300Sl Getting too hot...

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New Member
May 9, 2015
East Dulwich
1989 300 SL
R107 300sl going up to 95 degrees after idling for 25 mins, maybe normal!
However, coolant is bubbling out over the extension tank and causing the coolant to obviously leak & go down (unless there's another leak!). Am hoping just to replace the tank and cap as where the brass meets the plastic its not sealed. Has anyone experiences similar? I hoping the leak and slight over heating is due to the pressurised extension tank not holding pressure as seal leak etc but perhaps an underlying issue that caused the seal to break/coolant to bubble over perhaps... radiator or just a seal/pipe broken elsewhere. Any shared experience much appreciated. Has only just come out the garage for a distributor and fuel injectors doh!
Why are you idling the car for 25 minutes ??

Have you checked that the electric fan is kicking in as the coolant temperature rises ?
Cheers for reply. Idling to find the coolant leak... Which I think found as after 25 mins its bubbling out the top when it shoudn't be etc and couldnt see a leak underneath although haven't been able to raise it while checking yet. The fan at the front of the bay is turning on yep after it heats up.
Coolant level was 3cm under the line when cold, after 15 mins idling its at the line and after 25mins its reached the top and its leaking out. Thinking this is happening quick as the extension tank as now essentially not pressurised due to the cap/stem seal leak or crack. Will replace with new mb parts but, wasn't sure if this is due to the coolant system in general not being able to hold some pressure, ie rad problem or seal leak elsewhere etc. Too many possible problems eh!
Update in case anybody wondering! ...Fitted a new coolant ext tank, pipes and cap and flushed radiator with lots of coolant and water. It seems to be running cooler (didn't go above 90 driving round central London yesterday). I'm also, rather reluctantly, keeping one of the blowers on full hot at fan speed 2 to disperse some more heat. Both combined seem to do the trick, however not sure the blower method is going to feel great when driving round southern france in a couple weeks! I'm tempted to get a new thermostat or even another fan, if anybody has done this, but maybe being ott and it will be ok.
You shouldn't need to keep the heater on full heat. You seem to have checked & replaced quite a few parts, but have you inspected the viscous coupling on the main engine fan to be sure it's engaging when hot? You should ne able to hear when it's engaged. Mine exhibited similar issues till it was replaced.

On my 300SE which essentially is the same engine as yours, I was stuck in rush hour traffic last Wednesday in central Birmingham with the air con on. The electric fan only ran on low speed and the temperature gauge sat at a shade over 90 all the while.

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