Om603 Solving temp swings - PeachParts Mercedes-Benz Forum

JonW124 · #1 07-23-2019, 07:58 PM

I've owned a W124 wagon with an Om603 for years now. The cooling system has always bothered me. The #14 heads are known for cracking and I don't believe it's just a weak casting. Some have issues with them and some don't. I live in California were its usually hot and it's very hilly with long grades.

My car has never over heated but never stayed at the same temp when climbing hills. The temp swings range from 80c to just under 100c which in my opinion is a lot! My cars head has been cracked for quite some time. I finally found and bought a #22 and did a few revisions before I got it up and running.

First I drilled and tapped the thermostat housing to run a line to the drain plug on the block using ANS fittings. This routes water to the last cylinders and helps to get rid of hot spots. Second I gutted a thermostat and used the outer ring to re install the thermostat water neck with the rubber O ring. Then took the water neck on the head and cut and welded on a universal Thermostat housing that uses a chevy thermostat. Chevy T-stats are $8 at any auto parts store with many temps available. Versus the $30 Mercedes T-Stat.

The first week I drove without a clutch fan on the rad and even at stop lights it would take minutes for the temp needle to start to move off 82C. The highest it would see standing still at a light was about 87C. I have since installed my clutch fan but figured I'd post pics of how small my temp swings are now. My temp used to reach over 90C in no time climbing this same grade. With the new thermostat position at the top of the head my temp needle barely moves under throttle going almost 70 mph!

Also my cooling system was all new less than one year ago and I had 2 different engines with #14 heads that both showed the same temp fluctuation with the stock thermostat location. Only difference now is the #22 head and the thermostat relocation and coolant re-routing to the the rear cylinders from the water pump housing.

tjts1 · #2 07-23-2019, 08:07 PM

Get the smaller water pump pulley from the .971

I also run a colder 75c bmw thermostat. It never exceeds 90c with no clutch fan.

Diseasel300 · #3 07-23-2019, 08:21 PM

In addition to mentioned small pulley above, upgrade to the 606 turbo fan and clutch. Not only does it help the A/C, but it keeps the heat down. I've had 2 603 cars, I've never seen the temp break 100 even when pulling up a long hill (of which there are plenty around here) on a 110˚ day with the A/C cranked.

JonW124 · #4 07-23-2019, 08:33 PM

Quote:

Originally Posted by tjts1

Get the smaller water pump pulley from the .971

I also run a colder 75c bmw thermostat. It never exceeds 90c with no clutch fan.

I tired the smaller pulley and the colder thermostat but those are band aids in my opinion. In the winter my car would never reach operating temp. And if I turned the heater on my temp would plummet to 70C-75C. The colder thermostat makes the engine take a lot longer to get up to operating temp as well. Which is not what a diesel wants. I don't have to run a cold thermostat now to control engine temp. The original T-stat location is in a bad spot. I run a proper temp thermostat and my engine is up to temp in a 1/3 of the time it takes for the cold T-stat. Also the head is what matters most so the new location of the thermostat (which just about every engine has at the top of the head for a reason) keeps the head temps where they are supposed to be.

I see how a smaller pulley would be better but the water doesn't move much more so why spin the pump faster just for it to hit a barely open T-Stat? The restriction isn't the cooling system it self but the thermostat location not opening enough for proper flow. Cold water rushes in and closes the T-stat creating temp swings.

Anyways I've tried all those things and they don't perform well like this mod has. I could of saved a lot of time and money on cooling system parts because there was nothing wrong with the old stuff. Other than chasing a flawed thermostat location.

JonW124 · #5 07-23-2019, 08:41 PM

Quote:

Originally Posted by Diseasel300

In addition to mentioned small pulley above, upgrade to the 606 turbo fan and clutch. Not only does it help the A/C, but it keeps the heat down. I've had 2 603 cars, I've never seen the temp break 100 even when pulling up a long hill (of which there are plenty around here) on a 110˚ day with the A/C cranked.

