w114 loud knock from motor when cold, above 20 mph - Mercedes-Benz Forum

 
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post #1 of 9 (permalink) Old 12-07-2011, 01:55 PM Thread Starter
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w114 loud knock from motor when cold, above 20 mph

Thanks for any thoughts & suggestions, this one will be hard to reproduce for the mechanic given the temperature correlation.

1969 w114 (250 / 8), standard i-6 with the two Zeniths.

As the season has grown colder each morning, I have noticed a repetitive noise, I'd say best described as a knocking, coming from the engine compartment only at speeds above 20 mph and thereafter proportional in frequency to engine revolutions. This noise, until now, has been quiet enough to be concerning but only mildly so, because it goes away completely when the motor is fully warmed up (runs at around 160-175 F). I never noticed it during the summer months, even with the engine cold.

My morning ritual is as follows. I start the engine with pre-crank pump of the throttle, and allow the chokes to get the engine speed up before revving the engine down to a low idle. Takes it about 60-120 seconds. Always starts right up and idles very smoothly.

By the time I am taking off, the temp gauge is just starting to come off the low peg at 100F. During this time until it hit operating temp, I've recently heard the knock, but very low, and only until it warmed all the way up.

Lately it's been much colder, this AM was 30 F outside. (This is cold for California!) I started her up, she ran well and as usual through the warmup procedure. But once I hit 20 on my street, the same knock occurred except much louder this time, really disconcertingly loud. I immediately turned around and parked her back at home. The car was truly cold, as it had been about 4 days since she was last driven.

Would really like to hear thoughts on what the cause of this symptom could be. I have no other issues with the motor or drive system at all.

Thank you!

1969 W114 250 /8
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post #2 of 9 (permalink) Old 12-07-2011, 01:57 PM
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Thanks for any thoughts & suggestions, this one will be hard to reproduce for the mechanic given the temperature correlation.

1969 w114 (250 / 8), standard i-6 with the two Zeniths.

As the season has grown colder each morning, I have noticed a repetitive noise, I'd say best described as a knocking, coming from the engine compartment only at speeds above 20 mph and thereafter proportional in frequency to engine revolutions. This noise, until now, has been quiet enough to be concerning but only mildly so, because it goes away completely when the motor is fully warmed up (runs at around 160-175 F). I never noticed it during the summer months, even with the engine cold.

My morning ritual is as follows. I start the engine with pre-crank pump of the throttle, and allow the chokes to get the engine speed up before revving the engine down to a low idle. Takes it about 60-120 seconds. Always starts right up and idles very smoothly.

By the time I am taking off, the temp gauge is just starting to come off the low peg at 100F. During this time until it hit operating temp, I've recently heard the knock, but very low, and only until it warmed all the way up.

Lately it's been much colder, this AM was 30 F outside. (This is cold for California!) I started her up, she ran well and as usual through the warmup procedure. But once I hit 20 on my street, the same knock occurred except much louder this time, really disconcertingly loud. I immediately turned around and parked her back at home. The car was truly cold, as it had been about 4 days since she was last driven.

Would really like to hear thoughts on what the cause of this symptom could be. I have no other issues with the motor or drive system at all.

Thank you!
If you remove the Accessory belt and run it for TWO minutes, is it still knocking?

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 #3 of 9 (permalink) Old 12-07-2011, 02:13 PM Thread Starter
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If you remove the Accessory belt and run it for TWO minutes, is it still knocking?
I will check that and see - thank you for the suggestion!

Please let me know why you think this might be relevant, or which accessory you are concerned about so I can increase my knowledge.

Thank you!

1969 W114 250 /8
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post #4 of 9 (permalink) Old 12-07-2011, 02:18 PM
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I will check that and see - thank you for the suggestion!

Please let me know why you think this might be relevant, or which accessory you are concerned about so I can increase my knowledge.

Thank you!
You are trying to see if the knock is in the motor, or on the outside.

If it does not knock, we can go from there. But until we find out where the knock is coming from, anything added suggestion or information will be useless. Not to mention confusing.

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 #5 of 9 (permalink) Old 12-07-2011, 02:20 PM Thread Starter
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OK will check and let you know thanks again

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post #6 of 9 (permalink) Old 12-08-2011, 01:24 PM
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Knock Knock

Is this a distinct knock or a rotating squeal that gains in speed as you accelerate? My 72 250 had a similar problem. We live in the mountains of AZ. and have already had our fourth snow of the season. My problem started when the weather got cold. On this particular day my car started perfectly as it normally does. I always let it warm up off the chokes, then put her gear and headed to my shop. As soon as I got off the side streets and onto the highway, at about 25 MPH I started to hear a rotating squealing semi banging noise that got louder and faster as I picked up speed, my shop is only 3 miles from home so I continued to the shop where we immediately put it on the lift and found the rotating noise coming from the center prop shaft support and bearing. Ordered up a new support and bearing, changed them out and problem solved. Hope this helps.
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post #7 of 9 (permalink) Old 12-08-2011, 02:49 PM Thread Starter
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Thanks for this reply - I am investigating and will definitely consider this as it sounds possibly related, although I realize after talking with a few mechanically inclined friends that my sound would be classified as a "tap" rather than a "knock," as "knocks" seem much more intense, and this one probably wouldn't count as a "squeal," although it matches in other ways what you had.

If it's not already obvious, I am your basic noob when it comes to mechanicals but I am doing my best to learn. What is a "center prop shaft"? I assume and hope you are not referring to a piston connecting rod, because if you are my day just got a lot sadder.

Thanks!

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post #8 of 9 (permalink) Old 12-08-2011, 03:08 PM Thread Starter
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Nevermind, always google before you ask. I see what you're talking about now - thanks again for your message, always good to get this info early, as I was not thinking about the drivetrain at all for this issue.

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post #9 of 9 (permalink) Old 12-08-2011, 03:16 PM
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Also note that this noise also sounded like it was coming from under the hood. Driveshaft noise can transmit / travel all over the car. Good luck.
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