I need help to find out what is happen to my 68 250 - Mercedes-Benz Forum

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post #1 of 15 (permalink) Old 03-21-2010, 09:18 PM Thread Starter
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I need help to find out what is happen to my 68 250

I took my 68 250 out for a ride in January and was hitting speeds of 80 to 90 mph and after 30 miles of driving, it started loosing power a knocking sound coming from the hood so turn the motor off. I waited to check the engine oil and had gain more fliud and thinner than what was inside before.I started and it started drove with power again but there is a knocking from under the engine. Can some tell me what is wrong.
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post #2 of 15 (permalink) Old 03-22-2010, 04:43 AM
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try to flush out the old oild, and refill it...maybe ur old oil broke down ....? any signs of water in the oil?

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post #3 of 15 (permalink) Old 03-22-2010, 08:19 AM Thread Starter
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There is something wrong in the engine

I just started the car today after 2 months sitting and it started right up.The temperature is normal and I notice tiny bubles in the radiator and the engine oil dip stick. Now the engine is running very smooth on a normal idle, but when i go higher on the idle it brings out heavy knocking sound from the front buttom of the engine. What could this be?what should I do?
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post #4 of 15 (permalink) Old 03-22-2010, 11:13 AM
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knock

That may be a bad rod bearing knock, pull the valve cover off and check for a bad cam due to lack of oil. is this a 250S or SE(carbs or injection?)
Time for a used good motor.

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post #5 of 15 (permalink) Old 03-22-2010, 12:52 PM
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Could be the head gasket go to a shop and let's check if there is gas in the cooling water
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post #6 of 15 (permalink) Old 03-22-2010, 05:04 PM
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Could you determine if it is a valve train noise or a lower end noise? Completely different sounds. What's the oil pressure like? On my 250S I had a similar problem and I recognized it to be a top end noise. Oil pressure was fine, and the engine was running on all cylinders, so I just slowed way down and babied it home. Turns out it was some of the top end (rocker arm) oiling tube oil feed holes were plugged up with caked on oil. Checked the affected rockers and cleaned the tube and it's worked fine since.
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post #7 of 15 (permalink) Old 03-22-2010, 05:57 PM Thread Starter
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I don't know how to determine the oil pressure. The engine still starts normal and sound actually comes from middle of the engine to the bottom only when I step on the gas padel.this is a carburator engine. there is no signs of smoke or over heating.
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post #8 of 15 (permalink) Old 03-22-2010, 09:34 PM
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Joey, isn't the oil pressure gauge in the instrument cluster working? It should go all the way to the maximum when driving and down to around 15 PSI at idle, when the engine is warm. What does the oil look like? Is it foamy or milky looking, or is it clean, fresh and "oily" looking? A lower end (rod or main bearing) noise will be at engine speed (RPM) and sound/feel like a dull thud, almost like someone is hammering on something. It will be more pronounced under load, and a little quieter when you let off the throttle. So, on and off throttle the sound and volume will be different. A top end (valve train) noise will be at about half engine speed (RPM), higher pitched tapping type of noise. If it is a valve noise, and there are multiple cylinders involved, the noise may be continuous as opposed to intermittant when the affected cylinder valves of one or two are closed. All this being said, these engines are quite bullet proof and usually don't encounter major failures unless they have a zillion miles on them, they've been run low or out of oil, or have been grossly neglected over a long period of time. Let's check the easy stuff first....oil status (pull the dipstick to check) as indicated above, and oil pressure. If that's OK, pull the valve cover (two bolts) and take a look at the cam and valve train. Everything should be evenly wet with oil (all rocker arms and cam lobes) and nothing obviously broken or sloppy loose. The cam lobes should all me mostly shiny with maybe slight oily discoloration, and uniform...no flat, dull or worn looking lobes. You can jiggle each rocker arm. Some will be tight and some should be very slightly loose. None should be excessively loose. If everything is dry and crusty, you need to find out why. There is an oiling tube that runs the length of the engine and squirts oil onto the valve train. The oiling holes sometimes get plugged up. If it's dry you may want to have someone start the engine briefly to see if oil is coming out of these holes. Don't run it too long or you'll have a mess if it is oiling properly. If all that is OK, put the valve cover back on and loosen each spark plug wire at the spark plug. Use an insulated plug wire puller or non damaging insulated pliers and have someone start the engine. CAUTION: If you don't use insulated pliers, you'll get zapped real good. Have them accelerate the engine to the point where the noise is apparent, and then, one at a time, pull each plug wire, listen and then replace. The noise will go away or get considerably quieter when you pull the wire from the affected cylinder. Let's hope this is not the problem. Let us know how this all goes.

Last edited by mbbuff; 03-22-2010 at 09:38 PM.
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post #9 of 15 (permalink) Old 03-22-2010, 10:17 PM Thread Starter
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Thanks everyone , I will try all of the above and I will keep you posted on the progress
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post #10 of 15 (permalink) Old 03-23-2010, 12:58 PM
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I'm guessing head gasket...coolant in the oil and oil in the coolant.

Former MB: "Miss Juliana" - 1972 280SEL 4.5
Current MB: 2013 C250
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