No, the MB system allows the rear cyls to get much hotter than the front, that's why on #14 heads it's always cyl 4, 5 and 6 that crack. What the OP did is open up an external coolant passage from front to rear. It's a common mod when the 603/606 has to deal with more heat than it's capabable of handling.
https://youtu.be/F7GaCE7_aS4
As it left the factory, the 603 cooling system has massive temp swings when exposed to sustained high loads and heat.

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Actually, he didn't, because both ends of that hose are on the high side of the pump. If the hose flowed even a little, it would either be moving the very hottest coolant to the rear port, or it would be moving cool coolant away from the rear and into the radiator. The one thing it absolutely wouldn't accomplish is to add cool fluid to the rear. I'm guessing that only a small amount of coolant actually flows through that hose, and only when the thermostat is closed.

This is different from the mod in the youtube video, because in the video, the rear port is connected directly to the water pump outlet. That would have the effect of diverting a bit more flow to the rear of the block.

As I explained, the temp swings are an inevitable consequence of having the thermostat on the cool side. The thermostat has to be slow-reacting, or the system wouldn't work at all. You can produce more stable temperatures by engaging the fans earlier, but it's tough to get temps rock steady.

I think what you're looking for here is a strategic flow approach, as has been attempted on some Jaguar engines. This entailed sizing the ports in the head to restrict flow to the front cylinders, which allows more flow to the rear. It can be accomplished with a special head gasket, so it's possible to work out a retrofit. But this hose is just ludicrous.

Many modern cars use MAP controlled thermostats, where the "natural" temperature of the thermostat is 240F, and it's artificially heated with a resistance coil. This allows the ECU to implement a dynamic temperature range that's correct for each operating condition. "Normal" may be anywhere from 170-240F. So no gauges for you.

Personally, I prefer a steady temperature, as fluctuating temperature means fluctuating pressure, which eventually fatigues the radiator and heater core. But that's not where things are going.

97: I can't put my finger on the flow diagram at the moment. Coolant enters a blending chamber from the bypass, heater return and radiator. This is where the thermostat lives, reacting to the blended temp. From there it enters the pump inlet, and is pumped into a gallery along the right side of the block, and from there distributed through the block and head. The block drain is on the right side water gallery, all the way at the back. Here's the blend chamber:

this is all very interesting, I have a #14 head and its been replaced when I bought the car due to the PO walking away from it when he learned the shop wanted $1400 to replace the head (they also lost the original head when some metal hawk stole it off the shop floor). I got it as a mechanics lien and put a used head on it (after a valve job etc.) since then I've put over 40K miles on it and the block now has 305K on it.
So far the #14 head is working fine but I have a later head available in case.
Before I took on the cooling system mods described I would install an electric fan, I've been thinking of that but have other projects always getting in the way.
Oh, and a comment on temp fluctuations.
My OM604 cooling has temp fluctuations that are hardly anything compared to the OM603 but oddly my M104 seems to have a lot of temp variations, the engines use almost the same block and I assume the cooling systems are similar but I haven't really examined them closely. The diesel runs cooler of course. I'd like to take on the M104 and install an electric fan before I work on the '87 diesel.
DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

Have you pulled / inspected the M104 stat?

See my post 12 and 14 95 C280 Overheating

The M104 uses a stat that regulates water going to the rad rather than coming from the rad like the OM.

well, yes I changed the thermostat already. I updated the T-housing too at that time.
I wasn't aware of the different cooling systems. Shows you what little I know.

I have some Motul coolant additive supposed to lower temps but haven't put it in yet, I'm sorta waiting to flush the system and replace antifreeze. I am skeptical of claims made by most additives but this might help a little. I used a similar Redine product in another car and didn't notice any difference.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

No, that's incorrect. Those of us living in hot climates have been dealing with this problem for years. What the OP did is spot on and proven by his results.

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I'm with MxFrank on this one. But as a check I'd want to see a flow meter or temp gauge on this additional line. My guess is that the line is flowing from the rear cylinders into the mixing area rather than providing cooler water to the rear cylinders.

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I recondition w123/w126/w124/w140/r107/r129/ steering boxes!


1984 300D "Elsa" odo reset 6/2011 147k
1983 300TD "Mitzi" ~268k OM603 powered
1995 E300 "Adelheid" 262k [Sold]

Can you elaborate? MxFrank's flow picture seems to indicate that if the line is fitted to the port on the housing that dieselmeken drilled and tapped, the flow would push mixed coolant through the water pump scroll and then theoretically also into this hose.

If the line is flowing from the rear cylinders into the mixing area as you suggest, then that pressure differential would promote cooling to the rear cylinders by pulling coolant through the front cylinders to the rear faster.

It seems to me that one way or another this mod improves cooling. Dieselmeken isn't exactly a novice with these engines, either.

