W201 heater valve question
 

Just replaced my heater valve and I noticed that when I have it set to cold it works fine, but when I come to a stop and accelerate again it blows hot air for about 10-15 seconds then cools again. I originally thought vacuum leak, but it only does it when accelerating from a stop. Any suggestions or things to check?

Answer this question for me, does the defrost flap come open as well? I had a similar issue where the defrost flap was coming open part way sometimes even when I had it set to just the vents or feet, it ended up being a shot defrost pod. The way you can test the vacuum components in the HVAC system is to access the vacuum switchover unit that is behind the glovebox then use a vacuum tester to pull a vacuum and see if it holds.

You have a vacuum leak
 

When you accelerate the engine vacuum drops and the default is heater and defroster on, I guess its a safety feature. For the heat to come on both the water valve and the blend air actuator need to be enabeled, that is no vacuum.

Yes, I tested and tested everything and all the vacuum components are working. After looking at this diagram, I do see number 48 (orifice vented for 20 seconds) must be what causes it. As I said, upon acceleration, heat is present for about 10-15 seconds (it must be 20 seconds per the diagram). What I did was added a second Y-splitter and a one way check valve. This diagram is accurate, but my car has an economy gauge, the "drt" line, the "rt/gn" line, and the "mgn/rt" line getting a direct feed via intake/2-way check valve. I added the check valve to the "mgn/rt" line and it seems to have eliminated the 20 second vent and seems to not have changed anything else. I'll see how this holds up or if I notice anything else that changes.

Be sure the rt/gr line to #33 reservoir is not leaking. I have seen this line decompose when in contact with seam sealer as the line runs across the firewall

I gather this is a gasser, it's important to know that. At idle, manifold vacuum is very high. When you stomp on the pedal, it approaches zero. The heating system reacts to zero vacuum by supplying max heat. You have two components which keep this from being a problem: check valve 31 and vacuum reservoir 33. I would guess that either the check valve is bad or one of the rubber connectors is leaky as has been mentioned. To help you along, here's a photo of the vacuum reservoir, it's inside the left fender:

http://www.coolcatcorp.com/190Restoration/Mercedes23.jpg

I agree with Paulwho. That plastic line to the reservoir deteriorating is a very common problem.

If anyone is still looking for info on their heater, or parts of it; check out the below tech article.

Mercedes-Benz 190E Heater Core Replacement | W201 1987-1993 | Pelican Parts DIY Maintenance Article