'91 300E 115k mi
Main & aux fans work, reservoir is full with 50-50 anti-freeze, thermostat works: but it's still boiling.
When driving at speed, temp is normal: 80-90C.
Stop-n-go traffic or when parked: ~110C.
Once engine is off: 115-120C+ leaves puddle on ground.
Auxiliary fans kick on at about 100C. Happens whether A/C is on or not. Worse if outdoor temperature over 90F.
I replaced the radiator (side tank broke) last fall...picked the cheap one; could this be it?

 

I Would First Inspect the Radiator for any COLD spot.s.This would tell you if the radiator core is Plugged
Then Inspect the Fan Clutch..if it locks up when warm..if Not the Fan clutch is defective

 

One easy thing to check for (if you haven't already) on a car this age is the radiator cap itself. My '94 E420 was doing the same thing, boiling over, etc.--I checked the radiator cap and the seal had rotted. A new cap fixed the boiling over problem.

 

The only cap is on the reservoir/overflow tank. When I look at this design, I honestly don't see how this is a pressurized system. The overflow is neatly routed via a hose from the overflow tank to the right side front fender. Why wouldn't it vent out there all the time?
I will replace the cap and see what happens?

Checking for a clog in the raditor is a good idea too. Can someone suggest how to look for a cool spot on the radiator...very hard to get a hand directly on it...especially while running!

 

Not sure about the '91 300E. My engine is definitely pressurized, with warnings printed on the tank.

 

You can check the temp accross the radiator with a contact type or non-contact thermocouple/thermometer. I prefer the non-contact type. You point it, pull the trigger and scan accross the radiator and check for cool spots.

When you said you replaced the radiator before, the car didn't happen to overheat did it ? Those engines are prone to develop leaks in the head gasket, specially if overheated.

Hopefully that's not the case for you.

You can also purchase a dye kit that you can put into the cooling system and check it with a "black light type light" to see exactly where it's coming from. Check your local auto parts house for the dye kits. (or online)

Hope this helps

 

Not sure if the radiator cap helps or not, but just for general info, aren't all cooling systems under pressure when the engine is warm/hot? That's why you have to wait for the engine to cool before removing the cap, right? So if the engine boils over after running, either (a) there's way too much heat/pressure and it's defeating the seal, or (b) the pressure seal is bad. I think the vent is only to drain coolant if it's overfilled; otherwise coolant or steam would always run out when the engine is hot. Just thinking out loud! [sm=smiley25.gif]

 

I checked again and it is pressurized; the overflow hose to the outside is right at the radiator cap.
There is a sucking sound when the boiling is going on...pretty sure that is it now. Will replace it tonight and give results soon.
Thanks for the advice thus far!!

 

A new radiator cap solved the overboil problem...THANKS!

I still have the temperature rise to 110C on a hot (above 90F) day with the engine at IDLE. The auxiliary cooling fans do turn on to drop the temperature down to 95-100C. When driving, temperature is 80-85C.

What's normal temperature for idle speed?
If this behavior is not normal, what's next to check?

Again, THANKS for the help!!

 

I live in Arizona, so my '94 E420 runs at 100-110 a lot with a/c. I think anywhere between 80-110 is ok, especially in hot weather. Good news about the radiator cap--I love an easy (and cheap) solution!!