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1989 420SEL
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7 Posts
Discussion Starter #1
Hey guys, just seeking some advice. This afternoon I replaced the monovalve in my 89 420SEL, and sure enough the old one had a torn diaphragm. I used the BMW valve that some folks had suggested. Now, the heat is working, but I'm getting coolant leaking apparently from somewhere: as in white sweet smelling smoke coming from the immediate area of the monovalve as well as some from the air vents. (Coolant level is fine for now). Do you think I cracked the plastic valve housing?
 

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Moderator
1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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4,030 Posts
I am glad the heat now works, and am sad you’ve the new coolant leak...

Those screws were probably pretty easy to remove and as its plastic, I’d imagine you were careful not to over tighten,,,

Did you use the original rubber seal or the BMW one? Were they the same? It might also have been easy to transpose a washer or small detail perhaps?

While I agree it is likely related to the repair, I don’t feel its a cracked housing.

Fingers crossed here.

M.
 

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1989 420SEL
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7 Posts
Discussion Starter #3
Hey, thanks man! I was careful not to over-tighten. I used the BMW seal, and they were the same. As far as the order, I KNOW I have that right. John350 suggested that the rubber seal might have slipped. I'm hoping that's the case. I'll let you know for sure tomorrow, though.
 

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1989 420SEL
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7 Posts
Discussion Starter #4
So I THINK my problem was just coolant lost during the valve replacement, because I went out to my car yesterday and today and saw no puddles. When I started the car I could see none of the aforementioned smoke, noted no coolant loss on the valve and the valve housing was bone dry. Not seeing anything immediately suspicious, I let the car idle a bit, and warm up. I only saw the faintest whiff of white smoke drift up in nearly ten minutes of watching for it. So then I drove up and down the highway, got the car up to operating temperature, stopped for gas etc...about a 20 minute trip. No smoke and I had heating and cooling from the AC at all speeds. Now I'm the one with my fingers crossed! I'm feeling a little like the boy who cried wolf here, but maybe not. A big thank you to John350 and MBL87560SEC for taking the time to help me.
 

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1986 560SEL
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107 Posts
I replaced my mono valve a few weeks ago, due to lack of consistent heat. The job was pretty easy. One comment I had read previously was that you would get coolant leaking out of the opening when removing the unit, so the advice was to take some out prior to the job... which of course I did not do! So, in doing the job I really didn't notice that I had a pretty good amount of fluid leak out. When I started the car up I hit the panic button... white smoke everywhere, coolant smell, and when I looked on the ground under the car there was a huge puddle of coolant ! Yikes!!
Well I shut the car down and spent a good amount of time cleaning the garage floor and also the inside of the engine area where I also noted a good amount of fluid had found its way down! I examined everything looking for loose or cracked hoses, and went over everything I had touched, but found nothing. I started it back up and spent another 10 minutes searching for leaks to no avail. As I did this, the smoke went away, the coolant smell went away and everything looked good. My bad... I should have drained some fluid beforehand. All was and is well. And I have heat!!
 

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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4,030 Posts
Great you have heat now! From what you said here I too think it was merely residual coolant from the insert swap.

I forgot to mention I had vented the SEC cooling system before I took mine apart and then but a very tiny amount of coolant was lost opening the mono valve.

M
 

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1986 560SEL
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107 Posts
I did read a bunch of posts related to this before I changed it and others related differing stories. Had I vented to relieve pressure or even removed a cup or two with a turkey baster I would have had no problem. But all’s well that ends well I suppose!
 

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350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
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I found that if you do it when the system is cold, open the cap to relieve any perssure, then close it to prevent air from entering the system, you'll loose next to nothing when you remove the monovalve. If you leave the cap off coolant will flow out until the level equalizes.
 
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