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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.
Your radiator can either provide sufficient cooling or it can't. Drilling holes in the thermostat will accomplish nothing. If your thermostat is opening at 80c but your water temperature is over 100c you either have a coolant circulation problem in general or a blockage in the radiator. Drilling holes in the thermostat will allow "some" circulation when the coolant is below 80c. Inasmuch as your coolant is apparently never below 80c (after warmup) then drilling holes is a waste of time.
You can try an infrared thermometer to see if you have any hot or cold spots in the radiator, but being that you just had the radiator re-cored I'd guess your problem is circulation and not the radiator.

P.S. Retarded timing can cause overheating. Did you check the timing? Also a collapsed lower radiator hose can cause overheating.
 

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Discussion Starter #22
Your radiator can either provide sufficient cooling or it can't. Drilling holes in the thermostat will accomplish nothing. If your thermostat is opening at 80c but your water temperature is over 100c you either have a coolant circulation problem in general or a blockage in the radiator. Drilling holes in the thermostat will allow "some" circulation when the coolant is below 80c. Inasmuch as your coolant is apparently never below 80c (after warmup) then drilling holes is a waste of time.
You can try an infrared thermometer to see if you have any hot or cold spots in the radiator, but being that you just had the radiator re-cored I'd guess your problem is circulation and not the radiator.

P.S. Retarded timing can cause overheating. Did you check the timing? Also a collapsed lower radiator hose can cause overheating.
I'll check the timing. I had an Indie shop look it over before I had is smoged and one of the issues they dealt with was a high idle. I don't know what they did to it. The reason I wanted do drill holes was to improve circulation. I'm thinking drilling holes as big as possible, like 1/8" or 1/4" if there is room. Drilling several of those wouldn't help much? All the hoses are new. Everything is new.
 

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I'll check the timing. I had an Indie shop look it over before I had is smoged and one of the issues they dealt with was a high idle. I don't know what they did to it. The reason I wanted do drill holes was to improve circulation. I'm thinking drilling holes as big as possible, like 1/8" or 1/4" if there is room. Drilling several of those wouldn't help much? All the hoses are new. Everything is new.
No. Drilling holes will accomplish nothing. At 100c+ the thermostat is already wide open, making holes in the already fully open baffle will do nothing.
Does you aux cooling fan come on at 100c?
 

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I'll check the timing. I had an Indie shop look it over before I had is smoged and one of the issues they dealt with was a high idle. I don't know what they did to it. The reason I wanted do drill holes was to improve circulation. I'm thinking drilling holes as big as possible, like 1/8" or 1/4" if there is room. Drilling several of those wouldn't help much? All the hoses are new. Everything is new.
I would find out what they did to it.

RE Lean condition: Can you check the duty cycle of the fuel ECU components? If they did something to muck with the idle and fuel management systems, they might have defeated something which is leading to a lean condition.
 

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Discussion Starter #25
I would find out what they did to it.

RE Lean condition: Can you check the duty cycle of the fuel ECU components? If they did something to muck with the idle and fuel management systems, they might have defeated something which is leading to a lean condition.
I would have to look into how to do that. I richened up the fuel mixture and that seemed to help a little. Ignition timing is 19 degrees. I found a 75 degree thermostat and ordered that today.
 

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Mirv,

Consider a detergent flush followed by a citric acid flush - I was shocked how much gunk came out when I did this. Also consider a radiator recore with more tubes for more heat capacity since your displacement has increased. Finally, evaluate the health of your visco fan.

None of this is easy or cheap - I fought it for 4 years.
 

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Timing 19 degrees at idle? You need to check the spec. for your model. 19 is not correct.
 

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I would have to look into how to do that. I richened up the fuel mixture and that seemed to help a little. Ignition timing is 19 degrees. I found a 75 degree thermostat and ordered that today.
Find your proper timing spec for the engine (m117?). Set that first, once you've done that, Google myndex's kjet dianogstic diys. Really easy to do.

This is assuming you still have the the old fuel ECU components.

You might not pass smog with this though. I'd get ready for a visit to a smog referee to have them modify your smog requirements for the engine changeover.

Not the end of the world since you still have the old smog components and plumbing.
 

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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.
This is a bad idea. I am sure we have discussed it before. Do a search. And, have a look at the diagram of the cooling system in the shop manual and see how the thermostat works before you start drilling holes or removing it. Holes in the large valve will affect engine warm-up. Holes in the small valve will partially bypass rad and reduce cooling.

The thermostat temperature rating probably makes no difference. It only determines when the thermostat will start to open after the engine is warmed up. The thermostat will be fully open whether it is a 75 or an 80C once engine is at operating temperature. (not sure about this thermostat, but most are fully open when about 15-20deg above rated temperature. Allso don't know if open area for flow when open is same on the 75 and 80C thermostats. You don't want to restrict flow.

At this point, the main thermostat valve (larger one) should be fully open and the smaller one should be fully closed. This because the smaller one allows some coolant to bypass the radiator. Not all engines have that bypass valve and this may cause some misunderstandings.

Thermostats do have to be oriented correctly - There should be a vent in the main valve that lets air bleed out. It must be at the top.

