'90 300CE Run-Hot Issue - Didn't see same in Archives:
 

Folks -

Interesting situation, and I didn't see the same symptoms in the archives:

Car warms up and drives fine, temp to middle of the range no problem, probably 85-90 degrees C. In traffic (this is winter, mind you, ambient temp somewhere around 45 degrees F), car will heat up to a solid 110 degrees, until I turn the heater on full blast, at which time the temp drops IMMEDIATELY back to the temp as noted above (85-90). If instead I rev the engine, it drops some, but slower and not as far. Thoughts?

-Reed
1990 300CE
1984 300D-T
1970 BMW 2800

check the following

thermostat,

cooling fan operation-is yours an electromagnetic or the viscous type/-either way check that it is working fine.

if it is electromagnetic ,then you should hear a click when the fan engages-that is when the car is hot


if it is the viscous type-it should free wheel easily when turned by hand with the engine cold .

when at operating temperature(hot) -viscous fan,with the engine turned off,the fan should not spin more than a 1/4 turn

?blocked radiator.

my 190e used to run way to hot in traffic,-it turned out to be a faulty cooling fan switch-don't know if your car has a similar switch-it may be called an auxillarly fan switch in the u.s.

high speed aux fans should have kicked in and cooled it down. do a search and following the checkout procedures - make sure they are operating properly.

Hello hitch02, I have had similar problems with my 190 2.6. I changed the thermostat, fan clutch, and anti-freeze with 60\40 mix. Finally, I changed the radiator. Problem is now fixed. Now I have to tackle a leaking heater core. I will put that off for as long as possible.:)

Thanks for the input, folks.. Here's where I am (and I'm open to debate on these conclusions):

1. The fan clutch *seems* to funtion properly - does not freewheel at all, hot or cold - can usually spin maybe 1/4 turn, actually, not even.

2. The radiator is warm/hot all over at operating temp, so I don't believe it is misbehaving.

3. The fact that turning on the heater brings the temp down so fast leads me to think that the thermostat is acting right.

I'm left thinking that there's got to be a problem with the auxiliary fans. I don't guess this could be as simple/cheap as the pressure cap...? I'm also wondering if the cool harness would make a difference here, given the situation. Remember, outside temperature was in the 40's or 50's last time this really occurred, also the last time I was in in real traffic. I've been in some lighter traffic the last few days, and seen it reach almost 100 C, but nothing too exciting. I still think that it should reach an operating temp and stay there, and I've read differing opinions on this.

I will check out the fans, fuses, and relays this weekend. Any other thoughts?

Again, thanks for the replies.

-Reed
'90 300CE

a quick test for the high speed fans is to disconnect the plug at the blue painted sensor by the thermostat. the fans should go on at high speed. this is also where the cool harness is plugged into, so make sure that they are working first. i have seen my temps get up there in heavy traffic in the dead of winter (3hr NYC traffic jam). the high speed fans did cool it down but the cycle would then start over very quickly. addition of a cool harness immediately prevented temps from ever going over 100C. old thermostats will certainly add to the problem so i would change that and the pressure cap just to eliminate those possibilities.

jsmith -

Do you mean that the fans should come on full blast when I unplug regardless of temperature? If so, this is definitely the problem, as the fans do nothing when I unplug from the blue connector..

Let me know, and thanks!

-Reed

yes, it's supposed to. do a search on "high speed aux fan". it could be that the fuse is just blown. the high speed circuit uses 15A but has been updated to 25A. double check the threads that discuss this.