I've changed the coolant on my '91 300-SEL about every 2 years since I bought it in 1997.

On a couple of occasions, after changing the coolant, the upper hose would collapse while cooling down overnight.

I have done a search on this problem and am aware of the so called faulty pressure cap.

My question is why do these caps go bad from seemingly nothing more than the removal required to drain/refill the system?

The bleeding process I use works well - no overheating - air seems to purge. A 20 or so mile drive afterwards goes well - once again - no overheating.

I can remove the cap for the occasional topping up without incident. This problem only occurs when the coolant is changed. If the system had air which I suspect some will point to, the car would overheat and I would likely have a disfunctional heater. This doesn't happen in my case.

Thoughts appreciated.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Mike

I know you are quite knowledgeable about your car as you have replied or suggested to many of my posts. I have not seen the phenomenon you are describing on your car. As a matter of fact I changed the coolant on my car just a week back as part of the water pump replacement. One question is how are you bleeding the system. On my car there is a bleed scew(bolt) in the front of the engine to bleed the system. Are you using this for bleeding.

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SANDEEEP
89 300SEL
230,000Km

Assuming it only happens one or a few times after a coolant change and no more after that, it's probably an issue with your fill techinque.

It should be filled as follows:

1. SLOWLY pour in the proper quantity of Zerex G-05 antifreeze to obtain a 50-70 percent concentration of antifreeze in the final mix. System capacity is in your owner's manual. (Note: the block plug(s) should always be removed otherwise, much of the old coolant will remain in the system.)

2. Once the requisite amount of antifreeze is installed, continue with distilled water. Pour SLOWLY.

3. When the expansion tank reaches between about one-third and one-half capacity and will not take anymore liquid without the level rising, leave the pressure cap OFF, turn the cabin temperature control to maximum and heater blower to max speed if your model has a manually operated blower speed control, start the engine and maintain 1500-2000 revs. Leave doors or windows/sunroof open so cabin heat can escape. (The temperature control at max will circulate coolant through the heater core circuit to purge air, and having the blower on its highest speed will keep the engine from heating up too quickly and possibly boil over, even in hot weather. If you do the job in hot weather and the HVAC system does not detect a demand for heat, the heater control valve will remain closed and air in the heater core circuit will not be purged!!!)

4. The engine should be cold (or cool) when you install the antifreeze and distilled water. Once the themostat opens (at least 80C on most models), the level in the expansion tank should drop. Add distilled water to bring it back up to one-third to one-half full. Prior to the themostat opening, there is likely an air bubble at the top of the radiator. Once the themostat opens, this entrapped air will evenually be drawn to the water pump, circulated through the engine and heater circuits and purged through the vent line at the top of the thermostat housing to the open expansion tank, but it will take several minutes.

5. Monitor the coolant level and add distilled water to maintain the level. Also monitor monitor engine temperature. Install the cap when it reaches 90C or a little above and continue to run the engine at 1500-2000 or take it for a brief drive at low speed and load.

After the engine cools down the level should be checked and corrected as required. This may take more than one cycle, so check and correct the level on every cool down until the coolant surface in the expansion tank stabilizes at the correct level. Checking coolant level is also in your owner's manual.

If you observe hose collapse after this procedure and it continues without change, there is likely a problem with the vacuum relief valve in the expansion tank cap. Coolant system caps have both pressure relief valves and vacuum relief valves. Many auto parts stores have pressure cap testers,but I'm not sure if they can check the vacuum relief valve function.

Duke

SANDEEP:

Thanks for the kind words. Apparently I'm not as knowledgeable as I had hoped.

You can buy a t-stat housing with a bleed bolt on top. I purchased one of those from this site and the housing and bleed bolt washer were apparently substandard. The thing leaked badly, so I went back to the sealed t-stat housing that was on the car from the day I bought it.

The M103 motor has 2 plugs on top along side the drivers side - 14mm removes them. I used to refill thru the front plug as outlined in the 103 manual, but no longer do so. The last time I reinstalled these plugs(or small bolts), things didn't feel quite right. It almost felt like the thing was starting to strip out. I believe that psfred commented about this once, so he too ran across this issue. Didn't mean to get side-tracked from your original question just trying to be thorough.

OK - how do I refill and bleed? I use a technique that's been discussed at this site.

