Dealer Captive
 

HELP I have a 94 S350 thats running hot, the car is a Starmark that I bought it has 69000 miles and is flawless.
Took to dealer for running hot and upper radiator collapsing when cold dealer "supposedly pressure tested system" and said cap was bad I had checked cap before going and it tested fine. when I went to pick up car hose was collapsed, Now there scratching there heads and they want to replace hose and there not sure it that either. when cap is released hose fills out. What could be wrong. they claim its not the head.
funny the things there fixing are not covered under Starmark.

Need HELP

Bump for help

rdanz,

The expansion tank cap is supposed to have a pressure relief function to limit the coolant pressure to 1.4 bar, and a vacuum relief function to avoid the condition you describe. The vacuum relief is supposed to open at .1 bar under atmospheric pressure.

My conclusion is the cap is bad, and your dealer should be able to test it, as the test procedure and tooling is vividly described in the shop manual. There is no other explanation, as that condition is prevented solely by the vacuum release feature of the cap. When you tested the cap did you test it for its ability to open when the coolant system internal pressure is below atmospheric pressure?

Anyway, the overheating issue is leading to loss of some small volume of water, which is not surprising. You probably go over 1.4 bar and the cap lets steam escape. It is supposed to drain to an overflow reservoir in the front passenger side fender if you have a cap that you turn counter clockwise to a detent then push down and turn the rest of the way off. It would seem that reservoir might be damaged or the line clogged as well. The system design is intended to contain the steam that leaks off, as well as any water, and then, when the system cools and starts to go below atmospheric pressure, it draws the recovered fluid back into the coolant expansion reservoir.

There is another version with something called a two stage cap, which apparently allows the steam to escape under the hood. I do not think you have such a design. But the cap does the same thing to limit internal over and under pressure.

If the car is overheating it is most likely due to the thermostat, the viscous clutch on the belt driven fan, the thermal sensors that switch on the electric auxiliary fans, or the electric auxiliary fans themselves that are bad. It can be one or more of these items, or, if the coolant level is low you may also have some air trapped in the system.

I have had a 1991 350SD that had an engine that suffered and died from the bent rod issue. Although I was very disappointed with this I cannot report the car ever overheated, until after the engine was rebuilt. That happened in a snow storm that covered the entire radiator with packed snow, blocked all the airflow, and prevented the electric fans from working.

I hope this helps, and hope you got one that does not suffer from a propensity to bend rods. Good Luck, Jim

Try taking it to an independent shop and see what they can come up with. The dealer shops can always come up with things not covered, or find ways out from covering items. My dad’s Lincoln would be a good example; it blew a head gasket, which lead to the engine seizing. It was not covered because the shop said he put some water wetter in the coolant, and that voided the warranty.

I think Jim is on the right track. I have another thought also:
What if the line from the pressure tank (where the pressure cap is) to the actual expansion tank (inside the rear of the right front fender I believe) is blocked, pinched or kinked? that would give these results also. OR if the vent on the expansion tank itself is blocked? the expansion tank has the one line going in from the pressure tank and has a vent tube of sorts molded in to the tank.
I'm not sure what would block the vent line, deserves to be investigated.

Gilly

Update
The dealer now states they contacted Mercedes Factory Rep and he said "Normal Hose collapse due to fluid contraction when cold"
"If head gasket failed the hose would expand" and that a temp of 100 celsius is normal.
My question is how does fluid "contract"
and how many of you have cars that run at a 100?
Engine is clattering and milage has been droping steady.
the say to change to Hess Fuel as that theres nothing wrong with engine.

I have this in writing from dealer.

rdanz,

The coolant volume is part gas and part liquid. As the engine warms up the metal, liquid and gas respond according to Mother Nature. The gas response is the most noticeable, but the metal grows and the vapor pressure of the liquid increases. This brings about an increase in the internal pressure as long as the system is sealed. And yours is, up to a temperature and pressure limit.

As you leak a little steam, or water vapor to either the overflow tank or the ambient air under the hood, the volume inside the coolant system just changed. This leakage is controlled by the cap, which is really a two way relief valve. When activated internally to relieve internal pressure, steam or hot gas leaks until the volume change has reduced the internal pressure enough to fall be low the cap set pressure (1.4 bar, or 20.58 psi).

When the system cools off, the stuff that was steam inside the coolant system condenses and turns back to liquid, minus the volume that escaped. This involves a volume change on the order of a factor of 2000 as the steam returns to a liquid (steam is on the order of 2000 times the volume of the liquid water that it was before it changed state). Meaning if a liter of steam escapes it might only be a few drops of water (less than a milliliter), but to the inside of the coolant system, it is the equivalent of a liter of volume missing. This creates a vacuum, which the second "way" of the the two way relief feature on the cap is supposed to relieve. For some reason, this feature is ot working on your car and the soft parts of the system get squished by the fact that the internal pressure is now less than atmospheric pressure, meaning your hoses get squashed.

I would tend to agree it is not a real disaster, but it is not the way the system is designed to work, so they should fix it. Ask the guy if the system has a vacuum relief feature in the cap, and if there is an overflow tank. Better yet, lift the hood and look for yourself. You should see a small tube that is connected to the neck of the expansion tank (the tank with what used to be the radiator cap on it) under the cap. The tube should run behind the passenger side wheel well, through a partition, to the overflow tank that is out of view. If you do not have the overflow recovery system the tube will run to the ground below the engine. Show this stuff to the guy and ask him what it is for and how it works. If he/she cannot explain it see the manager, then ask for the district MB service representative.

As noted, MB has test equipment and a procedure for checking the cap. I think it would be less expensive to just buy another as the labor rate for the test is likely to exceed the cost of the cap by an order of magnitude. But the tube going to the overflow tank, or the tank itself could have a problem as noted above, so the cap may not be the culprit.

If your car did not used to run near 100 degrees Celsius, then the fact that it does now is not normal. This acceptance of 100 degrees as a normal condition is not with what my experience is for normal on many MB's for a long time. I would consider 85-87 degrees Celsius normal except under highly stressed conditions, like over 90 degrees Fahrenheit outside temps, and either stop and go traffic or pulling a trailor up a hill. But that is my experience. Go to the dealer and tryout a new car and see what it does on the same route you just drove, then ask the service manager why that car runs under 87 degrees Celsius under the same conditions yours runs closer to 100?

Like I said before, there are relatively few parts that can go wrong, and the prime suspect in my mind is the thermostat. MB coolant systems are more complicated than most and the thermostat is both a temperature regulator and a mixing valve. Either function can fail and disrupt the temperature regulation feature. Another source of trouble is the clutch that makes your belt driven fan turn. I have not seen this personally, but others here report it is a source of trouble. And then there are the electric fans and the sensors that turn them on. These are easier to check (let the car heat up, turn the A/C on and the fans should come on in stages and speeds) and easier to fix.

Good luck, and keep after the issue. Jim