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  #1  
Old 09-22-2016, 10:36 PM
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How reliable is loosening the oil cap as diagnostic tool?

I have a 617 engine shake at idle (84 300SD) and intend to do some diagnosing. I thought there was a compression gauge in the shop but can't (haven't) find it.

How effective is loosening the oil cap as a method of testing bow by and by inference, compression?

Also, I remember someone rigging an electronic method of checking IP timing but haven't found it. Anyone have it book marked?

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  #2  
Old 09-22-2016, 10:43 PM
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I think it's a silly test myself. What does it tell you? The engine has been driven around for 30 years. There's no precision measurement to be taken, no stats to compute, just "Oh, I guess it didn't roll off the assembly line yesterday after all."

I mean, I guess if you found one that didn't have that then you'd know it might be worth the fat roll of hundreds. That's about it though.

-Rog
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  #3  
Old 09-22-2016, 11:25 PM
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Quote:
Originally Posted by Rogviler View Post
I think it's a silly test myself. What does it tell you? The engine has been driven around for 30 years. There's no precision measurement to be taken, no stats to compute, just "Oh, I guess it didn't roll off the assembly line yesterday after all."

I mean, I guess if you found one that didn't have that then you'd know it might be worth the fat roll of hundreds. That's about it though.

-Rog
That, and the other end of the spectrum. If the cap flies off followed by plumes of oil spray, you have a ring or piston slop issue.
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  #4  
Old 09-22-2016, 11:35 PM
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Years ago there was a thread on here about fitting a 30 gallon garbage bag over the oil fill hole and measuring how long it would take to fill up the bag.

I'm not making this up, LOL.
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  #5  
Old 09-23-2016, 12:24 AM
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Essentially if your Engine runs the Car OK blow-by is only a nuisance.

I believe there is only 2 fixes depending on the cause.

If Piston Rings are sticking there is some possibility that instilling something like Marvel Mystery Oil (or something else) into the Cylinders and letting it work for a week or 2 might free up the Piston Rings.

If it is cylinder, piston ring and piston wear is the cause then the fix is to overhaul that area.

I believe a few have reduced blow-by by replacing the Valve Stem Seals but most including myself did not have that happen

The nuisance issue can be mitigated by piping the blow-by someplace other then inside of the Air Filter Housing; at least that is what I did on mine.
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  #6  
Old 09-23-2016, 01:34 AM
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How long does it take to start without glowing? That's a general compression test. Just make sure your valve clearances are ok of course...
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  #7  
Old 09-23-2016, 04:23 AM
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Blow a Bag of Blow By

AHHH.... here is the blow bag thread.


Engine blow-by test, Blow a bag of blow-by

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.

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  #8  
Old 09-23-2016, 05:59 AM
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Its a general test of condition of the motor. Less blowby is an indication of less wear but they'll run a good long time with lots of blowby. Its a point for negotiation too.
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  #9  
Old 09-23-2016, 08:33 AM
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Quote:
Originally Posted by t walgamuth View Post
Its a general test of condition of the motor. Less blowby is an indication of less wear but they'll run a good long time with lots of blowby. Its a point for negotiation too.
There is a test used by Volvo enthusiasts to cover the oil filler hole with a rubber glove to determine condition of the PCV system.

But nobody uses a garbage bag to do anything like that, that I know of.
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  #10  
Old 09-23-2016, 09:26 AM
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It was a link to a youtube video if I remember correctly.

Quote:
Originally Posted by Palangi View Post
Years ago there was a thread on here about fitting a 30 gallon garbage bag over the oil fill hole and measuring how long it would take to fill up the bag.

I'm not making this up, LOL.
I always intended to do that test but I was torn up about fixing stuff that would ruin my engine. Vacuum pump, injectors spraying a stream and cutting a piston in half and all the other drama we love so much.

I think I'll run that test when I get this 617 turbo running in the 1985 SE body.
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  #11  
Old 09-23-2016, 09:44 AM
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Take a strip of paper inch wide or less, hold it where it does not go completely across the opening, holding it with your thumb on one side. at an idle, it should 'flap' up & down, in & out of fill opening. If you have a lot of blow by, it flaps up more than it flaps down. Never tried it on my 300 but it worked on my old V-Dubs & MGA's. Simple enough to try, , , ,

-c-
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  #12  
Old 09-23-2016, 09:45 AM
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Thanks to Charmalu for retrieving that thread....

I would to add that MB actually has a machine which measures the blowby in the crankcase......

but that does not mean that the garbage bag test provides any useful information.
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  #13  
Old 09-23-2016, 11:03 AM
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Here's how Cummins does it on the 6BT's.

Measuring Blow by, The Real Test For Internal Engine Condition - Seaboard Marine

Quite similar to the plastic bag method but just a little more "professional".

The old school method was to remove the dipstick, place a tightly fitting hose over the tube, drop the end of the hose into a bucket of water and observe the bubbles. The amount and frequency of the bubbles would give you a pretty good indication of the amount of blow-by.

Small, steady, slow bubbles meant good
Erratic, uneven bubbles meant at least one weak cylinder
Frothing like an Alka-seltzer meant you have effectively created a 2 stroke from a 4 stroke design.
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  #14  
Old 09-23-2016, 11:31 AM
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I guess it would be a good way to check for blockage in the crankcase ventilation system.
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  #15  
Old 09-23-2016, 11:48 AM
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Not really sure how useful it is, the 77 240D engine that is being put into a 83 300D body for my dad has tons of blowby by the cap test but runs great, he says it has better performance than his 83 240D engine ever had. I could do 85-86 flat out in that car (the 77 the engine came from) with the auto, got it to 95 once.

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