I had the later Om606 fan, clutch and the small pulley on 2 Om603's before I swapped the #22 head on to the second engine with the cooling system changes. As stated they neither over heated but they sure didn't run a consistent temp. Those same parts are on my 2nd engine with #22 head and the temp doesn't move higher than the 2nd pic. It's high 80's here right now and my normal drive which used to show way higher temps in the winter don't budge and it's summer.

Also I ran no fan for the first week and its in the 80's here in SoCal. So the fan and clutch make no difference especially at speed.

tjts1 · #6 07-23-2019, 08:46 PM

Yeah the fan and clutch makes zero difference. Deleted it. Electric fans are far more effective.

h3ffe · #7 07-23-2019, 08:47 PM

Thanks for sharing this information.

I have struggled with this same problem as long as I've owned my S124.

Can you post some more pictures and/or go into greater detail on this process?

Very interested.

Quote:

Originally Posted by JonW124

I had the later Om606 fan, clutch and the small pulley on 2 Om603's before I swapped the #22 head on to the second engine with the cooling system changes. As stated they neither over heated but they sure didn't run a consistent temp. Those same parts are on my 2nd engine with #22 head and the temp doesn't move higher than the 2nd pic. It's high 80's here right now and my normal drive which used to show way higher temps in the winter don't budge and it's summer.

Also I ran no fan for the first week and its in the 80's here in SoCal. So the fan and clutch make no difference especially at speed.

JonW124 · #8 07-23-2019, 08:52 PM

Quote:

Originally Posted by h3ffe

Thanks for sharing this information.

I have struggled with this same problem as long as I've owned my S124.

Can you post some more pictures and/or go into greater detail on this process?

Very interested.

I'll Pm you

97 SL320 · #9 07-23-2019, 10:39 PM

Quote:

Originally Posted by JonW124

My cars head has been cracked for quite some time.

Did you find a crack that went from combustion to coolant or just a crack that didn't go to coolant?

Quote:

Originally Posted by JonW124

First I drilled and tapped the thermostat housing to run a line to the drain plug on the block using ANS fittings. This routes water to the last cylinders and helps to get rid of hot spots.

You are now moving hot water from the engine outlet to the cold rear cylinders when the stat is closed and might be moving cool water to the stat when open or not moving water at all. The only way to tell if water is flowing is to use a propeller flow indicator and watch it under all driving conditions. In a circulating system, it is all about pressure differential. https://www.mcmaster.com/flow-indicators

Another point is that the thermostat must be a restriction because a water pump needs to push against something to prevent / reduce cavitation and keep localized boiling to a minimum by creating dynamic pressure. This localized boiling is one reason heads crack regardless of brand.

Quote:

Originally Posted by JonW124

Second I gutted a thermostat and used the outer ring to re install the thermostat water neck with the rubber O ring. Then took the water neck on the head and cut and welded on a universal Thermostat housing that uses a chevy thermostat. Chevy T-stats are $8 at any auto parts store with many temps available. Versus the $30 Mercedes T-Stat.

If this is the same / similar MB stat as a gas engine, it will have a disc on the end. This disc blocks off the bypass when the stat is open so all coolant flows through the radiator and not just from the head discharge ( hot ) to the water pump ( cold ) . It is critical to have a bypass otherwise the coolant won't circulate when the stat is closed.

My 97 C280 had a broken bridge on the stat that caused it to be partly open but the disc didn't block off the bypass. The temps were OK on the road but would get hot at a standstill.

See my post 12 and 14 95 C280 Overheating

Quote:

Originally Posted by JonW124

The first week I drove without a clutch fan on the rad and even at stop lights it would take minutes for the temp needle to start to move off 82C. The highest it would see standing still at a light was about 87C. I have since installed my clutch fan but figured I'd post pics of how small my temp swings are now. My temp used to reach over 90C in no time climbing this same grade. With the new thermostat position at the top of the head my temp needle barely moves under throttle going almost 70 mph!

Where is your temperature sender? If this modified system is now measuring temps after the radiator, this won't be an accurate representation of actual max system temp.

