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"Hot Blooded" Aint Always a Good Thing!

Started by michaeld, 15 June 2006, 08:58 PM


Unfortunate Fortnight I'm having;

First I suffer a flat.  Somehow while the flat is being fixed, or immediately on the way home, my right side mirror glass comes off the mirror assembly.  Then my right rear tire (which was likely the original factory spare) shreds apart, causing me all kinds of worries on the drive.  Now, I am overheating!  Between you and me, I only blame my car for the overheating thing.  Still, she didn't leave me stranded.

I was driving from Palm Springs to Anaheim in the land of Disney and Hollywood.  It was 10:00 pm when I left, as I HAD to watch the Tuesday NBA final game.  I'm contentedly driving along, when - about 40 miles into the trip - I noticed I was getting hot.  I'd had my left hand covering the temp gauge from my view, and just didn't notice.  When I did, I was over 212 F.  I turned on the heater, but didn't seem to get relief.  I began looking for an offramp to pull off, but had just run into a looonngg stretch of empty highway.  My anxiety level climbed as my engine temperature went up.  I didn't want to pull off directly onto the shoulder of the highway, as I realized I would be in for a fairly long stop.  FINALLY I saw an offramp, and took it.  As I pulled into a Soup Plantation parking lot, I was very close to reaching redline :o ??? :'(

I saw no signs of coolant leaking, or steam, or oil in the coolant or coolant in the oil.  My coolant level - expanded from the high temp - filled the reservoir.  I waited for half an hour and took off the radiator reservoir cap; no hissing or boiling over.  I intended to wait another half hour.

That was when first one - and immediately after a second - police car pulled up to me hard and fast.  I had a 185 lb Rottweiler who was not happy to see the officers.  They asked me to secure him and then talked to me.  Believe it or not, the sight of the huge Rott frightened the two women who were still inside the restaurant I was in front of, so they called the cops on me.  How many burglars/muggers/rapists have brought their dogs with them?  Fortunately the police were friendly (after processing my ID) and even kept me company.  The adventures of Michaeld continue.

The list of possible cooling system culprits: radiator, lower hose, water pump, thermostat, fan clutch.  I can't imagine it is the electric fan, as the overheating occurred at freeway speed.  My radiator was replaced (a recore) 6 mos ago, so I would be surprised if that were the problem.  I saw no sign that the lower hose was collapsing.  The upper hose gets hot and solid (which tells me coolant is flowing and the thermostat is probably working).  The belts look good.  The fact that turning on the heater didn't seem to help me tells me I probably have a bad water pump.  On the negative side of the pump being the problem, the pump is not making any noises; nor is it loose; nor does the fan spin overly much when I spin it.  I have no idea how to diagnose a bad fan clutch (except by trying to see if the A/C blows cold, which I really don't want to do w/ an overheating engine).

If anyone has faced a similar overheating issue before (gradually overheating at 70-75 mph) I'd like to hear what was causing the issue.  I'm your typical DIY; I am okay at taking a bad part off and replacing it w/ a good part; I'm less good at diagnostic/troubleshooting.

I was freaked out about blowing my head gasket, but (THANK GOD!!!) I have seen no telltale signs - such as gunk on the oil cap, white smoke from the exhaust, or oil in the coolant (or vice versa).  I hear one could conceivably have a blown head gasket and not have any of these symptoms, but I am hopeful.
If I have a blown head gasket, I would have to say a tearful goodbye to my car; so please pray for my car, oh ye w116 faithful!

I am still trying to diagnose my problem.  As I said, I'm guessing water pump.  If I replace the water pump, I will replace the thermostat, the lower hose, and the pulley.  I'm debating whether to have a combustion leak test done.

My other issue is how to go about repairing the car.  If it's a water pump, I can do the labor myself  and get better components besides.  But that means driving back home 100 miles on a sick cooling system (home is not where the heart is so much as its where my tools are!).  If I do so, I will plan to drive 30-35 miles, then pull off for an hour, then 30-35 more miles, then another pull-off, then home.  I am thinking if I keep the system under 212F, I ought to be able to make it without issues.  What do you w116 wizards think?  I'd hate to have to pay twice as much to have some clown use inferior parts and not do as good of a job installing them!

P.S. When I was having a good hard look at my water pump, I was surprised at how small it was.  I once replaced the unit on my 70 Ford Galaxie w/ a 428 c.i. and that sucker was HUGE.  Is the pump a weak link in the system?

P.P.S. If I do the job, I'll post pics of the work for posterity.




Treat your radiator to a good flush and change your thermostat. It may be on it's way.

All the best.

