Yea, I missed the part about the radiator being replaced. In the spirit of stretching-it, what about the oil. Many years ago I had a Pontiac (Grand Prix) that would slowly overheat during highway speeds.

A guy at the local parts house suggested that the 10w30 oil was too thin. We added a can of STP oil treatment and it solved the problem. I later switched to 10W40 oil which had a similar (thickening) effect.

Don

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2008 ML350
2007 Honda Element
2007 Honda Accord
2009 Honda Civic

Donb,
Well, ya got me there; I HAVE been running Pennzoil 10/30 (a quality Pennsylvania grade crude, unlike the garbage that comes from the West coast). Unlike quite a few other members, I use oil rather than synthetics, but religiously change oil every 6 mos (and rarely ever go over 3000 mi in that period).

I have to agree that a lighter oil certainly could lead to greater chance of heat build up, but my overheating issue was far beyond anything minor; it screamed "something is drastically wrong!" Lighter weight oil could lead to minor increases in temp, but not what I saw.

The thing is, I don't have a mechanic I trust to accurately diagnose my problem. There are some gifted mechs out there; and then there are a lot of guys who earn money working on cars. Too many mechanics do the same thing that I would do; try one thing, and if that doesn't work, try something else. It's possible that engine build-up got into the new radiator core; but I could see this problem being the thermostat, the water pump, or something (worse) else.

The heater doesn't seem to lower the overheating temp as it should, so that might point to a bad pump; but then I felt the hoses and didn't feel pressure after the car got to operating temp, and that sounds like a thermostat. But when I took the thermostat out, it opened at 167F (but maybe not all the way? I don't know...). I do know that I hate randomly replacing parts; but at least if I'm the idiot doing it, and I don't have to pay myself labor, and can thereby afford to buy more parts, and increase the chance that I'll replace the right thing. It's such times that one realizes that skilled mechanics are truly God's gift to car owners.

I've heard that a blown head gasket can cause overheating (and vice versa); that a slipping trans can create overheating; retarded timing; air pockets in the coolant from improper burping; and several others. I had no idea that so many things could cause overheating! I've been hoping to come across something that said, "when you experience x set of symptoms, y is the cause." But so far, no avail.

I'm out of town, and therefore away from my tools. I will replace the thermostat, and hope that takes care of it. If not, I will limp it 100 mi home (pulling over twice for an hour break) and start eliminating candidates.

I still hope that someone has had a similar problem, and has "the fix."

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Love driving my '77 450 SEL!
124,000 miles

DRASTIC overheating on your m117 is the result of 3 possible things:

1) Not enough coolant
2) Blocked passageway to the rad (collapsed/kinked hose, bad t-stat)
3) Bad water pump

If your system is circulating an ample amount of coolant, no matter how hard your M117 is working or revving (within reason), it won't overheat if it's moving. If you overheat rapidly even going downhill with your foot off the gas, you need to check these 3. From the sound of it, #3 is most likely, especially if a low-quality pump was used.

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2024 CR-V Hybrid

Previous:
1972 280SE 4.5
2018 Durango R/T, 2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Pardon my ignorance guys, but does this car have a viscous fan clutch? That was the cause of my hot-running problem. While my article relates to a 95 W140, some of the info may help.

http://v12uberalles.com/fan_clutch.htm

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1995 S600, 1 of 618 (sold)

"Speed is just a question of money...how fast you wanna go?"

LONG LIVE THE W140!
Visit my Web Page at www.v12uberalles.com

pcmaher: It does, but I am 90% sure mine is shot to hell, yet mine NEVER overheats on the highway and barely gets warm in traffic. "Warm" as in the 212 mark. And I have modified my engine as well.

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2024 CR-V Hybrid

Previous:
1972 280SE 4.5
2018 Durango R/T, 2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Pcmaher,
Nothing to pardon; a bad visco fan clutch DOES cause overheating; but it usually does so UNDER 30 mph. I'm overheating more at higher speeds.

Tomguy,
What you're saying is what I had always thought prior to writing my "SOS" posts and looking things up on the internet. Since I was a teen, I'd always narrowed overheating issues to 4 things: coolant, radiator, thermostat, water pump.

As I've been reading, I've heard all kinds of alternative explanations. I have written to 3 Benz posts (as I was seeking a swift response) and you very well may be the first person to say, "I think it's your water pump."

