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I have a 1981 380SL and swapped a 500 SEC engine into it. Still trying to work the bugs out of it but it keeps overheating. Here in L.A. it's been hot and traffic is bad. I can drive it for 30 to 45 minutes and the temp slowly creeps up. Once it gets to 120, it wants to stay there. The water pump and thermostat are from the 380. They're both about 3 or 4 years old but only have a few hundred miles on them and worked fine in the other engine. New radiator cap and expansion tank. I'm going to look into getting a bigger core for the radiator tomorrow but wanted to know if anyone knows of other radiators that would fit and solve my problem.
 

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I thought the radiators were the same design between the two engines.

Has your thermostat failed or isn't opening up correctly? I'd also confirm if the temperature is 120 with an or temp gauge on the sensor housing.

If so, i would look at the thermostat first.
 

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I thought the radiators were the same design between the two engines.

Has your thermostat failed or isn't opening up correctly? I'd also confirm if the temperature is 120 with an or temp gauge on the sensor housing.

If so, i would look at the thermostat first.
The temp is correct. This thing was boiling violently yesterday. The expansion tank was bulging so badly that I thought it was going to explode. I think that thermostat is OK but I found an auto parts store nearby that has one so I'm going to replace it. I can't do any thing else with it today anyway and that just might be it.
 

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Sounds to me like the core needs replacing but you'd want to eliminate all other possibilities first. My 126 and my 107 both needed new cores and ran fine afterwards. Years of crud in the core restricts the water flow. :(
 

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The temp is correct. This thing was boiling violently yesterday. The expansion tank was bulging so badly that I thought it was going to explode. I think that thermostat is OK but I found an auto parts store nearby that has one so I'm going to replace it. I can't do any thing else with it today anyway and that just might be it.
Did you try turning on the heater? If turning on the heater brings the temp down then you know the radiator isn't providing enough cooling. Of course it could also mean that the thermostat isn't opening all the way.

If you have the time and feel like experimenting... remove the thermostat and drive the car with the heat on full. That will create a situation where all the cooling your system is capable of producing will be at it's full potential. If you can't maintain 85 to 90 C under those conditions then it's time for more radiator or a higher capacity water pump or both.

I'm assuming you already eliminated the viscous fan as the source of your problem. And while you're at it, make sure the aux cooling fan comes on at or just above 100 C.
 

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If I recall the M116 380 used a top tank brass radiator made by Magnetti Merrilli. The M117 in the 560SL used an aluminum side tank radiator made by Behr. Not sure what the Euro spec M117 500SL used and not sure if the 560SL radiator has more heat capacity. But these are worth looking into.
 

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Discussion Starter #7
Did you try turning on the heater? If turning on the heater brings the temp down then you know the radiator isn't providing enough cooling. Of course it could also mean that the thermostat isn't opening all the way.

If you have the time and feel like experimenting... remove the thermostat and drive the car with the heat on full. That will create a situation where all the cooling your system is capable of producing will be at it's full potential. If you can't maintain 85 to 90 C under those conditions then it's time for more radiator or a higher capacity water pump or both.

I'm assuming you already eliminated the viscous fan as the source of your problem. And while you're at it, make sure the aux cooling fan comes on at or just above 100 C.
I changed the thermostat and that helped a little. Drove it in stop and go traffic and after abouit 30 mins it hit 120 (with the heater on high) Got it home and found that I disconnected the electric fan switch when I changed the thermostat. Let it cool for a while, pluged the switch back in and started it up again. Fan came on after 100 but temp still climbed with the fan and the heater on. The viscous fan spins but how do I check it ? I know it's supposed to lock up at some point. If I take the thermostat out, will the o ring gasket be enough of a seal, or will I have to make a gasket? Have you tried driving one of these without a thermostat? I've been told that with some cars, running without a thermostat, the water will circulate so fast that it won't spend enough time in the radiator to cool. Everything is the same as it was before, just a different engine and it never overheated before.
 

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Did you try turning on the heater? If turning on the heater brings the temp down then you know the radiator isn't providing enough cooling. Of course it could also mean that the thermostat isn't opening all the way.

If you have the time and feel like experimenting... remove the thermostat and drive the car with the heat on full. That will create a situation where all the cooling your system is capable of producing will be at it's full potential. If you can't maintain 85 to 90 C under those conditions then it's time for more radiator or a higher capacity water pump or both.

I'm assuming you already eliminated the viscous fan as the source of your problem. And while you're at it, make sure the aux cooling fan comes on at or just above 100 C.
The thermostat also closes a return passage in the cooling system when it opens, so you need to plug that if you run no thermostat. The hole directly behind the thermostat that the round "bottom" plate closes is the one I'm talking about. I don't run a thermostat in my drift car and have a rubber test plug with a bolt in there. I also run a thermostat with the moving parts removed for a restriction.

Sent from my SM-J737A using Tapatalk
 

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I changed the thermostat and that helped a little. Drove it in stop and go traffic and after abouit 30 mins it hit 120 (with the heater on high) Got it home and found that I disconnected the electric fan switch when I changed the thermostat. Let it cool for a while, pluged the switch back in and started it up again. Fan came on after 100 but temp still climbed with the fan and the heater on. The viscous fan spins but how do I check it ? I know it's supposed to lock up at some point. If I take the thermostat out, will the o ring gasket be enough of a seal, or will I have to make a gasket? Have you tried driving one of these without a thermostat? I've been told that with some cars, running without a thermostat, the water will circulate so fast that it won't spend enough time in the radiator to cool. Everything is the same as it was before, just a different engine and it never overheated before.
I'd focus on the thermostat first. I'm willing to bet it wasn't put back in correctly. That's very easy to do.

The clutch fan and aux fan would help reduce a slightly hotter than normal engine, but if your temperature is going all the way up to 120, then that won't help reduce that.
 

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Discussion Starter #10
The thermost was put in correctly. Both of these engines use the same thermostats and water pumps. Do they use the same volume radiators? I'm thinking that they don't because cooling was never an issue with the 3.8.
 

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I've been told that with some cars, running without a thermostat, the water will circulate so fast that it won't spend enough time in the radiator to cool.
That's one of those urban myths that just ain't so. However, running without a thermostat creates the problem of too much cooling, preventing the engine from reaching full operating temperature.
 

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On my 380SL I had the situation where it would run at 100 to 110(deg) C even in cool weather. It turned out that my radiator core had hard water mineral build up. I replaced the radiator with a Denso brand all brass radiator and my temp gauge now stays pretty steady at 85(deg) C even in the hot So. Utah weather.
 

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Discussion Starter #14
The fan clutch is good so at this point I'm pretty sure that the radiator just isn't up to the task of cooling that engine. I found a local radiator shop that's going to look at the radiator and see if they can put a bigger core in it.
 

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I forgot the most important thing. While I was out test driving my car yesterday, I got a "Hey, Nice Car!" while stopped at a light.
 

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Well I had a bigger core put in my radiator and tried it out today. It helps but it's still running hot. I thought I would try it in stop and go traffic and it started flirting with 120 degrees. I had to turn the heater on and put in neuteral whenever I stopped to keep it below 120. While driving it hovers between 100 and 110. I'm going drill holes in the thermostat and see how much that will help. If it still runs hot I'll take it out.
 

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Hey Mirv, before you drill holes in the thermostat how's the aux tank cap? Is it sealing correctly? That might be contributing to your over heating scenario.

There's also a 70C thermostat you can also try. But i dont think drilling holes in the thermostat is going to help.

I'd do a flush first and refill with coolant before drilling holes in the cap.

Out of curiosity, could you have a bad head / intake plenum gasket?
 

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Hey Mirv, before you drill holes in the thermostat how's the aux tank cap? Is it sealing correctly? That might be contributing to your over heating scenario.

There's also a 70C thermostat you can also try. But i dont think drilling holes in the thermostat is going to help.

I'd do a flush first and refill with coolant before drilling holes in the cap.

Out of curiosity, could you have a bad head / intake plenum gasket?
Everything is new. Gaskets are not leaking. I have an 80 degree thermostat right now, can't seem to find a 70 degree anywhere. Drilling holes along the perimeter of the thermostat will allow more coolant to circulate , at least I think it will. At this point, if flushing it doesn't work, I'd rather drill holes in the thermostat than run without one. The only other possibility that I can think of is that it's running too lean. I haven't checked that yet but it was super clean when I just had it smoged.
 
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