Coolant just flooded my driver side floor!!
 

I think I may know what the problem is, but I revved my engine and coolant just flooded my driver side floor from the lower vent area. I hear a gurgling noise and my driver side windshield got fogged. I have a ball valve in place of my aux water pump to shut off coolant flow to the heater core as my heater valve has lost vacuum so I was getting heat all the time. I closed the valve to prevent coolant flow, but I'm not sure if coolant is able to still flow through the heater core. please see the picture. My question is: Is there a way to temporarily fix this problem? are my suspicions right, is it the heater core?? What are the dangers of this to the car other than loss of coolant and over heating? Please help.

Yes it's more than likely the heater core, or sometimes a loose heater hose clamp can allow coolant to spray in to the heater box, but pretty odd thing to have happen. Core more than likely.
You could just replace the aux pump with a piece of tubing, you don't really fix anything with the valve.
The heater valve (mono water valve) is electric not vacuum.
The mono water valve has a rubber diaphragm in it that commonly rips and can cause various problems, usually it is as you describe, unregulated full heat (fail safe design by the Germans so in a failure you will have heat, in case you need to defrost the windshield).
You can buy just the diaphragm repair kit, no need to buy the whole valve, and you should take it out first to verify it is torn in case I am wrong, you just need to remove the 4 small slotted screws to pull it out and check it.

Here is a link I found for you about the valve:
W126 Monovalve repair

how do I access the heater core? I already have my blower motor and wiper out. am I headed in the right direction?

Hate to tell you this dude but the entire car is basically designed around the heater core. Like "here is the very beginning of the 126 assembly line, Hans is taking the heater core out of the box and placing it on the assembly line conveyer".

Dash has to come out.

Gilly

See if this link helps at all:
http://www.peachparts.com/shopforum/diesel-discussion/146974-w126-evaporator-heater-core-r-r-diy-pictures.html

The 190's have luck with the evaporator in that it can be removed without dash digging....but the heater core, not as lucky.

It's very straight forward, but very time consuming....Just go steady and mark everything. I ended up with a handful of nuts and bolts with having no clue where they came from, even after bagging everything...Guess I wasn't good enough!

If you can, do the evaporator and all vacuum pods while you're in there. One time job.

On the 201 there are o-rings at the pipes that connect to the heater core. It is possible that you have a leaking o-ring and not a leaking core. The o-rings are accessible without removing the dash. On the driver's side the pipe comes in and elbows into the core up and rearward of the accelerator pedal. Pull the kickpanel out to access it. The passenger side pipe can be accessed after removing the glovebox.

Gilly...that was good! :P

Don't know why I thought it was a 126, it was kind of late though. The picture of the hardware store valve, I thought the engine was the right bank of a 117 engine is what I think I was thinking.

I had forgotten about the pipe nipples coming off and the o-rings on a 201.
I can't fr the life of me remember how the evap would come out without pulling the dash, but the heater core you do? Don't you pull the whole heater box out to get to either one??
Gilly

you live in CA. I suggest you live without heat if you can.

Just bypass or clamp off the heater core from within the engine bay.

Benhogan, Thanks. I did bypass the heater core, luckily for me, I do live in one of the hotttest areas in California where it rains maybe 5 or 6 days out of the year. I would like to remedy this at some point. I was able to pinpoint the leak, it is coming from the metal pipe that connects to the heater core. Is there a gasket or o-ring in there? it seems fairly accessible from driver side and held in by 2 bolts which I was able to remove, however getting enough wiggle room to disconnect the pipe from the core is another story as I do not know how far the metal pipe goes into the heater core. Any advise would be greatly appreciated. Thank you all for your replies and help!

You might want to check with Holmesuser01 from Asheville. I am pretty sure I read a thread he just recently did one on a 1984 201. I am not sure if they mirror a '91 or not.

Oh they are a fun item to replace. Several hours upside down for starters.

Hey guys.

The dash is not as horrible as one other car I did a core on, but it's close.

The whole dash basically has to come out so you can loosen and remove 4 bolts, and basically lift out the heater box. This is after disconnecting vacuum lines, etc, and in the case of my car, figuring out what the heck the previous owner did when they replaced the same core in 1999.

I put everything back the way its shown in the service manual.