atmosphere within the crankcase will remain basically static and unmixed with oxygen, thus preventing the formation of a combustible mixture (fuel/air ratio).......if a slight vacuum were within the crankcase it would suck air/atmosphere into the crankcase at various points and allow a combustible mix to form.....and then when a source of ignition presents itself....Boom.....

Trust me, it would make the housekeeping/cleaning of the engines if this were not so.....the slight positive pressure pushes oil out of the engine....

On the side of every industrial engine that I have worked on, there is a Warning Sign.....that reads something like this....."The engine must be stopped for a minimum of 5 minutes before opening the inspection doors"

In the case that I was involved with....MAK engine....we had fuel contamination of the lube oil......this was known, and an oil change was denied by the "powers that be".......I had gone on record "she's gonna blow up, sooner or later"......and she did....when it did, it blew open all of the explosion doors, it blew the gasketing from behind the explosion doors, it also blew the cylinder liner seals from 3 of the cylinders (this allowed the jacket cooling water to flood the crankcase, this interrupted cooling to the rest of the engine).....it also ignited the bilges, FIRE big time!!, and was the only time I ever had to drop CO2 into the engine room......was a very amazing event.....

SB

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG