Overheating '74 450SL

[sreams]

This one has me stumped, so I thought I'd ask to see if anybody has any thoughts on it...

I have a '74 450SL that overheats. It does okay in traffic and at lower speeds with temps hovering at or near 185 deg, but cruising at anything above about 75MPH on the highway results in the temp gradually rising up to the 210-220 range (and higher if I'm not careful). This happens on 85+ deg days.

So... I don't think airflow is the issue. Obviously there is plenty of airflow at 80MPH. The AC isn't currently charged, so that isn't contributing to the problem. The AC condenser is straight and free of obstructions. I've replaced the thermostat (twice for good measure) and the water pump. I've made sure the belts are all good and tight and not slipping. When I drain the coolant it is completely free of oil or rust. It is a nice clear green. With the car running, the upper hose gets nice and hot while the bottom hose remains lukewarm (until it overheats, of course). Timing is spot on (5deg ATDC at idle with vac connected, 7 BTDC with it disconnected, 33-34deg BTDC max advance at high rpm).

Any help would be greatly appreciated.

-S

[michaeld]

sreams,
What you described sounds a lot like what I experienced. It turned out to be my original radiator, which was 40% blocked.

In my particular case, the car ran fine for low rpm trips and shorter freeway jaunts: it wasn't until I made a long drive that I saw it start climbing past that 200+ mark.

I got a new core and everything was hunky dory (at least, for several months when I started to experience a different overheating issue).

I'm still not sure yet, by I'm HOPING that my new issue (which started only gradually on freeway drives) is fuel-mixture related. When I passed my last smog test, the CO% measured at 0.00.