Overheating Solution?

[Longtooth]

The 250SL has an oil cooler (water/oil heat exchanger) mounted under the Fuel Injection Pump beside the block.  

As for differences in engine heat/cooling capacity between the 250SL (M129 II engine) and 280SL (M130 engine), the 280SL's increased bore required the cooling water jacket between Cyl pair 2 & 3 and between Cyl pair 4 & 5 in the M129 to be omitted in the M130 engine.  The M129 had an 82 mm bore while the 280SL's bore is 86.5 mm.... both engines used the same stroke (78.8 mm). Another difference is that the 280SL's fuel injection pump uses oil from the block... whereas the 250SL uses it's own oil supply. The 280SL also uses a different cam design than the 250SL but I don't have access to the specs on valve opening closing timing for the 280SL so can't determine if the difference in cam designs adds or reduces to heat build-up.

The 280SL's increased engine horsepower also meant it produced more heat... larger bore means more piston wall area for friction of rings on cylinder walls increased by 5.5%.  The elimination of the water jacket between the 2 cylinder pairs on the M129 engine to accomodate the increased bore on the M130 engine also decreased the conductive metal volume between those cylinder pairs as well as the others... which decreases the thermal heat carrying capacity from the cylinders... so a double whammy in terms of removal of heat from the cylinder wall on the M130 engine (280SL) relative to the 250SL's engine.

Most of the heat removal from the M130 engine cylinders is removed by the water passeges in the head... which by the way is also the hottest portion of the cylinders --- nearest the combustion chamber and exhaust passages in the head.

I don't know what MB design engineer's did to increase the cooling capacity for the M130 engine however... I'd guess, but have no documentation, that coolant flow rate and volume would have had to be increased (larger capacity water pump).  I don't know the 280SL coolant capacity but the 250SL uses 12.9 liters, while the 230SL's uses 10.8 liters (a 19% increase in coolant volume on the 250SL over the 230SL).  

It's possible, therefore, that from an engine design point of view the 280SL's increased bore and eliminated cylinder coolant jackets to accomodate it coupled with the increased heat generated in the higher horsepower engine means that it has less margin with any restrictions or reductions in coolant or pump efficiency to maintain coolant temperatures with lower (idle) rpm and hot exterior environment. The lower rpm with car at standstill means less air/water heat exchange at the radiator and reduced coolant flow volume thru the block and radiator at the same time.  Unless the 280SL uses ~20% more coolant and a larger capacity pump (greater volume flow rate) it's likely that the 280SL (M130 engine) would be more prone to heating coolant to higher temperatures with low engine and car speeds than the 250SL.

Ref: Engelen's book, pages 176, 180, 181, 182 and Service Manual, Modification Oct. 67, 0-3/7 , 0-3/8 (i.e. specs).

Non-thread comment:  I guess that may be another reason why the early 250SL's are the crem de la crem of the W113's.

[thelews]

quote:Originally posted by Longtooth
Non-thread comment:  I guess that may be another reason why the early 250SL's are the crem de la crem of the W113's.

  In my creme de la creme, the temp gauge is practically pinned at 180 no matter how cold or warm outside.

[Ziggy]

quote:I agree with your sentiments but it does seem that the early 280’s were quite susceptible to getting a bit hot and bothered and MB changed the head design on later cars which seemed to cure the problem, they did bring out a kit for the early cars but those that have tried it say it’s not much good and very expensive. I have replaced everything possible in mine and added numerous upgrades but all to little avail. As I don’t think MB changed the pump design it seems to be more a circulation problem which, so far at least, the electric pump seems to have resolved. Having said that it also seems unlikely that my car behaved this way when new and I do know that the problem is not A/C related as I had the same issue before it was fitted so something must have changed over the years but quite what, apart from fuel, I don’t know.

I have an overheating problem as well, when idling in drive. In N the problem doesn't seem to occur. Maybe I have the wrong waterpump pulley, or using a pulley with smaller diameter would cure the problem. I'd like to try that.

Does anyone know of a part number or engine type which uses a smaller pulley? I know most Mercedes engines from the 60's and 70's all use the same 4 hole water pumps.

[Benz Dr.]

You get an auxilary water pump from a 123 or 126 and mount it in one of the lines.
 These things work by pushing coolant through the heater core on 126 cars while the car is sitting at idle. The regular water pump on the engine doesn't have enough power to feed all the coolat lines on these cars.

Should work great on our cars and could be set up to turn on at a certain temp or throttle position.

[Q45Denver]

Dr. Benz:

Would I need to install a heater bypass in order for the aux pump to work in hot weather? Is there a temp sensor in the block I could connect it too?  
Also,  would you have any ideas as to what caused my M130 to start smoking after I overheated it?

quote:Originally posted by Benz Dr.

You get an auxilary water pump from a 123 or 126 and mount it in one of the lines.
 These things work by pushing coolant through the heater core on 126 cars while the car is sitting at idle. The regular water pump on the engine doesn't have enough power to feed all the coolat lines on these cars.

Should work great on our cars and could be set up to turn on at a certain temp or throttle position.

[Benz Dr.]

Get an auxillary watter pump from a 123 or 126 and plumb it in line with a heater hose. Since it's electric you can hook it up to work only at idle or at higher coolant temps.

[Dick M]

quote:Originally posted by Benz Dr.

Get an auxillary watter pump from a 123 or 126 and plumb it in line with a heater hose. Since it's electric you can hook it up to work only at idle or at higher coolant temps.

Dan,
 what is your take on doing the heater bypass...really effective...cause problems?
Thanks for your many contributions..

[Markus]

Hello all, I have for the past year been enjoying (testing)operating temps @ 180F instead of freaking out as the gauge heads north towards red.

I made two T fittings, one from 11/4" steel pipe, and 3/4" pipe, (inline upper S radiator hose) and the other T made from 3/4" pipe (inline to heater) mounted the water recirculating pump from 123 or 126, utilized a 4 pole working relay, connected to the lower terminal of the throttle body switch.

It has been my intention to put this in the R&D section with a step by step including pictures, just trying to find the time. I can say that after doing everything else (4 row radiator, new fan clutch, water pump, totally rebuilt and cleaned engine) this is the ONLY solution that has actually worked (for me).

Mine is a late model 280sl with automatic and a/c, overheating is a common problem and I have read most of the posts. Your radiator has plenty of cooling capacity. The problem is that the heat is not being taken away from the cylinder head quickly enough at idle or stop and go traffic. I am now able to drive the car in Dallas 96F ambient(120 off the pavement), and temp 185-195F in stop and go.

It works so well, that I rarely look at the gauge anymore! The only drawback, it now takes about 7-8 min for the car to warm up where the idle drops, instead of 3-4min. A small price to pay. In cooler temps, I just disconnect the electric water pump so that it doesn't run at idle.

[Q45Denver]

Markus:

Would I need to install a heater bypass in order for the aux pump to work in hot weather? Is there a temp sensor in the block I could connect it too?
Also, would you have any ideas as to what caused my M130 to start smoking after I overheated it?

[glennard]

Was ist los(Penna Amish)?  Chock one up for shady tree engineering.  Will those Germans ever learn?

[Benz Dr.]

I've never had to do this set up so I'm not the best one to ask.h However, logicaly you would have a benifit from moving coolant through an area that has little flow at idle. The 123 coolant pump would be ideal for this job and it's designed to take the heat. The problem is that there's little coolant flow around the back of the head and block when the engine is idling and there's no air flow around the oil pan at all. Roughly 40% of your cooling is done by oil running through the engine and that heat has to go somewhere once it's produced. This is why the 250SL has the absolute best system when it comes to cooling - it has the biggest rad of all three cars and it has the water/oil heat exchanger. This is not an oil cooler but it is a heat exchanger .

[Benz Dr.]

I would also look at putting a 79C thermostat in for summer use. In very hot weather my car just starts to hit 82C while driving at higher speeds. It never gets much above 84C while sitting for over a 1/2 at idle. Standard trans cars also run much cooler.
Any car with auto and AC should use the 79C thermostat.

[Longtooth]

Benz Dr. -- fwiw, the 250SL water/oil heat exchanger is called an "oil cooler" in the MB parts description.  I just today looked into availability from both my local MB dealer's parts dept and MB CLassic Center in Irvine.  Both locations documentation blow-ups parts list and describe it as an "oil cooler".... since it's purpose and reason d'etre is to cool the oil.

If you bypass the oil cooler ... i.e. connect the input and output coolant lines to one another so that no coolant enters the exchanger, the engine heats up more at speeds when the engine's working harder... i.e. oil runs hotter because the engine oil isn't being cooled at all by the coolant thru the heat exchanger.  

This was a quick and dirty design fix to the 250SL engine's heating due to it's increased displacement over the 230SL's, even though the M129 (250SL engine) has coolant flow between 2 pairs of it's cylinders.  The problem with the 250SL's oil cooler was that it was insufficient for the 280SL (M130) engine's oil cooling requirements which lost the coolant space between the 2 pairs of cylinders due to it's increased bore plus it added hp and thus higher thermal dissipation over the 250SL's engine.  The 250SL's heat exchanger is also in a difficult place to remove/replace if it developed a crack... i.e. higher labor content.... hence the change to the 280SL's oil cooler (on the sides of the radiator) covered the issues well enough for an engine without it's coolant passages or oil passeges obstructed -- but with the 280SL engines now approaching or exceeding 40 years of use in varying degrees, these passages are often-times partially blocked... especially the coolant passages... by calcium deposit build-up over time... not to mention rust related obstructions.  Hence, the acceptable and required level of cooling in the 280SL's cooling capacity is reduced significantly in this circumstance.  The addition of a fan at the radiator to augment the air/water heat exchange at the radiator compensates for the ruded coolant volume flow thru the engine.... especially at idle or low engine speeds without car's motion forcing additional air volume thru the radiator.... as well as lower coolant volume flow at idle.

[DavidBrough]

Following what appears to be a successful cure for my overheating I thought it worthwhile posting what I have gleaned about “normal” operating temperatures and what MB say about it. The table below I think gives a good summary of what happens when but I can give no guarantee as to the accuracy of individual temperature gauges, which may vary:-

Deg F   Deg C      Action
135   75      MB noted minimum normal operating temp
174   79      Summer thermostat opens
180   82      Marking on most gauges (considered normal operating temp by many)
187   87      Winter thermostat opens
194   90      Viscous fan starts to fully lock
203   95      Viscous fan fully locked & MB noted max normal operating temp
212   100      Next mark on gauge above 180 dot
239   115      Red dot and MB noted max allowable temp and pressure cap blow point
250   121      Top mark and overheated

Most gauge marks are therefore 180 – 212 – 239(red) – 250 or 82 – 100 – 115(red) – 121 and MB say that normal operating temp is 135/203 or 75/95 with stints up to 212/100 acceptable and usual in heavy traffic.  The maximum permissible temp is the red line at 239 or 115, the limit of the pressure cap. However, if you reach this level, the cap will blow and the water will boil so don’t go there.

The upshot of all this is that our cars can and often do operate up to the dot above 180 i.e. 212 or 100 without concern, just don’t let yours go above it.

I now have a 95/90 temperature switch for my electric fan and pump which keeps things neatly under control.

David Brough
1969 280SL Auto with A/C and 3.46 Axle

[Ziggy]

As per Dr. Benz' advice, I took an electrical water pump off a 124 and will install it soon. I am thinking of a seperate switch, just to see which is more effective, the electric water pump or the fan.

[DavidBrough]

Hi Ziggy,

I tried a small 20ltr/min pump in the heater bypass line and found it made little or no difference for me. The pump I have in the bottom radiator hose is 80ltr/min and has a much more profound impact on the temperature. It cuts in at 95 degrees and removes 5 degrees in about 10 seconds. I think it will take a bit longer with higher temperatures but the fan on its own struggles to reduce the temperature by much at all and just can’t cope when things get really hot. In addition, the pump will also work with the engine turned off to eliminate any chance of heat soak which is also very reassuring.


 

David Brough
1969 280SL Auto with A/C & 3.46 Axle

[waqas]

quote:Originally posted by Q45Denver

Would I need to install a heater bypass in order for the aux pump to work in hot weather?

It would appear so, as where would the coolant go if the heater was switched off? The real question is, does it make better sense to bypass just the the heater (thereby re-circulating), or from the back of the head to directly to the upper radiator hose?

quote:Is there a temp sensor in the block I could connect it too?

I'm sure there's such a temp sensor on the M130, but is there such a thing on the M127 and M129 engines?  Do these sensor signals have any hysteresis?  This would be useful in order to avoid rapidly switching the pump on/off when the temp is floating around the trip point. Or am I over-analyzing this? (don't answer that)

[DavidBrough]

If you want to install a pump in the heater circuit you will need to also install a bypass line for it to work with the heater off. The one I tried just fed back to the normal water pump housing by the bottom hose and wasn’t very effective. A feed to the top hose may be more effective but would require some degree of additional modification and would also bypass the thermostat.

