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  #1  
Old 08-02-2020, 03:00 PM
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rebuild a VCV?

Has anyone tried to rebuild the no longer available VCV on top of the injection pump of an 82 300D?



The valve leaks through the valve seat. Also through the opening around the actuating shaft that the spring is attached to inside the valve, although I don't think that's a problem.





The valve proper has a metal base which is crimped over the plastic top where the vacuum line attaches to it. I should be able to cut the crimp off and access the inside of the valve to repair the valve seat. I'm not sure how I'd put it back together, but I think a little silicone sealant should do it since there is a plastic ring that snaps over the valve to hold it in place.





Anyone find a workaround besides converting to a manual transmission?

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  #2  
Old 08-03-2020, 07:16 PM
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So I did verify that the VCV is the offending part. I took the valve apart and did a smoke test. You can see the holes in the diaphragm and where the smoke is leaking out.

This is the bottom of the valve proper. As you apply the accelerator, a linkage comes up and moves the arm on the outside of the valve body up, which turns the shaft inside the valve body. That raises a lever that pushes up on that brass part with the spring crimped inside it, which somehow unseats the valve and lets the vacuum out. Looks like something inside pushes down on that ball.

I'm not sure what that material is. Some kind of rubber I'm sure, but it's Mercedes voodoo rubber, so I'm wondering if there might be something better to seal it with. And, what material would make the best diaphragm.

If there was a dealer nearby I'd swing by and see if I couldn't get someone in the service department interested, they might have some secret Mercedes sealant that would plug the hole up for another 38 years.

I really think the best thing is to open it up and see about replacing the diaphragm.

Any thoughts?
Attached Thumbnails
rebuild a VCV?-holes-diaphragm.jpg   rebuild a VCV?-holes-diaphragm-ll.jpg   rebuild a VCV?-smoke-test.jpg   rebuild a VCV?-ball-check-thing.jpg  
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  #3  
Old 08-03-2020, 07:27 PM
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Thanks for posting this. Eventually this will become an issue for everyone.

I’ve heard of people using that liquid electrical tape from Home Depot on motorcycle carburetor diaphragms. I’m not personally familiar with its durability.

Too bad there isn’t a way to disassemble the entire thing and replace the diaphragm with a sheet of viton.
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  #4  
Old 08-08-2020, 12:03 AM
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Quote:
Originally Posted by Dkirk View Post
So I did verify that the VCV is the offending part. I took the valve apart and did a smoke test. You can see the holes in the diaphragm and where the smoke is leaking out.

This is the bottom of the valve proper. As you apply the accelerator, a linkage comes up and moves the arm on the outside of the valve body up, which turns the shaft inside the valve body. That raises a lever that pushes up on that brass part with the spring crimped inside it, which somehow unseats the valve and lets the vacuum out. Looks like something inside pushes down on that ball.

I'm not sure what that material is. Some kind of rubber I'm sure, but it's Mercedes voodoo rubber, so I'm wondering if there might be something better to seal it with. And, what material would make the best diaphragm.

If there was a dealer nearby I'd swing by and see if I couldn't get someone in the service department interested, they might have some secret Mercedes sealant that would plug the hole up for another 38 years.

I really think the best thing is to open it up and see about replacing the diaphragm.

Any thoughts?
For that little bit of a hole spray it off with brake cleaner and let is sit for 2 hours to be sure it all evaporates out of the rubber. Take a bit of silicon sealant the same type as you would use on your engine and fill in the hole with it and wipe it so it is nice and flush as it would have been if the real rubber was there. Then let it sit overnight and cure and the next day see if you think the results will work.

Silcone sealant usually gules stuff together if the areas have been degreased.

If you have any black wind shied sealant or stuff for sticking the rubber seals on doors that might also work.
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Old 08-08-2020, 08:12 AM
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Quote:
For that little bit of a hole spray it off with brake cleaner and let is sit for 2 hours to be sure it all evaporates out of the rubber. Take a bit of silicon sealant the same type as you would use on your engine and fill in the hole with it and wipe it so it is nice and flush as it would have been if the real rubber was there. Then let it sit overnight and cure and the next day see if you think the results will work.
Well, it wasn't a little bit of a hole! That thing is wore out! I think I will try this though, maybe with some door gasket adhesive. That seems like it would retain maximum flexibility. I tried sealing the holes with some RTV Blue, but it's difficult to get a nice thin coat, and I think I need to retain that feature in the middle of the gasket, that hump there, and the diaphragm is only 0.3mm, the rtv cured wasn't as flexible as the diaphragm itself.

Obviously, the thing to do is replace the diaphragm. I don't think I can do that and retain the hump in the middle of the diaphragm though. So whatever I use is going to have to stretch frequently.


Quote:
Too bad there isn’t a way to disassemble the entire thing and replace the diaphragm with a sheet of viton.
Having trouble locating Viton sheet in small sizes and .3mm thickness. I did locate some nitrile coated nylon. Do you think that would suit? I'm wondering if the nylon will stretch. I guess I'll order some and find out while I'm looking for Viton.

Thanks for the suggestions!
Attached Thumbnails
rebuild a VCV?-valve-top.jpg   rebuild a VCV?-valve-side.jpg   rebuild a VCV?-diaphragm-l.jpg   rebuild a VCV?-diaphragm-ll.jpg  
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  #6  
Old 08-08-2020, 09:12 AM
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Looks something like a motorcycle carb diaphragm, but I suspect most of them are too large in diameter.

https://www.carburetordiaphragms.com

But if you found one that works, you would have lots of grateful admirers.

Peter
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  #7  
Old 08-08-2020, 07:21 PM
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Exclamation I.P. VCV "Fun"

This is a gear thread, thank you ! .

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