Weird 240D cooling system issues... solved? Maybe? - PeachParts Mercedes-Benz Forum

anghrist · #1 04-17-2012, 03:13 PM

Okay folks,

I've become pretty well versed in the way that the old '77 240D 4-speed manual behaves when it comes to cooling. I've

-replaced the radiator
-replaced all thermo sensors/switches
-degreased
-citrius flushed multiple times
-replaced thermostat(s)
-replaced aux fan

Now I can typically tell what's going on with the system after a few interesting incidents.

-The system will generally run just above 80C (81-82C est.) when cruising on the freeway 65-70mph.
-Speeds of 70-75mph (on flat ground) will cause temps to climb to 90-92C.
-Speeds of 75-80mph (on flat ground) will push temps to 93-95C, but generally not over 100C.
-If the system starts to get low on coolant, air pockets will form and the water pump will not flow fluid at idle (temp will increase quickly).
-In the past there was a coolant smell in the cabin on days of over 70F ambient air temp.

Now for the fun part. I took this car in for general maintenance, one of such items was a coolant flush. Drove the car home and everything was fine. Drive on the freeway and everything is as ?normal?

One thing that has changed is no more coolant smell. It's as if there is no vapor leak that I had seen in the past. Also, the car seems to have issues with loosing coolant??

Bought some drinking water to refill the radiator and let the engine cool for a couple of hours. Checked oil level and it was fine. Then I looked at the radiator cap.

The cap that is on the car is rated at 13PSI (0.896 bar for the mathematically challenged).

The caps that came on these cars was rated at 1.2 bar, and there are 1.4 bar rated caps available.

My guess is that the cap is not holding enough pressure to keep all of the coolant and pressure in the system. The vapor that had previously been blowing out at the heater core connections is now (and previously) blowing out of the rediator cap. This causes a loss of coolant, and a need to consistantly refill the system every week.

My plan of action is to replace the radiator cap with a 1.4bar version and reseal/replace the heater core during my HVAC rebuild. That should keep all of the pressure and coolant in the system and will likely end up showing me which hoses need to be replaced under the hood.

In the short term, I will likely loop the heater core and valve out of the engine coolant loop and replace the radiator cap.

Any thoughts, opinions, strong feelings, or lewd comments?

vstech · #2 04-17-2012, 03:59 PM

the 240 has a pitiful small radiator compared to the turbo models, but still should not be overheating or even climbing past 95, so your thermostat could be opening too late. you might want to test your stat with a thermometer in water and watch what temp the beginning to open point is, and what temp fully open is achieved. my bet is the stat is beginning to open at 87 and fully open only after reaching 95. many of the "80C" stats are actually rated at 87 for some reason...

fully loaded (flat out at 8omph) 240 staying below 100C is not bad.

mjquillici · #3 04-18-2012, 04:33 PM

I recently had a problem where my engine temperature would fluctuate and generally increase as I drove faster on the freeway. One day, I realized that the fins on my oil cooler were ~50% clogged with grime. I blew it out, and she now runs consistently at 90 degrees. I wish I'd noticed that in the first place. . .