Engine blow-by test, Blow a bag of blow-by - Page 2 - PeachParts Mercedes-Benz Forum

vstech · #16 03-14-2010, 08:31 PM

Quote:

Originally Posted by leathermang

I hope you guys are having fun playing around with these garbage bags because they are not giving you any useful information.

Hmm. really?
blowby filling a bag is nothing worth measuring?
K.
what's important? the origination of the blowby? the amount of oil suspended in the vapor? MB has a device to measure volume of blowby... wouldn't a trashbag chart equal some sort of ruler?

it's gotta be better than those that plug the tube with thumb and time how long it takes to stall the car...

leathermang · #17 03-14-2010, 09:39 PM

Quote:

Originally Posted by vstech

Hmm. really?
blowby filling a bag is nothing worth measuring?
K.
what's important? the origination of the blowby? the amount of oil suspended in the vapor? MB has a device to measure volume of blowby... wouldn't a trashbag chart equal some sort of ruler?

it's gotta be better than those that plug the tube with thumb and time how long it takes to stall the car...

I did not say that plugging the tube with your thumb and seeing how long it takes to stall the car is of any use either...

but just for the sake of discussion...

Lets say we standardize the size of the bags...
and the amount each guy takes to cover the oil fill hole..
and the barometric pressure on the day it is taken..
and the temperature of the blowby ...
and the exact rpms the engine is turning when the measurement is taken..

So TEN guys do this and post their results.. and we agree those test conditions are equal... and they each tell you the amount of time it took to blow the bag up to an agreed upon tightness...

So the reports come back :

10 seconds
30 seconds
45 seconds
1 minute
2 minutes
3 minutes
4 minutes
5 minutes
6 minutes
7 minutes

So you describe to me what you know either about the bunch, or any one particular person or WHATEVER... convince me there is some useful information in there to or about someone's car...

In the mean time... if someone wants to know the condition of their engine they should try a compression test... which they can compare to the specs listed in the Factory Shop Manual... and if the compression shows any fault then a leak down test should isolate the problem to either the upper or lower portion of the engine.

pch2021 · #18 03-14-2010, 09:57 PM

1.The correct test for engine wear is always based on oil consumption.

2. Where in the factory service manual does it refer to using a garbage bag?

3. Diesel all have some blow by. Especially at higher mileage. I have seen plenty of engines that have blow by, but all use minimal amounts of oil [1qt/1000 miles or so]

4. What's the next test? Sticking a potato in the exhasut and seeing how long it takes the engine to blow up?

vstech · #19 03-14-2010, 10:03 PM

naw, oil consumption is pretty vague also.
is the motor leaking oil? is the motor smoking?
with the garbage bag test, you are filling a known size container from the output of the vent tube.
of those 10 results, I'd say the last 5 are holding the bag wrong, and their test is invalid. the first 3 have problems that need to be looked into with a compression and leak down test. 4 and 5 sound normal to me.

7 minutes to fill the bag? wow, that's one tight motor!

leathermang · #20 03-14-2010, 10:08 PM

LOL, I agree completely...

whunter · #21 03-14-2010, 11:09 PM

Quote:

Originally Posted by leathermang

In the mean time... if someone wants to know the condition of their engine they should try a compression test... which they can compare to the specs listed in the Factory Shop Manual... and if the compression shows any fault then a leak down test should isolate the problem to either the upper or lower portion of the engine.

I half agree..
IMO; Dealing with blow-by, you need a compression and leak-down test for best diagnostic data.

Further data directed to other members..

In a perfect world:
Every mechanic would also pull the pre-chambers and inspect the cylinder bore, piston head, and valves with a Micro Inspection Camera.
Some shops do offer this, but charge extra $,$$$.$$ for the service.
Most shops have no desire to offer this diagnostic service...

Beyond determining minor or serious, trying to quantify blow-by is a zero value issue.
You know it needs to be fixed.

This is a good option to try:
ADDING MARVEL MYSTERY OIL® TO ENGINE OIL
http://www.marvelmysteryoil.com/index.php/site/faq/

Danger, not recommended:
I have seen shade-tree mechanics salvage and/or wreck diesel engines with water sprayed into the intake.
My general experience has been 2/3 wrecked, 1/3 survived running better for a year or two before catastrophic failure...
It can work, but the odds are against you..

pch2021 · #22 03-16-2010, 08:09 PM

I've decided to retract my terrible opinion on this very insightful and wise test!
Here are the reasons I was wrong in the first place:

1. How foolish I am for not assessing the longevity and performance of my engine with a plastic bag! Daimler Benz engineers should have been doing this! They obviously had no clue how to diagnose an engine!