1. After draining coolant and doing whatever sort of flushing one cares to do, close up 19mm drain plug on passenger side of block.
2. Reconnect drain hose on bottom of radiator - passenger side. I do not drain the radiator thru the front radiator drain plug as it's all but impossible to get to.
3. Remove t-stat housing cover - 3/10mm bolts. Remove t-stat and gasket. Slowly pour in a 50/50 mix of MB coolant and distilled water(please! - no arguments about coolant brand/water type) until coolant comes to the top of the housing. Refit new t-stat with jiggler valve in highest position. Refit new gasket - install t-stat housing and 3/10mm bolts - torque sufficiently.
4. Slowly add 50/50 mix to overflow tank to complete coolant refill.
5. With cap off overflow tank, start engine and ensure heat is set to max position. This was actually done before the draining process began.
5. Monitior temp. gauge. Temp gradually climbs to just above 80 degrees centigrade. Blip the throttle a number of times to about 3000 rpm or so. Allow to idle for a bit more. All in all - idle time is about 30 mins. Temp gauge never goes over 82C-83C.
6. Shut off engine. Reinstall cap. Go for test drive - approx 30 miles. Temp gauge never goes over 82C-83C. Does not overheat at idle.
7. Return home. Allow to cool a bit. Coolant level down a bit in overflow tank - not unusual as I've seen just about every car I owned do this after a refill. Top off coolant. Drive car again a few miles. Return home. Allow to sit overnight.

Upper hose collapsed the following morning after cooling down overnight.

That pretty much covers the flow of events.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

If you don't turn the temperature control to max to allow coolant to flow through the heater circuit, it may take quite some time for the entrapped air to purge. If you do the job in the early summer air could be entrapped in the heater core circuit until the HVAC system determines that heat is required and opens the valve. This could take MONTHS, and any entrapped air can rapidly increase corrosion in the affected area, which could cause premature failure of the heater core.

I have never removed my thermostat housing to refill the coolant or found it necessary to install a "bleed valve". It's totally unecessary if you slowly fill the system through the cap opening and allow it to self bleed with the temperature control on max.

I think on the the biggest "problems" guys have with coolant changes is not taking the appropriate steps to allow the heater core circuit to self bleed!

If you follow appropriate procedures, which includes filling the system slowly to minimize air entrapment and allow the ENTIRE system to self bleed including ensuring that coolant flows through the heater circuit, you should not have subsequent problems due to insufficient coolant level.

Duke

Duke:

Thanks for the reply.

I went back thru my notes. Whenever I do a project like this I write down start times, how long 'til upper hose starts getting hot, etc. etc. I monitor the process form start to finish.

The car idled for 40 mins. The temp gauge was at max and as the temp gauge rose, the heater started putting out a good amount of heat, leading me to believe the coolant had gone "full circle". On a lot of cars, a sure sign of an air lock is no heat, quickly following by a rapidly accelerating temp gauge. This never happened.

MBDoc once wrote(I haven't been able to find the thread) that on a certain model of car that had the 103 motor, elevating the front was needed to bleed the air out of the cooling system.

If I have air in the system, why are my operating temps good -> no more than 82C-83C?

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

If you are using the proper fill and bleed procedures and are still seeing collapsed hoses, the likely suspect is the cap vacuum relief valve. Could be that just the act of removing and installing it causes a malfunction on the next several cycles, and then it works properly again until the next time you remove it.

You might also want to check that the vent line from the top of the themostat housing to the supply tank is not clogged. Try removing it at the expansion tank (and remove the tank cap too) and see if you can blow through it. There will be some resistance, but the coolant in the system will be able to expand into the tank.

Get the cap tested for for both pressure or vacuum relief operation, or just buy a new one and see what happens.

Duke

I went ahead and put a new cap on this am, drove it 15 miles; then garaged it before going to work in the pickup. Should know if this solved the problem when I get home tonight. Better half wants her car back, so I didn't have time to trouble-shoot as much as I would have liked to.

The last time this happened, a new cap resolved the issue, so I'm hoping this new part does the trick.

Thanks to all for the replies.

I'll update this either tonight or tomorrow morning.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

When I replaced my water pump, I refilled coolant into the top of the water pump with the t-stat out. The top radiator hose was still disconnected and I filled the radiator through the top hose and then connected the top hose. I added coolant to the reservoir. I started the car and turned the heat on MAX. I let it run a bit and then briefly removed the front-most 14MM plug on the cylinder head to release any air. It's been doin' fine since.

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2012 E350
2006 Callaway SC560

As mentioned earlier, I replaced the expansion tank pressure cap - part # 124 500 04 06 yesterday morning. The vehicle has been driven and cooled down twice since then. No more collapsed upper hose.

Onward.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"