The radiator core is 7 mos old; the water pump and thermostat are less than three years and 4000 miles old. The coolant level was fine, as judging by the clear overflow tank. I note that the old thermo was a Wahler 75C (I purchased a Behr unit); I have no reason to doubt that the current water pump is a quality OEM replacement unit. I purchased a lower hose, because I ALWAYS replace related inexpensive routine items that are under pressure/load when I do a bigger job - especially if that item could possibly be causing the problem (I did not see the hose kink when I looked at it).

As I don't have a gauge to measure V belt tension here, I was loathe to remove the alternator; and that made getting the thermostat housing off a major pain (shoot, this job is a pain anyway; it has got to be the most maintenance-unfriendly system I've ever seen, as I never had to drain a rad and remove alternators to take off a thermostat on any other car I've ever owned). But my plan now is to replace the thermostat and lower hose, add water (just to test the system), flush the system (it it seems to be circulating), and then replace w/ a 50/50 mix. If I still have overheating issues, I will pull off every 30-35 miles during my 100 mi drive home, and eventually be able to replace the pump.

On the subject of water pumps, which unit? I saw a Graf and an Airtex for about the same $$$, any opinions on which is better? Is there a "THE" pump?

When I replaced the rad core, I had no qualms; it was the original radiator. Rads go bad; no worries. But now, I'm looking at parts that SHOULD NOT BE worn out or bad yet. THAT DOES worry me. Why would I have to replace these components so soon after last time?

__________________
Love driving my '77 450 SEL!
124,000 miles

I have an update on my temp issues. I believe that members should write posts telling everyone what the problem WAS so that others can benefit from the thread.

Thus far, I have replaced the thermostat, upper/lower hoses, and the pressure cap. I still seemed to be running hotter than normal, but I have never gotten as hot as I did the first night (when I got to near 250F). Currently, in the heat - without the compressor on - I'm running a little over 180F; it will go up close to the 212F mark at idle at a stop light. This is in the day, in the 110F heat of Palm Springs. At long idle, she seems to go up to about 212F and stay there.

I had a pressure backflush today, figuring it was routine maintenance. The mechanics took a look at the system after the job. They said the water pump and fan clutch were operating fine. If anything, they thought I might have a problem w/ the auxilliary/condenser fan.

From that shop, I immediately drove the car to the shop that replaced my radiator core 8 mos back. They let the car idle for about 20 min and told me pretty much the same thing: the system seems to be running fine, but you might have an issue with the auxillary fan. Last week, I had the opportunity to speak with the owner of MotorWerks - which works on a lot of old Benzes - and he said that older Benzes often run hotter, and he also seemed to zero in on the auxillary fan.

This leads me to some questions:
1) My auxilliary fan DOES come on; but... it doesn't seem to stay on very long; sometimes it starts to turn and then stops; it doesn't seem to come on when the car is getting hot at idle; and, I've had the A/C on the last two days, and pulled over and checked, and it wasn't spinning at all. Does anyone have a clue when these infernal things are supposed to work?

2) I would like to begin by replacing the 212F fan switch.
where is it located? I have a 77 450SEL. Is it mounted on the dryer - which is located on the condenser to the immediate right of the fan? If so, my switch has two prongs.

3) Does an improperly functioning auxillary fan seem to explain my car's running at about 180-185F (maybe plus a couple of degrees) and idling at near 212F?

4) How hot does your car operate in city driving in hot weather? (Keep in mind, I'm in Palm Springs; last week we got to 122F. A COOL day in July/Auguse will run you 106 plus.

5) Does anyone know of a fan switch rated below 212F, and if installing one would be a bad idea? I would be willing to trade a little power for cooler operation.

I'm actually wondering if I really have a problem, or if I'm just paranoid. I may have had a bad thermostat, improperly burped the system when I refilled, and subsequently corrected the issue. But meanwhile my auxillary fan (or the 212F switch) was going out. In any event, I would much rather mess with an auxilliary fan than with a water pump install.
Mike

__________________
Love driving my '77 450 SEL!
124,000 miles

it does sound like you are on the right track with the aux fan.

have you cleaned the connections on the temp sensor for the aux fan"

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I don't have specific exerience with that gasser, but, my opinion is that such temperatures are normal. The diesels, with the older radiators, will reach 212°F. at idle, after a freeway run, with ambient temperatures in the '90s. I'd be chasing an overheat for sure if I had to suffer 110°F. ambients.

I'll also suggest that the replacement of the thermostat, hoses, and pressure cap had no bearing on the results. I'm guessing that you previously had air in the top of the head and you were successful in removing same on the last cooling system repair.

I don't see any issue with running that engine at 212°F., indefinitely, provided that the system pressure holds and you are running proper coolant.