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Old 09-09-2002, 03:24 AM
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Question Monovalve?

I have a coolant leak in my 1987 420SEL. I took it to a local mechanic and he replaced a few hoses along with a few other little things that needed to be done. (a.i. change the oil and so on) The changed hoses seemed to help untill the motor got hot. I was driving around the city and the engine temp got pretty close to 100. On my way home the coolant light went on so I stopped to fill it up. The next day after a 45 min drive I parked the car and looked under it. I noticed a slow stream of antifreeze comin out.
I took the car back to the mechanic to see what was happnin. The guy told me that the leak was from my monovalve. He didn't give me a price becous his supplier was closed and I took the car back.
What kind of money can I expect to shell out for this repair? Should I pay for it? After all I took the car to him once and over $300 later I still have the same exact problem! Is this somthing I can do myself CHEAPLY?
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Old 09-09-2002, 08:34 AM
j shepardson
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if you do the repair yourself it'll cost you about 30.94$ for parts through fastlane.

to locate the monovalve open the hood and look to the right of the battery(looking at the battery towards drivers side) on the back fire wall you should see a square box/bracket attached there with a power plug hooked into it that is where the monovalve is stored all you have to do is unscrew the screws and pull out the valve insert new valve and tighten screws fairly painless.

the shop will prob charge you full price of valve somewhere in the ballpark of $100 plus labor min one hour at $85 and hour plus tax and whatever else they stick ya with.

best of luck

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Old 09-09-2002, 03:50 PM
Bob Boyer
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Again, exactly where is monovalve (precisely; what is its precise function, and how do you know if it is malperforming?
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Old 09-09-2002, 11:14 PM
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HEY! Thanks for the help shepardson ! My wallet thanks you
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Old 09-10-2002, 06:39 AM
Join Date: Nov 2001
Location: Cape Cod Massachusetts
Posts: 1,427
Lightbulb Here it is!!!!!!

BB, look here (on a 126)!:

Your symptoms are not the usual failure mode for a malfunctioning valve piston, the $30 part. That failure default mode is continuous heat, more at idle less at speed!I have heard of valve bodies being replaced, but it a pretty simple and sturdy part so if replacement is required a used part will be the most economical and a part that has been looked over and is up to spec should last forever. I guess your valve body itself could be damaged, corroded, plugged etc.
I have not heard of a coil failure but you can get that part used for sure and possibly new (the coil may only come as part of a new valve body?) Could be a control problem from a faulty ACC?

There are also three coolant hoses ( plus! two smaller hoses going to the windsheild defroster radiator) and their respective connections that may be the root of your problem. Once you see where the monovalve is located and the connections and how they need to be made, you will understand why a pro will not do the job for less, its a pain that will test your patience and resolve working in that space.

If you dive into there you might want to check out the auxilliary circulation pump that will be eazier to access with the monovalve assembly removed! Its just to the right (facing rearwards) and directly under the ECU! Maybe easier to take both out together? Good Luck!

This is another perspective,

126 Windshield Defroster Heat Exchanger ?

Last edited by Billybob; 09-10-2002 at 06:47 AM.
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Old 09-10-2002, 08:24 AM
j shepardson
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not a problem, hopefully the leak will be a simple repair even if its not really the valve itself what I would do is try to trace down the leak first, it is very well possible the mechanic did not tighten down the hoses quite enough or possibly may have not changed one that may be cracked and leaking, possibly a needlepoint sized hole in a radiator fin. several possibilities if the leak is coming from the front i would rule out the monovalve.
Though winter is coming so It may be a good idea to change it now (if it's going south)
one good way to tell if its bad is go for a drive one day when its cool out (late at night early morning) turn on the heat. if you turn the heat off while its running then turn it back on but no heat comes out then the valve is more than likely bad also another way is once driving and the RPM's hit 4000 the heat should cut off altogether causing you to have to turn off the engine to restore heat....

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