JonW124 · #10 07-23-2019, 11:05 PM

Quote:

Originally Posted by 97 SL320

Did you find a crack that went from combustion to coolant or just a crack that didn't go to coolant?

Both #14 heads had cracks that went into the coolant and caused pressurized cooling system.

You are now moving hot water from the engine outlet to the cold rear cylinders when the stat is closed and might be moving cool water to the stat when open or not moving water at all. The only way to tell if water is flowing is to use a propeller flow indicator and watch it under all driving conditions. In a circulating system, it is all about pressure differential. https://www.mcmaster.com/flow-indicators

Another point is that the thermostat must be a restriction because a water pump needs to push against something to prevent / reduce cavitation and keep localized boiling to a minimum by creating dynamic pressure. This localized boiling is one reason heads crack regardless of brand.

Routing coolant to the rear cylinders to help flow has been a common practice for years with certain engines that have had issues with cracking heads. The rear cylinders always seem to be were the heads cracks on these engines. All the super turbo diesel guys do it and have supposedly proven it helps when boosting insane amounts to keep heads from cracking. It helps there engines live and multiple company's sell kits for this very purpose. I figured it couldn't hurt since I already had the fittings and only had to purchase the hose. Any little bit helps right?

If this is the same / similar MB stat as a gas engine, it will have a disc on the end. This disc blocks off the bypass when the stat is open so all coolant flows through the radiator and not just from the head discharge ( hot ) to the water pump ( cold ) . It is critical to have a bypass otherwise the coolant won't circulate when the stat is closed.

My 97 C280 had a broken bridge on the stat that caused it to be partly open but the disc didn't block off the bypass. The temps were OK on the road but would get hot at a standstill.

See my post 12 and 14 95 C280 Overheating

I had tired multiple thermostats and the only one that kept a consistent temp was a way colder thermostat than stock. Which took a long time for the engine to reach operating temp and made the engine run way to cold when I had to use the heater. I get how Mercedes wanted there system to work but it didn't work well in my opinion. The stock thermostat couldn't react quick enough when under load causing temp swings. None of the thermostats I ever tried were stuck or broken either. My engine only increased temp when climbing even the smallest of hills. I've never had an issue with temps at a stand still. Thats a separate problem as you probably know (fan clutch).

Where is your temperature sender? If this modified system is now measuring temps after the radiator, this won't be an accurate representation of actual max system temp.

The temp sender is still in the head. Which is where every good system temp should be measured from. Almost every single car manufacturer puts the thermostat up top at the front of the head for a reason. It makes the most logical sense and reads the temp most accurately. I think Mercedes had a space issue and thats why they chose to move it.

Mxfrank · #11 07-24-2019, 12:27 AM

Seriously?

The line from the engine outlet to the block drain functions as a bypass that routes hot coolant back around the block. It's unlikely to flow much, because both points are on the high side of the pump. But it has a value that you don't understand. Without this hose, there would be no circulation at all around the thermostat wax capsule, and it would take forever to open. Even so, I bet there's barely enough flow to get the thermostat started from cold. Unless you've drilled the thermostat flange, and in that case you can seal up the hose.

Gutting the thermostat leaves both the bypass and the radiator passage wide open. Luckily, the radiator passage is 50% larger than the bypass, so the system is dominated by unrestricted cool flow from the radiator.

Relocating the thermostat to the top of the system makes it more responsive to head temperatures, which happens to be where the temp sensor is located. So it looks better to you. But the design of the stock cooling system places the thermostat on the cold side, where it regulates the temperature of circulating coolant. The purpose of doing this was to prevent thermal shock from cold coolant hitting the hot block.

The way the Mercedes system is supposed to work is that hot coolant is circulated continuously around the block. When the thermostat opens, cool water is blended into the bypass flow and hot water is pushed out the top of the engine So the engine is never directly exposed to cold flow. A consequence of this design is that the thermostat sits between the hottest and coolest water in the system. It has to be built with high hysteresis, otherwise it would immediately shut when cold flow from the radiator begins. In plain English, the stock thermostat has very slow response to transients. Fluctuation. You get that with this model.