I don’t think there are any existing temperature switches that could be used but there are a number of devices available that are a simple fit into the top hose to control fans and pumps. I had the top casing of my radiator modified so a standard temperature switch would screw in and it works fine. Temperature switches usually operate with a throw of 5 or 10 degrees depending on application and I’ve found that an on at 95 degrees and off at 90 degrees works for me. I first had a 95/85 but once on that almost never switched off so I tried a 92/87 but that was still an over high cut off so I ended up with the 95/90.

David Brough
1969 280SL Auto with A/C & 3.46 Axle

[paulr]

Hi David

I'm sorry to keep bringing this up, but can you give us your definitive thoughts on the use of an additional switchable electric water pump, without the extra fans etc. If one's car is working "normally" with everything having been checked, like radiator and fan yet still in City hot traffic gets worryingly hot, does the fan make a difference ?

Both Roger Edwards and Kevin O'Keefe wont fit one or even consider the heater bypass and just say that our cars dont need it but traffic was different in those days and although everything has now been done to mine like HE radiator just cleaned out, fan bought up to scratch etc it still gets hot when sitting in traffic and I just can't keep turning up the heater and revving the engine.

Fancy fitting one for me?

[Q45Denver]

If your cooling system is marginal you may also try running straight distilled water and some Water Wetter should be good for about 10 degrees.

[Ziggy]

Paul, I have a radiator with extra cooling core, heater bypass, electrical fan and recently build in an electrical water pump.
The summer is over here, so I haven't been able to test it really well but it seems the waterpump on it's own (has seperate switch) seems to do little in a traffic jam. Combined with the electrical fan (put on at the right moment) it seems to be able to keep the temperature at around 90 degrees C in D. In N I don't need the fan or the waterpump, even though the revs are only about 200 rpm higher..

Do you have the newer type oil pan? This is supposed to make a noticable difference. I'm planning to install one in the near future.

[paulr]

Hi Ziggy,

What does the oil pan do, where is it and how does it contribute to cooling ? Would you mind explaining please.

p

[DavidBrough]

A finned oil pan sounds interesting and would certainly add to the cooling capacity but I doubt it would make much difference when stationary as there would be no airflow. What you need is either extra water flow or air flow or both.

I found the electric pump fitted in the bottom hose to be excellent even cooling the engine without the fan when the car is stationary. Once on the move it’s not really needed as normal air flow always brings the temperature down below 90 even on hot days. I’m so impressed with it I’m going to remove my electric fan and put the standard viscous unit back as I think that would be the ideal set up. All I have to do is work out the plumbing as it’s more tricky to fit with the standard fan. I’m also going to try changing my existing 79deg thermostat for an 83deg one to stabilise the temperature a bit as I’m so confident in the ability of the pump. I think we may sometimes think these cars should run cooler than they do because we’ve mostly fitted 79deg thermostats and forget that when new UK cars came with an 87deg thermostat so normal running temperature would be 90c or 194f not the widely quoted 180f which is just 82c. A 79 deg thermostat can give some wide fluctuations in operating temperature as the system is effectively overcooling in some circumstances and this is what fuels overheating concerns. The issue is just controlling temperature rise when there is little or no natural airflow through the radiator.

Ziggy, I see that your electric pump didn’t seem to work well, did you fit it in the heater bypass or the bottom hose and what make and capacity is it? I tried a 20ltr pump in the heater by pass and it made no difference at all but an 80ltr pump in the bottom hose works very well.

Paul, sorry to hear you are still having problems. I would be happy to help you fit an electric pump once I’ve worked out how to fit it with the standard fan. If you still have my number give me a call or e-mail me.


David Brough
1969 280SL Auto with A/C & 3.46 Axle

[Ziggy]

The new type oil pan is deeper than the old, increasing the amount of oil in the engine. I'm not sure by how much but I thought it to be approximately 1 liter. This is important as something like 50% of the cooling is actually being done by oil.

As per Dr. Benz' advice, I took an electrical water pump off a 124 type. It is a Bosch I think, don't know the flow per minute but it is only a small pump which can not be fitted in the radiator hose, therefore it had to be fitted at the heater bypass.

I can see why a big 80 liter/minute pump in the radiator hose would make a big difference.

[Q45Denver]

Any one know the approximate flow rate on the stock water pump? Hpw big a job is replacing the oil pan?