Overheating at warm up

[Mathias Schroeder]

Dear fellow Pagoda lovers...

I am the happy owner of a US 1970 280SL Automatic A/C, which I have had for 2 years now. Until recently it had not had any issues with overheating.
I decided it was time to change the coolant and the Thermostat.
I now face the problem that the car is overheating very briefly during the warm-up period. It happens very shortly for about 10-20 seconds only, then it seems like the thermostat is opening and the coolant is during its job. It never did this before I changed the thermostat, so I figured that it was a faulty thermostat and ordered a new one which I changed this weekend. Same result:-( Shortly overheats, and then after 10-20 seconds, the temperature drops to around the 180 mark, and stays there until the cars is left for cooling over-night.
I mostly think that its some air trapped around the thermostat that makes it open to late, however I can seems to get it to bleed out anymore air from any of the tricks I have read about here.
Also, I noted that the old thermostat had 4 holes drilled in it, which I guess was there to get the flow happen faster (I read somewhere in this forum, that this could be a trick to keep the engine cooler at all times).
Here is what I have done so far:

- Changed the thermostat twice (with no holes in the new ones)
- Back flushed the cooling system
- Changed the coolant
- Bleeded for air from the top hose on the WRD
- Bleeded for air from the inlet on the heater system
- Bleeded for air, parking the car uphill (steep), Cap off the water tank, warm-up car and top of coolant

Does anyone have a great trick to resolve this issue, like getting the air out somehow?

Thanks in advance for all of your comments!

Best wishes
Mathias Schroeder
Denmark

[tel76]

As you already have a new spare thermostat I would drill a small hole in the top plate and fit it, you could replicate the four holes that were originally there, but I think four is OTT.

[ja17]

Be sure to top-off the coolant since air has left the system. Sounds like you are just a little low on fluid.  Second guess would be the incorrect thermostat.

[ejboyd5]

Drill one or two small holes in the rim of the thermostat - problem solved.

[ctaylor738]

My technique.

Drain radiator and then refill all but a quart/liter through the left side heater hose.  Then unfasten the top radiator hose, lift it up,  and slowly pour the remaining coolant into the hose until its full.  Reconnect the hoses, start the car, and let it run with the pressure cap loose.  Top up coolant in tank after engine warms up.

[Rolf-Dieter ✝︎]

Hello Mathias,

Welcome to the Pagoda SL Group

Some good pointers from members above, hope they help you out. My overheating problem was caused by a faulty fan clutch that was changed out. I don't think that is your problem, I just thought I mention it. Good Luck!

[alchemist]

Most likely is an air bubble in the coolent. Raise the front of the car from both side and run the engine with the filler cap off and watch the level of the coolant. Watch the temperature gauge it should rise above 185F and then it should drop. If this happens, it means that your thermostat is working.

[Mathias Schroeder]

hi all

thanks so much for your help. I drilled 2 small holes in the edge of the thermostat and the problem was gone:-)

Thanks again

best
Mathias