The Mercedes thermostat has a 50% larger poppet than the typical "Chevy" thermostat. That may or may not be helpful in normal driving. But it will certainly improve flow at higher RPM's. With your setup, I'd be tempted to experiment with a Robertshaw 333 (aka Ford 351 Cleveland thermostat), which has a larger flow path. You'd have to find a real Robertshaw, not a Gates or Stant substitute.

So in summary, you have successfully turned your Mercedes cooling system into a Chevy cooling system. At least it works, I've seen worse.

JonW124 · #12 07-24-2019, 01:50 AM

Quote:

Originally Posted by Mxfrank

Seriously?

The line from the engine outlet to the block drain functions as a bypass that routes hot coolant back around the block. It's unlikely to flow much, because both points are on the high side of the pump. But it has a value that you don't understand. Without this hose, there would be no circulation at all around the thermostat wax capsule, and it would take forever to open. Even so, I bet there's barely enough flow to get the thermostat started from cold. Unless you've drilled the thermostat flange, and in that case you can seal up the hose.

Gutting the thermostat leaves both the bypass and the radiator passage wide open. Luckily, the radiator passage is 50% larger than the bypass, so the system is dominated by unrestricted cool flow from the radiator.

Relocating the thermostat to the top of the system makes it more responsive to head temperatures, which happens to be where the temp sensor is located. So it looks better to you. But the design of the stock cooling system places the thermostat on the cold side, where it regulates the temperature of circulating coolant. The purpose of doing this was to prevent thermal shock from cold coolant hitting the hot block.

The way the Mercedes system is supposed to work is that hot coolant is circulated continuously around the block. When the thermostat opens, cool water is blended into the bypass flow and hot water is pushed out the top of the engine So the engine is never directly exposed to cold flow. A consequence of this design is that the thermostat sits between the hottest and coolest water in the system. It has to be built with high hysteresis, otherwise it would immediately shut when cold flow from the radiator begins. In plain English, the stock thermostat has very slow response to transients. Fluctuation. You get that with this model.

The Mercedes thermostat has a 50% larger poppet than the typical "Chevy" thermostat. That may or may not be helpful in normal driving. But it will certainly improve flow at higher RPM's. With your setup, I'd be tempted to experiment with a Robertshaw 333 (aka Ford 351 Cleveland thermostat), which has a larger flow path. You'd have to find a real Robertshaw, not a Gates or Stant substitute.

So in summary, you have successfully turned your Mercedes cooling system into a Chevy cooling system. At least it works, I've seen worse.

Yes I understand what Mercedes tried to do with there cooling system on the Om603. In theory it sounds good but in real life it doesn't work well. Which is why nobody makes there cooling systems that way. Remember Mercedes made almost all there other engines cooling systems like mine after the modification I made. Which happens to be similar to Chevy and almost all other car manufactures across the board. And none of those engines have huge temp swings or cracked head problems like the Om603 is known for. So I don't see any harm in trying something different since the original didn't work as well as they intended.

97 SL320 · #13 07-24-2019, 07:38 AM

Someone please post a coolant flow diagram from the factory manual so the OP can edit in his mods.

In a " Chevy " type system, the engine is subject to thermal shock during initial warm up. This is evidenced by temps rising then plummeting when the stat snaps open and a cold slug of water exits the rad and hits the engine. This is how one can assess the thermostat opening temperature without taking the stat out.

As noted in Mxfrank's post, the MB system blends cold coolant into the mix reducing this thermal shock. With this Chevy mod you have lost that effect.

Some GM cars went so far as to run the heater outlet to the water pump inlet side rad tank rather than the inlet of the water pump. This tempers the rad water so the effects of thermal shock are reduced.

tjts1 · #14 07-24-2019, 10:33 AM

Quote:

Originally Posted by 97 SL320

As noted in Mxfrank's post, the MB system blends cold coolant into the mix reducing this thermal shock. With this Chevy mod you have lost that effect.

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.

Mxfrank · #15 07-24-2019, 12:49 PM

Quote:

Originally Posted by tjts1

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.

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: