I’m going completely insane over this. This is regarding my (new/old) 84 300D with almost 200,000 miles on her. This car has run warm consistently since I bought it 5 weeks ago. But now it is running even hotter.

When I buy a car, one of the first things I do is a thorough cooling system flush and then I replace all the hoses and water pump, which I did. (I believe in preventative overkill, to a certain extent). The system was already clean, as the flush water was not very discolored at all. I used an OEM type water pump (Meyle – is this an actual OEM supplier?)

The radiator was original, and Last Friday it succumbed to the “neck crack” syndrome. Luckily, I caught it immediately, so I didn’t fry the engine. It *did not* overheat. I replaced the radiator with a new Behr OEM type unit. The first replacement thermostat (Wahler) was defective, and I replaced it with a new dealer supplied OE one. It is positioned correctly, with the arrow pointing up. I filled her up with a 40/60 mix of phosphate free antifreeze + Water Wetter. The auxiliary fan functions properly. The viscous fan clutch is O.K. The metal bypass line is clear. There is no air trapped in the cylinder head, since it is self-venting.

The temperature gauge is reading accurately, confirmed multiple times with an infrared non-contact thermometer used on the engine block & cylinder head. The new (2nd) thermostat was checked in a pot of water, and it starts opening at 180 degrees.

After all this, the engine runs at 97- 99 degrees on the gauge, no matter what the load – light load, medium load, A/C on, A/C off, 60 degrees ambient, 90 degrees ambient, etc. You get the picture. Can you spell F-R-U-S-T-R-A-T-I-O-N? I guess that I should be satisfied that it is at least *stable* at 97-99 degrees, but I am ready to try a THIRD thermostat. I can’t think of anything else.

<< UPDATE Since I first wrote this: Today I ran the car on the freeway in hot weather with the A/C on, and also in traffic, and the temp went up to 103-105. >>

IF the head gasket were leaking into the cooling system, I don’t believe that the cooling system would retain pressure in it overnight, which it does. Unless there is such a phenomena as a one-way leak (?!).

To reiterate: The system was flushed thoroughly. I now have a new radiator, new hoses, new OEM pump, new thermostat(s), proper coolant, a functioning visco clutch, a functioning auxiliary fan, a clear bypass line, and this thing still runs HOT. I am at my wits end. Plus, there is a terrible feeling of deja-vu happening here – when I first discovered MercedesShop.com, I was having cooling system problems with another car, hence my chosen name: RunningTooHot.

Anyways, I hope that someone can point me in a direction that perhaps I’ve overlooked. I like to delude myself into thinking that I’m a fairly competent DIY’er, but this is making me feel like a moron.

Thanks in advance for any suggestions (other than selling the car, thank you :) )

RTH

. . . pretty thorough so far
 

I can understand your frustration but 100C is only 212F, so that's not too hot at all!

Maybe try to turn the aux fan(s) on a little sooner with an external resistor, and that should lower it to around 95C or so.

No water? Maybe that's the reason...

I may catch some flack from saying this but I would try running it without a thermostat and see what happens. Also, try 100% water. I have seen holes drilled in the outer plate of thermostats to help cooling issues. I would only do this for testing purposes and would not consider it a long term fix (especially the 100% water idea) unless removal of the thermostat makes it run "normal".

Water could be one factot as pointed out above.

The other one - is to see if the fan actually kicks in when the temp heats up. It could boil down to a simple fuse.

At highway speeds the fan should have no effect on cooling since the "ram air" due to the vehicles speed is sufficient unless the airflow across/through the radiator is obstructed. So I wouldn't pursue that. I lived in Dallas for a while and it was not uncommon for the car to run at 105 C and hotter in traffic in the summer.
I'm not familier with diesels except in principle, but if a gas engine runs too lean it will run very hot; is this true also for a diesel???????

Does this car have an auxiliary water pump?

It's the aftermarket water pump. Go get a reman pump from the dealer - last I checked it was priced at $25. Compare the impellers between the MB pump and the one you got. I bet there's a dramatic difference. The factory pump has a cast impeller with very tight clearances to the water pump housing. After market pumps I've seen use a smaller cast impeller, sometimes with fewer vanes. The really bad ones use a stamped sheet metal impeller. These are guaranteed to add 10C to your coolant temperature :)

With a little practice you can get the water pump out of an old 300D in 20 minutes...

It's normal for coolant temperature to run as high as 100C if you're working the car hard on the highway, a/c blasting, and hot weather. However, it should stay near 90C in gentle stop & go traffic.

Thank you everyone for the help!
 

Thank you everyone for the help! I greatly appreciate it! I’ll address each one of your replies individually, in the order received.

JimF: I know that the later model cars such as the ones you own run hotter, and 100 doesn’t seem out of line, but it is not normal on a diesel of this vintage. I actually did turn on the fan manually *just* to see what would happen, although I knew in advance that this was going to be futile. It was.


Rainmaker: Please. I recognize in retrospect that my grammar was incorrect, but then again I was not an English major :) . My writing “I filled her up with a 40/60 mix of phosphate free antifreeze + Water Wetter” meant that it was a 40% anti-freeze / 60% water mixture PLUS Water Wetter. Seeing that the cost of Water Wetter would work out to about $75 per gallon, this form of ignorance would have been not only just plain stupid, but very expensive as well. I guess that *everything* needs to be ruled out though….


Engatwork: The way that these systems are engineered is such that the thermostat does not work in the typical manner. It functions to open the passageway to the radiator as well as acts to close off the bypass circuit at the same time. Running this type system without a thermostat will not cause ALL the coolant to go through the radiator with each circulation, whereas on (most) other cars it would.


Ymsin: Oh how I wished that were the case. But much to my surprise, I discovered that on this car there is NO thermo switch sensing the coolant temperature for fan activation. Really. It is in the wiring diagram. It kind of blew my mind, but then again, that seems easy to do lately :) . The auxiliary fan is switched on ONLY via the temp sensor at the A/C receiver-dryer. And it does work – the fan comes on with the AC.


Intruder: Absolutely correct about the ram air effect. But, there is no blockage to my radiator. My gas engine cars behave exactly as you describe. The diesel should not be this hot under light loads & low ambient temperatures. Diesels run lean inherently, but that is another thread altogether. It does not make them run hot in the same manner as a gasoline engine.


Yal: Yes it does have an auxiliary water pump. It’s primary purpose is not for engine cooling however. It circulates water to the heater core.


Jcyuhn: I saw the 'Pep-Boys special' water pump with the stamped sheet metal vanes. I passed on that! I can only imagine the problems with cavitation that pump would cause, although American cars have used that type since the dawn of time itself. The pump I put in does have the cast impeller, but I did not actually measure the diameter. I am desperate enough that I am going to go get a dealer supplied pump & try it out. Too bad I ***** canned my old original pump with the official star on it – the core charge is $67! Maybe Mercedes prices it’s pumps so low to act as a sort of “education” for us DIY types. In other words, “Come to us for parts, they’re not *that* expensive, and if you already tried an inferior product, we’ll bend you over a barrel for not having OUR core!” (Just a conspiracy theory for those paranoid types, as well a for entertainment purposes.) ;)

If anyone else out there wants to take a stab at this, *Please* do!

THANKS AGAIN EVERYONE!
RTH

thanks RTH for the heads up as to the circulation operation. Another thought did come to mind too. One day while driving with the temp up to the 100 mark turn the defroast on full blast and watch the water temp gauge. I can make the one in the E320 drop at least 10 dC by doing this. Believe me, I don't leave it on long. Anyway, it is my opinion, that if you can't make it move doing this there is a problem with the sensor/gauge system.

Well everyone, I bought a Genuine Mercedes Benz water pump today from the local dealer (sorry Fast Lane – as quick as you are, I wanted it NOW, plus you don’t have the OE pumps.)

Call me anal, but I had to determine the differences between the two pumps. The impeller diameter is the same. The design of the impeller is *almost* the same, except for a more squared off 45 degree edge to the ends of the blades on the Meyle. The blades themselves are MUCH thicker compared to the OE pump. Then again, the rebuilt OE pump may have a lot of material missing from the impeller compared to when it was first cast. Another seemingly critical (and probably MAIN) difference is that the end of the impeller (not the shaft itself) is about 2-3 mm shorter on the Meyle pump. This will equate to an increased gap between the pump impeller & the rear of the pump housing which will decrease its efficiency. 1/10th of an inch may not sound like much, but we’ll soon see. Stay tuned…. I’m going to go button it up now.

And Engatwork, I tried the heater method of cooling her down, and it did not make much of a difference, at least not as much as if should. It went down a bit & jumped right back up again. The sensor & gauge system is working perfectly, substantiated by using the infrared thermometer. (A nice little addition to the tool box, if I may say.)

IF this doesn’t work, I’m going to throw in the towel on this. I hate to admit defeat, but if this car wants to do an impersonation of Chernobyl, so be it. I just hope it doesn’t melt down when I’m on an extended trip.

Thanks again everyone!
RTH

Here are some more longshots to add to your bag of worms; sorry in advance. The transmission is a possible cause by putting more load on the engine (thus higher temp) than usual. The pressure pumps inside the tranny can put excessive loads on the torque converter which the engine must overcome. It's a long shot.

The same can hold true for any of the accessory items, especially the a/c compressor. But, if you've ruled out the radiator, the water pump, airflow blockage and water passage obstructions I think you might want to consider these.

Another one is an obstructed exhaust but you probably would notice the change in sound and the loss of power.

It's all I can think of.

I was desperately grasping at straws, and to no avail. The OE pump did not change a thing. I am completely stupefied by this car.

All I can hope for is that one of the ‘Super Techs’ here can pipe in on this exasperating situation. There is no substitute for experience. Someone that has witnessed the idiosyncrasies of dozens or even hundreds of these cars may have an idea. Perhaps one of their customers has had a similar problem; perhaps it is an early sign of greater problems yet to come, such as a head gasket in the early stages of failure? I don’t know.

Additional information: This does not seem to be a problem with cooling capacity per se. For example, when the gauge was reading about 98, I checked the temperature of the *hot* side of the thermostat housing, the infrared thermometer read 99 degrees. The (cooled) water return pipe going from the bottom of the radiator to the thermostat housing reads between 60-65 degrees; so the radiator is cooling the coolant. This would seem to indicate that the thermostat is not opening properly, but I’ve now had THREE different thermostats in the car. What are the odds of 3 bad thermostats in a row? Rather astronomical. If only I had such ‘luck’ with those odds consistently, I’d play the lottery!

Thanks Again Everyone!
RTH

RTH:

I'm certainly no pro, but am curious about this situation.

You mentioned replacing the radiator with a Behr OEM.

Is there any possibility that the capacity or the flow characteristics of this unit are different than it's predecessor? Just a thought.

This is a stab in the dark, but here goes. Does the intake manifold look like it has ever been off? Can you see RTV silicon or the likes around the edges. It could be quite possible that the flow could be restricted due to carelessness with the use of such products. I had a 72 vette that I had to go back and "clean up".

Good luck,

Steve