I'd love more information if you, SL, or MxFrank can provide some, as I'm no expert. Thanks to all for engaging in a valuable discussion and sharing information!

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1983 300D Midnight Blue
4 speed, W115 intake mani, non-EGR exhaust mani, KKK K26, 3 pc Euro bumpers, Lo-spec Euro headlights, AL129X, hubcaps, custom A/C

1987 300TD Smoke Silver
Euro headlights, thermostat relocation, coolant bypass mod, rebuilt Becker + Jehnert + Helix audio, OEM Oris roof boxes and surfboard racks, Euro towbar

1983 300TD Reed (Moss) Green / Dark Olive MB Tex Euro spec

1983 300TD Silver Blue / Blue cloth Euro spec, OM648 and 722.6, 15" hubcaps, W126 S2 brakes

The water pump is pushing water directly into that tapped line coming off the thermostat housing and pushing water to the rear cylinders. I've read it's supposed to help avoid hot spots on the rear cylinders and since it's pushing water through from the cold side I'm sure that helps as well. I didn't come up with this mod but it has been put to use for many years with the Super Diesel guys and multiple company across the pond sell it for god reason. It's been tested so I don't really need to question or wonder if it works.

Can you say altering engine temps to control emissions on the test rollers then changing the operating temps in normal use so as not to cook the engine in the long term?

Thanks. It looks like the temp sender in the pic is measuring cooled water after it leaves the radiator and / or suction side of the pump, this won't give an accurate representation of maximum engine temps. With the original stat gone, the bypass is in constant operation adding to the mess.

I'd be very interested in temps just prior to the new stat and at the radiator inlet, these are the temps that really matter.

I do have the same mod as posted in the Youtube video. Before I changed the position of the thermostat all my cooling system was replaced (trying to cure these temp swings) only to get the same results. I tired 2 different brand radiators. Installed the Om606 water pump and pulley along with Om606 fan and clutch (which don't help anything but idle or slow speed temps). I would drive up a grade or hill in San Diego and my temp would go from 82C to just under 100C rapidly. So I'd pull over and check the radiator. Half would still be relatively cold like the coolant wasn't flowing enough. I tried 4 different thermostats with the same results thinking they could be sticking or broke. After the thermostat relocation my radiator has even heat through out very quickly. The engine gets up to temp right away and stays consistent no matter how I'm driving now.

So my question would be why you believe the temp swings before where real and "normal" but can't believe the stable temps I'm seeing now aren't accurate with the same sensor?

Also the smaller Chevy thermostat works great cooling 500 plus cubic inch Chevy engines. It maybe smaller than the Mercedes T-stat but I think it can handle a 3.0 liter engine producing 150 hp.

Here's a pic of my thermostat housing.
IMG_3166 by jon allie, on Flickr

I don't think is this an apples to apples comparison. Diesel engines produce more internal heat energy than gasoline engines, on that alone I'd think that that the cooling systems on a diesel would be greater than that of a comparable gasoline engine.

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I recondition w123/w126/w124/w140/r107/r129/ steering boxes!


1984 300D "Elsa" odo reset 6/2011 147k
1983 300TD "Mitzi" ~268k OM603 powered
1995 E300 "Adelheid" 262k [Sold]

I owned a 540 cubic chevy engine that used the same thermostat I'm using in my Om603 now. My 540 BBC made 685 hp to the wheels and creates a ton of heat. I understand diesel engines create a lot of heat and the turbo really adds to that. But I drag raced that engine and drove it hard and that small thermostat had no issues. The point I was trying to make about the Thermostat size is that it doesn't matter if the cooling system is efficient.

What you're saying is that a higher flow restriction is not a problem, but that's not a true statement. We could get into the diameter of the hoses, the pump size, the flow rate of the coolant, etc. The size of the thermostat DOES matter when you're keeping everything else the same outside of the size of the thermostat.

You cannot compare a V8 gas engine to a 3L diesel just by saying "same same," nor can you compare a daily driver with a car designed to drag race.

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Onus probandi incumbit ei qui dicit, non ei qui negat

I recondition w123/w126/w124/w140/r107/r129/ steering boxes!


1984 300D "Elsa" odo reset 6/2011 147k
1983 300TD "Mitzi" ~268k OM603 powered
1995 E300 "Adelheid" 262k [Sold]

I guess I'm not be clear. I didn't say both engines were the same. My point was that they both create a lot of heat even thought they are different. The size of the Mercedes thermostat really doesn't matter because it doesn't open enough where its located because it gets hit with cold water and closes. The chevy thermostat gets hit with a more consistent water temp so it doesn't open and close as much. If they made an inline Thermostat that used the bigger Mercedes T-Stat then I would have used that. The location of the T-Stat makes more of a difference than the size.