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1993 Mercedes Benz 300D 2.5 Turbo, 1957 Chevrolet Bel Air Wagon, 1958 BMW Isetta 300
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24 Posts
Discussion Starter · #1 ·
I recently discovered that my '83 240D is missing its auxiliary water pump. I just need the car running and driving for this weekend so things like heat and A/C do not matter.

I've seen that some people have bypassed the auxiliary water pump with a simple hose that leads to a metal pipe (or "hard line"). Problem is, I don't seem to have a hard line either.

My question to everyone is:
1. Do I have to have something connecting the bottom of the thermostat (where the inflow for the auxiliary water pump comes from) to the other end of the hard line (which I'm guessing is the heater core?) for the car to run and keep cool?

2. If I do not use the heat or A/C does this mean the aux water pump/heater core will not pull any coolant from the thermostat so I can temporarily plug it up so coolant doesn't spill everywhere? I know this seems like a silly question, but when I drove the car onto the trailer when I bought it, no coolant leaked out which makes me question if coolant is sent to the aux water pump/heater core all the time.

I only had coolant spill out when I was checking the trailer brakes with the car on the trailer.

Any help would be appreciated. Thank you!
 

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1981 W123 300D non turbo, 1992 190E 1.8 <=> 2.0
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6,581 Posts
If you don't use the heater core at all then you need to block the pipes at the engine at both ends...

...but winter is coming!

The metal pipe you are missing under the manifolds needs to be metal because of the heat in that area - well I assume that that's the reason Mercedes made it that way.

If you are not worried about originality then I reckon some steel 15mm heating pipe would do the job - though I think the original part has an outside diameter closer to 12mm. If I see that part in the garage I'll measure it for you.

As far as I can make out the auxillary pump is only fitted to vehicles with air conditioning systems. My non A/C 300D doesn't have it.
 

· Registered
1993 Mercedes Benz 300D 2.5 Turbo, 1957 Chevrolet Bel Air Wagon, 1958 BMW Isetta 300
Joined
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24 Posts
Discussion Starter · #3 ·
So where are the places on the engine that I need to block? Is one of the places the bottom spout of the thermostat?

Or do I absolutely need to get the hose in there for flow to something thats not the heater core?
 

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1981 W123 300D non turbo, 1992 190E 1.8 <=> 2.0
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6,581 Posts
I think the safest thing is to repair the system properly / put in your own metal pipe and get a bit of rubber pipe to fit.

If you look at chapter 20-005 in the FSM you can see the coolant flow system in schematics and photographs. The part that would need to be blocked off for what you suggest would be before the heater control valve - again I think that this is safest rather than just relying on the cabin control lever acting on the control valve. It only takes someone who doesn't know to move the heater control valve and you loose (all of) your coolant!
 
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