My engine manual doesn't cover the 500SEC which apparently had an M117.965 engine? I am assuming cooling circuit is similar to the earlier M117s. That is a much bigger engine than the original. Do you know how the 500SEC and 380SL radiators compare? Presumably you do have the 500sec pump and fan (if they are different)

I didn't understand your timing figures. But if engine timing is too retarded, it will run hot.

This video shows how a thermostat with bypass valve works. Shows warmup first with big valve closed, mid point with some bypass and some coolant to rad, Full cooling with bypass closed and all coolant to rad.

 

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The only question I have is does the 380 water pump push the same volume as the 500 pump? Flush the entire cooling system, pressure test the system, and, possibly, have the radiator boiled out and leak tested. You may need a new rad, or, at least, a re core if it can be done.
 

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Discussion Starter #31
I appreciate all of the help. I'm going to try to address all questions, comments and suggestions here. I didn't check the timing correctly. I just hooked up my timing light, without disconnecting the vacuum. I'll check it correctly tomorrow. Coolant volume of the 500 SEC is 13.7 quarts and the 380SL is 13.2 quarts. Not much difference. I don't know the difference in radiator size but since others have done this swap (and I don't recall anything about cooling issues) I didn't expect this issue. When I had the new core put in the radiator, I told them what it was going in and what engine it would be used with. Both engines use the same water pump. Pump is 4 years old but has less than 1000 miles uf use. I'll get an infra red thermometer and check for hot spots. I'll also do the Tide and citric acid flushes. There's a store around the corner from me that sells citric acid so I'll get that and the thermometer tomorrow. I'm reading opposing views on drilling holes in the thermostat so I'm just going to do it so I'll know for sure. I've got more than one thermostat so if it doesn't work, I can just swap it out.
 

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I appreciate all of the help. I'm going to try to address all questions, comments and suggestions here. I didn't check the timing correctly. I just hooked up my timing light, without disconnecting the vacuum. I'll check it correctly tomorrow. Coolant volume of the 500 SEC is 13.7 quarts and the 380SL is 13.2 quarts. Not much difference. I don't know the difference in radiator size but since others have done this swap (and I don't recall anything about cooling issues) I didn't expect this issue. When I had the new core put in the radiator, I told them what it was going in and what engine it would be used with. Both engines use the same water pump. Pump is 4 years old but has less than 1000 miles uf use. I'll get an infra red thermometer and check for hot spots. I'll also do the Tide and citric acid flushes. There's a store around the corner from me that sells citric acid so I'll get that and the thermometer tomorrow. I'm reading opposing views on drilling holes in the thermostat so I'm just going to do it so I'll know for sure. I've got more than one thermostat so if it doesn't work, I can just swap it out.
Just a thought but when I had the dual chain conversion done (20 years ago) the shop I used applied gobs of silicon sealant to seal the water pumpt. My car was eventually taken of fhe road for another reason and the car sat unused for the next 20 years. Yesterday I removed the left side head in search of the cause of a loud clacking sound comming from the engine and I found medium sized balls of the same silicon sealant that was used on the water pump, lodged in the coolant passages in the head. Something to think about if silicon was used on your water pump replacement.
 

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I'm reading opposing views on drilling holes in the thermostat so I'm just going to do it so I'll know for sure. I've got more than one thermostat so if it doesn't work, I can just swap it out.
If you drill any holes, only drill them in the large main valve. Those would only affect your warmup. Don't drill any in the smaller diameter disk. That would make things worse. This is not an opinion - just simple hydraulics. You don't want coolant to bypass the radiator.

Make sure you have the correct thermostat and gasket for the thermostat housing on the exact engine you have. I seem to recall there being some differences in either thermostats or gaskets. Memory is not so good! Someone else may recall.
 

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If you drill any holes, only drill them in the large main valve. Those would only affect your warmup. Don't drill any in the smaller diameter disk. That would make things worse. This is not an opinion - just simple hydraulics. You don't want coolant to bypass the radiator.

Make sure you have the correct thermostat and gasket for the thermostat housing on the exact engine you have. I seem to recall there being some differences in either thermostats or gaskets. Memory is not so good! Someone else may recall.
Drilling holes will accomplish nothing other than to increase warmup time and destroy an otherwise good thermostat.
 

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Drilling holes will accomplish nothing other than to increase warmup time and destroy an otherwise good thermostat.
That's what I have told him (twice :) ) And so did you. But, where warm up is not an important factor, drilling holes in the big valve that is open anyway when car is warm, won't hurt much. It also won't help with overheating problem once valve opens. But if intent to drill holes, that's the part to drill them in :)

  • make sure the thermostat is the right one with the correct gasket and installed correctly
  • make sure timing is correct so the heat is not due to an incorrectly adjusted engine.
  • make sure that the visco fan is the correct one and that it is working
  • make sure nothing blocking air flow across the radiator (like blocked A/C condenser?)
  • make sure water pump is sound and has original pulleys and rpm. Some cheap aftermarket pumps have been reported to fail early.
  • make sure block passages are clean.
  • make sure heat is not caused by exhaust gas leak into coolant. Pressure tested yet?
  • Hoses good (not collapsed)
  • 100C switch works and aux fan comes on at over 100C
  • Check block doesn't have air lock. Park car on slope with front up and remove or loosen AAV until front top of coolant passages are full.
Just a summary. Maybe others can add.
 
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