That is normal on a properly functioning cooling system in cold weather.

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'83 300D, 126K miles.

True that there is a bigger difference if cold weather but same applies in hot weather, just that the difference is not as pronounced.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Fastest way to tell if a heater core lacks water flow / is clogged / valve shut is, with a hot running engine, to feel the heater hoses. If they feel about the same temp, there is flow. If one is hot and the other cold you lack enough flow. A good heater core throws off way less heat than you would expect. When clogged / restricted , water dwells in the core longer allowing the water to throw off more heat.

If the upper rad hose is constantly hot and lower ice cold, you may have a water circulation problem. RE: thermostat open , water pump not moving water , thermo siphon effect occurring.

Even with a properly operating system, if air temps are low enough the upper rad hose should get hot then colder as the thermostat cycles. If the upper stays hot, the lower should get warm / hot as temps stabilize.

hose out of back, drivers side of block - 147 dF
hose at coolant recirc pump - 123 dF
lower radiator hose - 95 dF
upper radiator hose - 167 dF
upper part of radiator - 188 dF

I will be looking into the damper operators next.

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Jim

IDK about this. The way I see it: When clogged, water dwells in the core longer, but with less flow, it is cooler and gives off less heat, not more heat.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

These are reasonable numbers and does not indicate a clog. Is this the same car where the PBU, monovalve and aux water pump have just been replaced?

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

To clarify on the low flow, is isn't that the air coming off the heater core is hotter. It is that the water loses more heat making the return hose much colder than the inlet.

Every substance has a thermal transfer rate, the longer that hot water stays in a heat exchanger, the more thermal energy it will lose. With a rad or heater core, we don't want the drop to be too great because the cold water will cause thermal shock as it reenters the engine.

Still trying to sort this one out. I've checked the vacuum actuators and they all seem to be working.

The ONLY time I get heat (good and hot) is right after start up after it has been shut down (while up to operating temperature). BLows good and hot for a period of time and then cools back down. Does not seem to matter if monovalve is hooked up or not.

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Jim

I'd put a piece of pipe or ball valve in place of the mono valve for the purposes of testing, I'd also tend to remove the aux pump from the circuit as well as it may be presenting a restriction. Is there any chance the plug leading to is was flipped causing it to fight coolant flow? Does unplugging it help?

Having heat for a short time points towards low flow,

Great talking with you.


.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
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1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

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I'm about ready to throw the towel in on this one. Only time I can get heat is after I have driven car and gotten it up to temperature then turn car off, wait a few minutes, turn ignition to on position (no start) and if blower fan is on it will blow good and hot. Crank it up and shortly thereafter it quits blowing hot. I have flushed from hose on drivers side head rear through core, monovalve and aux pump without any obvious pluggage or restriction issues Something else I noticed is when filling the cooling system with coolant, after I have finished "burping" the cooling system and am continuing to add coolant I only see a very small of flow out of the upper radiator nozzle (the level in the coolant reservoir starts rising though). Most times I get good, steady flow out of this nozzle while pouring coolant into the upper hose (while it is still hooked up at the engine end) on every other 300D I've done this to. I may pull the water pump and radiator to see if I can find blockage somewhere else. The cooling system works great when it comes to keeping the engine between 85 and 90 dF.

Oh yea, the monovalve is disconnected during all this testing.

Anything else to look for or inspect please comment. tia

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Jim

Disconnected as in wire off or removed from the heater hose and a pipe installed in it's place?

You have very little flow through the heater core. There isn't enough water flowing to keep incoming heat ahead of out going heat.

Do you have a spare heater core from any car? If so, plumb it into the heater circuit, rig up a fan and run the engine. If it heats consistently the problem is on the cars heater core / air flap. If no heat, mono valve / aux water pump or something else that causes low water flow.

Does the original heater core have a 2 pass system? RE: Inlet and outlet are on the same end tank with a block between them, the other tank is what connects the two. There might be a failure of this block allowing water to bypass the main part of the core. When you shut the engine off the entire core heat soaks until you run the fan again and everything cools off.

Looks like monovalve(s) are not worth a durn. Put bypass in and it will run you out. Got three monovalves, including one recently off ebay, and none of them work.

Attached Thumbnails

 

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Jim

None of the 3 monovalves tried is a genuine Mercedes? Is that the next step? If a new Mercedes monovalve still does not solve your problem, it may be due to too high a pressure coming out of the outlet of the heater core.

This is my theory:

The heater core has one inlet pipe and 2 outlet pipes. I believe the purpose is to slow down the flow and decrease the pressure input to the monovalve.

The much discussed monovalve spring pushes against coolant flow, i.e. once the solenoid is energized (flow blocked), and subsequent de-energized (flow open), it is the spring that provides the opening force, not the solenoid. If the spring is weak or if the coolant pressure is too great, the monovalve remains shut even even though power is removed. If the heater core is blocked, e.g. one of the 2 output pipe is blocked and or both are partially blocked, it could create a higher than normal output pressure.

I have tested a used genuine Mercedes monovalve with city faucet water pressure (1 ft long hose between monovalve and faucet valve). With water flowing freely through the monovalve at first, flow stops upon energizing the monovalve. But upon de-energizing the monovalve, it remained closed= no flow- result of the pressure being stronger than the spring. I tried with the faucet opened with very low flow with the same results. I do not know what city water pressure is but I think it is around 45 psi. I do not think the water pump in a 617 generates that high a pressure. Since I do not have a water pressure regulator, I am going to repeat the test with a 50 ft long hose (to slow down the pressure buildup) between the faucet and monovalve using low flow and see if I get different results.

It would be interesting if you can Tee in a pressure gauge at the monovalve inlet and measure the pressure on the faulty car and compare it another 617 with a fully functional heating system. It wouldn't be that hard to set up and may shed some light on the situation.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Got a new one coming from MB. Will let you know how it works out.

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Jim