300CD Running hot on highway

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If I were at home already, I would buy a new radiator in a second, but I could also have my local mechanic to look into the problem as well. Since I'm in the middle of a road trip, I'm not in a place where
there's an available knowledgeable mechanic or readily available parts. I can get a Nissens or Behr replacement radiator new for about $250 from multiple places online.

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Thanks for the overwhelming response here... I was on the road all day yesterday, so didn't have a chance to check the posts. Here's the situation as it stands now.
1. Got the radiator flushed and coolant replaced. Coolant was very rusty. Got on the highway and the car ran cooler than it has been (c. 100C)
2. On the highway, car only got up to 120C twice, even with A/C running. About 30 seconds blowing the heater and it comes back down to 100 or even lower and stays there. I can cruise down the road with the A/C on. Problem essentially seems to be fixed at this point.
3. Coming up to Monarch Pass on Highway 50 in Colorado, and the car's still running great. Mountain driving, but the car seems to be handling it just fine. Temp is showing about 100C. I get to the summit, which is about 11000 feet in altitude, turn into a scenic overlook to take a break, and as I'm turning steam starts coming from under the hood and coolant starts overflowing. Temp gauge still isn't over 120C.
4. Only after the car overheated on the pass do things change. On the way downhill, the car continues to heat up, and blowing the heater is to no avail. In fact, the temp gauge TOPS OUT at the BOTTOM of the mountain and the heater stops working - just cold air. Even with the temp gauge at max, there is only a small amount of coolant overflow when I pull over, but the auxiliary fan is not turning.
5. After the car overheats, I'm limping down the highway with the heater on high. The air alternates between being cold and blowing hot. I can tell when the heat is about to start blowing because there's a WET, SUCKING sound from behind the dashboard before the air gets hot. When it's hot the temp comes down, but the hot air only lasts about 60 seconds.
6. I stop the car for about 45 minutes, top off the coolant, and when I start back up the heater works fine and the operating temp stays between 80 and 100C, the coolest it's run all day. With the heater on high, I limp down the road to where I am. Currently, I'm stuck in Grand Junction, CO on a Friday before Memorial Day weekend, with a car that can't make it down the highway without overheating, and I need to get back to Los Angeles. Any thoughts?
Thanks for all the help. Austin

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Put more water than coolant... I'd put in 65% water and 35% coolant... so in your case, just put water in. I think your coolant ratio is 50/50 or 60% or higher... don't do that...water is the main heat dissipator.
Sounds like your radiator cap is bad.... change it with any generic one you can get cheap.
I would run to Autozone to borrow their coolant pressure tester... they loan it out for free...
Get a rag and cover the radiator car with it... release the pressure slowly... once no pressure, remove the cap.
Hook up the gauge and pump to 20 PSI... does the pressure hold for say 5 minutes or 10 minutes? If yes, then you are good.
If no, do you see any leak... where coolant are dripping? Look under the car. If no leak and pressure is dropping, then it is bad news... possible head gasket.
There is another possibility if you got zero leak and zero pressure drop... that somehow your PCV system is clogged... and engine has to work harder to run... thereby more heat... I am not versed enough on W123 or older diesel PCV system... perhaps other will chime in.
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Changed the pressure cap, the old one was pretty decrepit. Haven't had a chance to pressure test the system yet. Noticing that the car is having more difficulty than usual idling when cold.
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If there was a lot of rust and crud in the cooling system you may have already fouled a new thermostat. If it was me, I would take the thermostat out and put the housing back in empty and go for a drive and see if that fixed things. If it does, then you know the problem is a crapped up thermostat. Any small fleck of junk can cause the thermostat to jam up on the rod that it slides on when the bimetal thermal part of it tries to expand/contract.
Did you flush the system with it running and hot and the heater core open? Did you flush it for a long period of time, like half an hour with the motor running?
Paul
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I'm bringing it in to a professional this morning, but I'll be sure to have them check the thermostat. He described my symptoms to me and said he thought it was probably the auxiliary fan not turning. I had the radiator flushed at a Kwik Lube, and it didn't take more than half an hour altogether, so I'm sure there's more flushing that could have been done.