With best regards

Dallas, TX USA.


Michaeld, what about your oil? 
It's obvious I know but I can't recall you saying that it was OK.  Mind you, my first thought was your water pump and/or thermostat. 

But then again, you waited half an hour before removing the rad cap.  Being on the reservoir as well I don't think you'd notice hissing as if the cap was on the rad itself.  There's a number of points you raised which suggest the pump is working so maybe oil level is another consideration.  Good luck, hope it's something simple.
1973 350SE, my first & fave


I checked my oil before leaving along w/ all my fluids (which I always do when ( take this 100 miles drive).  I also changed my oil less than two months ago.  I am wondering if I should change sooner than the usual 6 mo interval due to the fact that I ran so hot.  What do you'all think?

The clear reservoir tank is a big plus in assessing coolant situations, I believe.  Keep in mind that the entire system - reservoir as well as radiator - is pressurized.  When I got out of the car the first time it overheated, the reservoir was filled to the brim due to the increased expansion of hot water.  I waited half an hour, and had no problems opening the reservoir.  The second time, I was in a hurry and only waited 15 min before trying to open the system; I was cautious enough to use a shop towel, but it started to boil over.  Next time, I'll follow my new half hour rule.

On every other car I've ever owned, I would just replace the thermostat, but on this car (according to the manual) I will need to drain the radiator.  It does appear that the upper hose is unfortunately lower than the radiator or reservoir.  As I just replaced the radiator, I just have a hard time believing that it needs any service (any thoughts on that?  How likely is it that a 6 mo old radiator would become clogged?).  I would rather not spend $25 on coolant more than once.  If I replace the water pump, I will replace the thermostat as a matter of course (along w/ the lower hose and the pulley).

In terms of draining the radiator "just enough" to replace the thermostat, does anyone know about how much that would be?  Given the fact that my coolant got so hot, I am inclined to want to replace it one way or another; but I don't want to replace it - replace the thermostat and discover it is not the problem - and then have to replace the coolant all over again when I do the water pump.  If I just replace the thermostat, I want to be pretty darn sure that is my only problem.

Also, could it be my fan clutch?  I hope not, as it is actually the most expensive cooling system component (short of the rad itself).   

The Warden

I don't think it could be the fan clutch. If the fan clutch was unhappy, you'd be overheating at slow vehicle speeds (with not much air going through the radiator) but would be okay at freeway speeds. One of the reasons to have a fan clutch (instead of a direct-drive fan, like older cars do) is so that the fan doesn't pull in air (and rob the engine of that much horsepower) on the freeway when wind force actually pushes more air into the radiator than the fan can pull. In fact, because the fan can't pull in as much air as the wind can push air in at freeway speeds, I've been told that a fan that's spinning at engine speed can actually INCREASE operating temperature some, due to turbulence caused by the fan. But, at the same time, it wouldn't make the engine overheat like that...

I'm wondering if the water pump shaft might have sheared? I seem to recall something like that having happened, the water pump shaft is spinning freely but the impeller isn't turning.

Personally, unless the water pump is expensive (I think it's in the $40 range for an OM617; don't know how much yours would be, though), I would summarily replace both the water pump and the thermostat  (and both hoses, just to cover all your bases) and thoroughly flush the system. Last time I flushed my cooling system, I used two flushing compounds. One was meant to be run in the engine very briefly (basically until the engine gets to operating temperature, then for maybe 10 minutes), and the other was meant to be run in the engine for between 100 and 200 miles. I used the brief flush first, then used a garden hose to flush out the block as much as I possibly could (with the block drain plugs removed), and I also removed the radiator from the car and flushed it out separately. Once I was convinced I had all of the stuff out of the block, I put in the other flush compound and drove to L.A. Once I got to my dad's, I drained the system, flushed it thoroughly again (for the third time; I had done this before putting the first compound in as well), then put in my water/coolant mix. This worked wonders for me...a previous owner had put a thing of Bar's Stop-Leak in my car's cooling system, that seriously clogged things up. That flush job removed any trace of the compound...

If you do something similar, you'll get an idea of whether your engine's still overheating or not, without needing to waste coolant.

Just an idea...hope it helps. Good luck!!


Thank you for your explanation of fan clutches; frankly, it was better written than anything I found on the net!

As of this moment, I am figuring that my water pump is the most likely culprit.  I'd like to rule out everything else BUT the pump, thermostat, v-belt, or hose considering that I will replace them if I replace the pump.  I suppose it would be prudent to flush the rad at that time as well.

In order to do a good diagnostic, I will need to get the car hot - something I'm loathe to do more than necessary.  So at the moment, I am planning to limp the car 100 mi home (making 2 planned hour-long stops along the way to cool things back down) where I can work on the car in my garage.

The cheapest quality pump I've found so far is a Graf unit for $68 w/ free shipping (tho I did find a reman of unknown origin for $54 on Napaonline).  The lower hose, thermostat, and belt will run me another $50, but it seems prudent to do these at the same time.  If I remember correctly, a 50/50 mix of coolant will require 4 gal of antifreeze at about $8 a gallon - so to have to drain more than once is literally sending money down the drain.

Frankly, if I haven't damaged my head gasket, I will consider myself as having gotten off pretty darn cheap!


Check your overflow tank. If cracked, it won't hold pressure and high temps can be the result.  Also the tank cap should be marked 14.  A radiator cap that comes off the shelf from Napa doesn't belong on a Mercedes. 

You say six months ago the radiator was done. Who re-filled the coolant?  These cars are suseptible to air pockets in the cooling system.  This guess is far fetched, given the length of time since the coolant was done, but if your drives have been short and low speed, it's possible to still have a bubble introduced at the refill. The metal at the air pocket will get so hot that when the coolant finally does get circulated to it, it will boil.

I know the car is designed to have the cap at the highest point of the system and that theoretically, any bubbles should be purged by the fact that the bubble seeks the high point in the system, but personal experience has taught me to put the front right wheel on a curb to elevate it further and to rev the engine to a constant 2500 or so after the thermostat has opened and the car has come to operating temp.  Carefully remove the cap before revving it.  The revs will bring your water pump to life and any bubble should be able to escape to the tank after a few minutes with the thermostat opened.

My experience with air pockets had the temp gauge swinging wildly and rapidly to overheat indication, so this may not be your deal at all, but it will be good to check it when you start sorting this out.  Also be sure and clean your temp sensor. Dirty equals high resistance and inaccurate readings.

Your system needs coolant for the lubrication and anticorrosion and boiling point increase properties it supplies, but 50/50 coolant and water is probably overkill for Palm Springs in the summer.  Higher water concentration will remove more heat.  If you were in Anchorage, I wouldn't  recommend 30/70 coolant to water, but you can get away with it in summer where you are located.

Because you have seen an indicated overheat, your thermostat should be replaced without giving it another thought.


Informative post on the overflow tank (which I have incorrectly been calling the reservoir tank).

I did an unofficial pressure test by letting the car get so hot; I was close to redline at the time I finally got a chance to shut the car down.  The pressure was high enough that the overflow tank was completely full.  There were no leaks in the system.  I did a genuine pressure test at the time that I replaced the radiator core (6 mos ago); it passed then as well.  The tank and cap are both genuine MBz.  I agree; there are parts that you can buy at Napa, et al, and there are parts you shouldn't.  The cap is a part you shouldn't.

Your write up of "bubbles" was also interesting; I have read that bubbling was a sign of a bad head gasket as well.  I'd sure rather have the problem you're describing than THaT.  So far, I haven't noticed bubbling.  As for my driving: I don't put a lot of miles on the car, but the miles I DO put on are mostly freeway, and mostly longer drives.

As for replacing the thermostat, yes, I completely agree; my principle is to replace less expensive related parts whenever I replace a more major part.  So if I replace the water pump, I'll also replace the thermostat, the lower hose (at least), and the belt.  I'd rather mess with it once and be done with it.  (If I think it's just the thermostat, I would not replace the water pump, but I would replace the lower hose and the belt).


Just to pose a question...if you have a bad temp gauge, will it read at all?  It sounds like michaeld has a real cooling issue, but I am curious. 


A bad temp guage might display zero, get stuck on some temp number, or just be off.  Thus far, my gauge has been spot on, and I trust its "findings."  The best way to verify an accurate gauge is to have a non contact thermometer on hand, as I do - but still 100 miles away where it does me no good!

Styria, I am frequently one of those "theoretical guys" that the REAL mechanics rightfully love to shred to pieces.  My readings on thermostat diagnosis involved checking the TOP hose for pressure and heat.  Is it actually the bottom?  Drat and double drat!  In any event, one way or another, I came to the same (possible) conclusion as you; the thermostat may be the only culprit, and is a good place to start.

I've been driving a rented car the past several days, and haven't wanted to push my old girl.  But yesterday I started her up, and let her warm.  I let the car get over her standard operating temp, and then turned on the heater.  Nothing.  That normally indicates a bad water pump, so I'm told.  But then I checked the top radiator hose (though according to Styria I should have checked the bottom hose) and felt no pressure in the hose.  As I had been doing that intermittently since I started the car, I believed I should have had something more substantial.  As for the coolant level in the overflow tank, I saw no movement (which I understand could be either the pump or the thermosat, or even a clogged rad).