2. How stupidly vague oil consumption figures are! Shame on Daimler Benz , in their finite wisdom, for telling us that if an engine burns X number of quarts in so many miles, there must be an issue.

3. I thoroughly expect an engine compressing its fuel at 21 Bar to keep all of the combustion pressure and gas in the cylinders. If it can't, and is pumping it all into the bag, it must barely be running! I should rebuild my engine until it pumps nothing into the bag.

4. What if we equipped all the engines with blowbyometer bags? We would be able to judge them by how quickly the bags fill up. We should have done this sooner.

please forgive me for discounting the valuable diagnostic test!

79Mercy · #23 03-16-2010, 08:11 PM

Pierre, I am truthfully offended by your lack of ethical insight. How dare you....

vstech · #24 03-16-2010, 08:17 PM

ohhhhkayyy,
I get it.
MB official diagnostic procedures are correct.
the thing is, someone buying an old MB, has very little in the way of proof for discerning a motor's worth. blowby is a finite issue that can be measured. true true true, it's far from a definitive meter, but truly, it's gotta be better than the "teakettle" test...
I'll hardly use it myself, I have the tools and skills to do a compression test, measure the sag of the motor mounts, and inspect the various parts of the car to see what it's really worth.
but
does everybody have these skills and tools?
howmany have a trash bag...

pch2021 · #25 03-16-2010, 08:18 PM

Best compression test: Start with no glow plugs.

vstech · #26 03-16-2010, 08:33 PM

Quote:

Originally Posted by pch2021

Best compression test: Start with no glow plugs.

yup, starting (or attempting to start) a diesel with out using the glow plugs (or even disconnecting them to ensure they are not afterglowing from a cut wire...) will sure show a motor with perfect compression right away.
but a motor with good compression 350 or so on all pots when it's been 30 at night, and attempting to start it will fail 99% of the time. what did that show?
IDI motors need glow plugs to start.
starting an older motor without using the plugs is not wise.
starting a cold, STONE cold not run in 12 hours cold, with a single 10-15second glow will show safely how good the motor starts. it'll show weak pots, (by exibiting stumble, and smoke)
how often do scammers have the car totally warmed up before you get there, so it will start smoothly... even if you specifically insist on a cold startup.
I got an SD that was stone cold, started up great with no glow cycle, but the motor smokes like crazy. I'm sure it has bad valve seals, or crap injectors, it might even have an out of time IP or a badly worn timing chain.
all of these things I can evaluate.

leathermang · #27 03-16-2010, 09:12 PM

UH>>>>Starting without glow plugs in...
BE SURE TO CLEAN OUT ... PROBABLY REAM AND CLEAN THAT GLOW PLUG HOLE SO NO JUNK GETS SUCKED INTO THE BORE....

kerry · #28 03-16-2010, 09:50 PM

I don't think anyone ever argued that the garbage bag test was a substitute for a compression or leak down test. If my aging memory serves me correctly the garbage bag test was invented as a substitute which was being advocated on here about 8 years ago or so. It was to plug the PCV hose and count how many seconds before the engine shut down from the internal pressure pushing out the shut off diaphragm. People pointed out that that particular test could either blow out seals or be useless if seals had already failed.

leathermang · #29 03-16-2010, 11:24 PM

The ''purpose' may have been to give an alternative to the plug the hose test... but it provided nothing useful at all...

kerry · #30 03-17-2010, 12:26 AM

Quote:

Originally Posted by leathermang

The ''purpose' may have been to give an alternative to the plug the hose test... but it provided nothing useful at all...

Not yet, but the database is pretty small so far. It's not out of the question that if the database expands, useful information may be gained. For instance, if we were to learn that a car with a one minute garbage bag started fine at 10 below zero and a car with a 3 minute garbage bag didn't or vice versa, that might be helpful data. Or, if the database developed to the point of a clear correlation between bag speeds and compression readings that data could also be useful. Whether data is useful or not cannot be definitively determined in advance.