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What an adventure!
If it were my car I'd look for kinked hoses, remove the thermostat, ensure the cooling system is full, check that the lube oil is at the full mark and start driving. The rough idle is most likely due to the altitude, not the overheating.
When you return have the water pump checked.
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It has been quite an adventure, T.G. I'm sure I'll make it back to LA at some point...
On May 26, 10:43 am, "-->> T.G. Lambach <<--" <"T.G. Lambach at NoHamorSpamcomcast.net"> wrote:

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Although the mechanic is making his living fixing cars and may be completely honest and trying to do his best, he is off base about the auxilliary fan. Like Tiger said, the air blast from moving down the highway exceeds anything the fan can do and the auxiliary fan is to help the AC condensor more than remove engine heat. If your auxiliary fan fails, it can easily result in your AC compressor destroying itself, but engine overheating is almost surely something else.
I have had my 300 TDT for many years. Over time it began to run 5-10 degrees C hotter than I was used to seeing it run. I tried many things to get back to the normal operating temperature because on long grades and on very hot days (I live in Inyo county, the home of long grades and Death Valley) it would overheat, even blowing out all the coolant at the top of Towne's pass in Death Valley NP one hot July afternoon. I ended up coasting down to Stovepipe Wells for water.
Nothing I tried with the cooling system seemed to help. I tried the water wetter products, and they helped some. I have always thoroughly flushed the radiator and replaced the coolant on an annual basis, so flushing and coolant didn't make any difference at all. No matter what I tried, the temperature gauge just wouldn't stay down near 180 F like it used to.
Then, unrelated to trying to solve the heating problem, I replaced my fuel injectors. In addition to making the engine run a lot more powerfully, the heating problem also went away. Another diesel mechanic and were talking about that and we figured out that what must have been happening was this; the springs in the injectors were tired after 24 years and 360,000 miles. Since the springs were tired the injectors were firing too soon, having the same effect as advancing the timing, which will make your engine run hot.
Unfortunately a valve stuck open on my engine recently and I decided it was time to have the engine remanufactured at Metric Motors. The engine is there now. It will be expensive, but far less than a new Benz.
The Mercedes is a wonderful vehicle, but has unique engineering and if someone does not have extensive experience with them they will have trouble diagnosing problems.
Please let us know how this shakes out for you and good luck getting home across the Mojave. It has been pretty cool at night still.
Paul

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Wow... that's great info Paul. Even I learned something today. I think this is the case with 'Eyeball Kid"
Since his is lower mileage... I assumed... and maybe his timing is just too advanced... so he will need a MB diesel specialist to do his injection timing adjustment.
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I have a 300D (83 Euro non-turbo) with 420K miles on it. I've had to have the radiator 'rodded' twice since I got it at 80K. Both times, it's like driving a new machine (regarding the normal operating temp and response). Flushing is nice, and should be done every other winter or so, but mechanically cleaning the radiator makes a significant difference!
FYI Jim James L Szatkowski, PE snipped-for-privacy@jlsce.com

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Thanks for that suggestion, Jim. I'll have it done when I make it back. Looks like the most immediate problem is a the fan clutch on my radiator fan. The car is driveable, but I'm going to avoid the heat of the day until I get the clutch replaced.

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Fan clutch has no bearing on highway speed... fan clutch does the job when under 25MPH... once you past that, the air flow will exceed that of the engine fan.
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I second Tiger about the fan clutch - irrevalent at speed. You car's problem is a plugged radiator - insufficient coolant flow through the cores (or insufficient air flow through the mesh). As you know running the heater adds cooling capacity; this proves the radiator has insufficient cooling capacity.
The faster you drive the more BTUs need to be shed via the cooling system so slow down to gain some margin, especially when on a long grade.
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Off the top of my head if the coolant isn't being cooled and at this agre I'd change the rad, water pump, known good (tested!) t-stat and hoses just on principle.
I've been through this. You replace the weakest part of the cooling system then the next weakest part of the coling system is now the weakest part.
Fuggit, renew the important bits and stop worrying about it.
I've sen some pretty bad injectors but never seen them overheat a car with a decent cooling sytem. They should be renewed too, as they get old then squirt rather than spray and can hols pistons. They're only food for about 100K miles.
Having said that my gut tells me just renweing the rad would probably fix this.
--
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
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Well, I made it back to LA from Grand Junction! Drove overnight and the only real heat built up on the long inclines. I cranked the heater and lowered my gear and it made it in one piece. When you're blasting the heat, the sunroof is a godsend for letting heat escape out the top of the cab.
Thanks for that insight into a possible fix, Paul. I think I'll change the rad, since I know it wouldn't hurt, double-check the thermostat and hoses, and have the injection timing checked/adjusted. Still have to get that fan clutch fixed anyway...
On May 29, 1:33 am, snipped-for-privacy@news.vrx.net (Richard Sexton) wrote:

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