This left me confused, as I saw what I interpreted as mixed signals.  It was as if BOTH the thermostat and the water pump were bad.  But what's the likelihood of that?  I therefore decided to replace the thermostat, and then see what happened after that.  I ought to be able to replace the thermostat with the tools I have at hand.  If someone has replaced the thermo before, they could recommend how to best seal the water pump--thermostat housing joint (i.e. just use the sealer, or apply some sort of "gasket grease" compound).

I am modifying my post w/ the following:

It looks like I will have to completely drain the radiator, as it appears that I need to remove the lower hose in order to gain access to the inside bolt (of the 3 bolts on the thermostat housing).  On the plus side, the radiator drain is so conveniently located that I will likely be able to easily catch/save/re-use the old coolant - at least for the purpose of testing whether replacing the thermostat does the trick.  [On every other car I've ever owned (5 US and 1 Jap.), the drain plug was located so that it splashed through an impossible-to-clean shroud cover (or hole in the bumper assembly, or you name it), rendering the coolant unusable].  As I need to remove the lower hose anyway, I will replace it when I do the thermostat job.

As a P.S. I have had two people - both knowledgeable diesel guys (and one a retired professional transmission shop owner) tell me that it is very important to use MBz replacement thermostats, rather than garden-variety replacements.  Their explanations as to why made my eyes roll w/ the technical details, but I will follow their advice and "buy Benz."

s class


When you fit a new thermostat, if it is a genuine benz item it will come with a rubber O-ring which is all you need to seal the joint provided the alloy castings are not too pitted.  Disconnect the hose to the top of the thermostat housing and prise out the themostat.  Carefully clean away the crud that will hava accumulated where the outer flange of the themostat must seat.  Insert the new thermostat, then its O-ring, then the cover (which likewise must first have been cleaned and de-crudded.

Some may disagree, but I have never neededany additional form of gasket or sealant when doing benz thermostats.  BTW I just did this job on my '92 500SEL this weekend. 

Good luck, Ryan

[color=blue]'76 6.9 Euro[/color], [color=red]'78 6.9 AMG[/color], '80 280SE, [color=brown]'74 350SE[/color], [color=black]'82 500SEL euro full hydro, '83 500SEL euro full hydro [/color], '81 500SL


Hey, dragging my problem back into the present.

I took my thermostat off (a real pain w/o removing the alternator, but I somehow managed), and performed the boiling water/thermometer trick; it opened at temp (mind you, it opened gradually between 165-175; no "popping").  I think this is the first in my life I was seriously disappointed when something worked RIGHT; I was really hoping it was just the thermostat.

The next question is, what IS it?

Symptoms: gradually overheating at freeway speed over about half an hour.  When I was overheating prior to buying a new radiator core (I was told it was 40% blocked), I was overheating over an hour to about 190F or so - and staying there.  Now it is overheating faster and there is no limit but redline.  It is hard for me to believe that the rad could be bad 7 mos and 3000 miles after the recore.

But looking over my service history, the PO replaced the water pump and thermostat w/ quality OEM units in Oct 2004 only 4000 miles ago.  If it IS a bad water pump (it isn't a bad thermostat), what could cause the unit to fail so quickly?

I posted my request for troubleshooting to mercedesshop/com to their tech support because it is much larger and I would get more "hits" more quickly.  As of last check, I got 3 responses: one said bad radiator (but see my above); one said retarded timing could cause overheating; and one said that a dirty condenser could cause the problem.  I had no idea that bad timing could cause overheating.

Well, shoot.  More possibilities.  But I would really HATE to replace the water pump and thermostat and NOT fix my problem!  I looked over my service history, and saw that the PO had a brand new condenser installed in '95 at 108000 miles (only 19000 miles ago); and had the timing professionally set at 124000 miles in '04.

I am of the impression that a bad fan clutch/Visco fan would cause overheating at under 30 mph, but not at freeway speeds.  Bad thermostat ruled out.  OEM Water pump less than 3 years and 4000 miles old.

Is there something else that could cause overheating at freeway speed?  A bad temp sensor?  Evil supernatural forces?

Like I said, I would be one whining, crying, gibbering maniac if I spent the $ and time to replace the pump and got nowhere.  At the same time, I have the cooling system partially dissasembled, and can't check things like the timing unless I put it back together (and I would hate to put it back together, only to have to take it back apart).  This is your DIY rock-and-hard-place scenario.