My 1987 420SEL has had this slow heat and low heat in Winters for many years. It has 155k+ miles and run fine. The heat comes up slow and the heat is not enough to keep the car inside warm. The temp dial is turned to the max warm and fan is on the normal speed. The heater starts to show warm air in about 10 minutes (after the car being on road for many minutes and coolant temp indicator is pointing at 60C-70C) with low heat warm air. It is barely enough to clear the windshield frost in server Winter days. Air circulation appears to work fine in all modes.

The thermal stat appears functioning: I start engine and put my hand on the coolant return hose, which is cold until the engine warms up and the hose would suddenly warm up.

So, where do I start checking and looking to resolve this slow heat and slow heat problem?

Please advise.

 

Hello and welcome to the forum! Have you checked the heater core itself? It may be time for a replacement.

 

Thank you for the suggestion.

Checking and replacing the heater core is a big job and I would leave for the last. I recall in reading someone mentioned a heater control mono valve (in the engine compartment?) but I don't know where it is located. This heater control valve has a mesh filter/shield that is easy to be plugged up and is suggested to check in the first step.

I need advice on where this heater control valve is located and perhaps how to remove it.

 

Hello - take a look at the link below to an earlier thread where this is discussed.

Remove Mono valve with pic's - Mercedes-Benz Forum

 

Thanks for the lead on the mono valve.

 

The thread is not letting me make a reply post with 2 photo inserts - waiting for moderator review?

What I wanted to say is that I checked my 1987 420SEL booklet and found a diagram showing a device similar to a mono valve (device code 463). Along side of this device is another unit (device code 467, which assembles the look of a control valve that has a mesh screen tube) which seems to go inside of mono valve device 463. The booklet diagram doesn't show the location of device 463. And I looked over the engine compartment under the hood and could not find anything that close to the look of a mono valve.

I hesitate to take things apart without a known target. My 1987 420SEL is a very old beast now: plastic components have become fragile to tools and replacements are not easy to find these days, ...