Low compression on 4,5&6 - Page 2 - Mercedes-Benz Forum

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post #11 of 20 (permalink) Old 11-06-2011, 06:53 AM Thread Starter
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Originally Posted by flyboyike View Post
If you suspect the carbs, do a dry test (no fuel) and see if the difference is just as dramatic.
Could you elaborate a little on doing a dry test,,, pull the fuel lines from the carbs? Remove the carbs?
How would having gas vs no gas change the compression?

Yesterday I did a "dry" test, as I put no oil in the cylinder. I will do a wet test today to check the difference. My engine trouble shooting book says if the compression rises after having added a squirt of oil to the cylinder, the issue is rings, if no change, then valves/head gasket/carb adjustment.

Thanks for your direction.
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post #12 of 20 (permalink) Old 11-06-2011, 09:01 AM
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Could you elaborate a little on doing a dry test,,, pull the fuel lines from the carbs? Remove the carbs?
How would having gas vs no gas change the compression?

You could just crimp the fuel line. I'm sorry, is your question basically whether there is a compressibility difference between straight air and air/fuel mixture?

Felicita e un bicchiere di vino con un panino.
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post #13 of 20 (permalink) Old 11-06-2011, 03:26 PM Thread Starter
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After reading more, I have come across my issue. I made the mistake of not holding the accelerator pedal down while doing the test. I will redo the test again and report back.
Thank you so far flyboyike for asking me the right questions to make me think. It is not the compressibility of fuel and air vs. air, as much as it would be the carburetor restricting the motors ability to breath freely. (because of my error)
I think that it does show also that the carbs are out I sync. I plan to eventually rebuild/tune up/ or convert to webbers. Which is the reason for checking the compression first.
Again, I will report back with my findings.
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post #14 of 20 (permalink) Old 11-06-2011, 03:37 PM
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How exactly are you doing the test for compression?

Top five reasons I posted in your topic:

1. You're talking about god, there has never been any god of any kind. EVER!

2. I just have a general dislike of you.

3. I am trying to tell you how to fix your Mercedes and you are not listening.

4. I'm flirting with Jody.

5. You're just to dumb to own a Mercedes, and should go buy a Honda.
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post #15 of 20 (permalink) Old 11-06-2011, 03:44 PM Thread Starter
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How exactly are you doing the test for compression?
All plugs out. Cold motor. Coil lead removed. Using a standard gauge type tester. One at a time. 8 revolutions of the motor for each. Battery cranked evenly throughout all cylinders. It was a brain fart for not stepping on the gas to open up the carbs. Was reading it needs to be at a minimum, half open throttle.
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post #16 of 20 (permalink) Old 11-06-2011, 04:02 PM
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Originally Posted by alvector View Post
All plugs out. Cold motor. Coil lead removed. Using a standard gauge type tester. One at a time. 8 revolutions of the motor for each. Battery cranked evenly throughout all cylinders. It was a brain fart for not stepping on the gas to open up the carbs. Was reading it needs to be at a minimum, half open throttle.
lol. I see you know what you are doing.

Some people also forget to take out the plugs. Ahh.. the mis-information that is the internet.

Might help if you try lubing up the cylinders a little.

Top five reasons I posted in your topic:

1. You're talking about god, there has never been any god of any kind. EVER!

2. I just have a general dislike of you.

3. I am trying to tell you how to fix your Mercedes and you are not listening.

4. I'm flirting with Jody.

5. You're just to dumb to own a Mercedes, and should go buy a Honda.
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post #17 of 20 (permalink) Old 11-13-2011, 05:34 PM Thread Starter
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I know enough to get myself in to trouble :0)
I went out and retested just cylinder number 5 at WOT and got the same result. I pulled the valve cover off to recheck my valve adjustment (It was my first on a benz) I noticed some bad wear that i didn't notice the last time. (it is bad enough that it was there the last time, but went unnoticed. What I'm saying is that it couldn't have occurred because I adjusted the valves. I didn't change them that much!!!) I will upload a picture of the #5 exhaust valve. all the others look fine (nothing out of the ordinary like #5).

I have it in the back of my head that I should just pull the cam out and lay some beads of weld and grind it on the bench grinder!! After all it is a $1000.00 car with low compression.

A local guy has a 71 250 with a m130 (same as mine) for cheap enough to pick it up for parts. I would have to weight my options if I do buy it, as far as do i do a full motor/tranny swap or just a head swap or ,,,,,
old man Gottlieb lead me!! :0)
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post #18 of 20 (permalink) Old 11-13-2011, 05:55 PM Thread Starter
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post #19 of 20 (permalink) Old 11-13-2011, 07:17 PM
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Oooo flat cam lobe,
Looks like a time to pull the head for the complete valve job and cylinder wall check-up. and on and on until it is correct.

Brian O
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post #20 of 20 (permalink) Old 11-14-2011, 03:41 AM
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Originally Posted by alvector View Post
Cold motor.
I always learnt that compression tests should be done with a warm engine.

A compression leak test should reveal the culprit of the low compression, but you need a supply of air (from a air compressor) and a reduction valve so the air pressure is at around 2 bar.

A worn cam in itself would not cause low compression, it can restrict air flow a bit if it is a intake valve cam, because the valve doesn't open completely.
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