W124 Coolant System Pressure Gauge - PeachParts Mercedes-Benz Forum

Jeremy5848 · #1 01-27-2014, 11:40 AM

Last summer while I was diagnosing a coolant system pressure leak (turned out to be a bad pressure cap), I decided to put a pressure gauge in the cooling system of my '95 E300D. I was interested in how high the pressure got, did it get higher on hot days, did it continue to rise after the engine was shut down (due to "heat soak), etc.

Since I didn't need to see the gauge while driving, I decided to install a simple mechanical gauge under the hood. My first location was the expansion tank.

That worked and it was easy to see but I had a hard time getting the joint to not leak, it was obviously not "stock" in appearance, and I was afraid I'd accidentally "whack" the gauge with a tool and break the tank. So, I replaced the tank with a new one (it was old and yellow anyway) and moved the gauge to the rubber hose running between the expansion tank and the radiator. (For safety, I used a brand-new OE Mercedes hose.)

Now the gauge was down out of sight but still easy to see if you knew where to look. It was leak-tight and worked great. To test the "heat soak" theory I took the car for a drive and photographed the gauge immediately after stopping and then once a minute. The results show that there was no heat soak effect. (I need to repeat this on a hotter day.)

Jeremy

ah-kay · #2 01-27-2014, 11:44 AM

Much ado about nothing. You will get as high as the cap pressure specification.

funola · #3 01-27-2014, 12:03 PM

Would have been easier and cheaper to tee the gauge in at the neck of the expansion tank. no cutting of hoses necessary.

gsxr · #4 01-27-2014, 12:10 PM

Quote:

Originally Posted by ah-kay

Much ado about nothing. You will get as high as the cap pressure specification.

That wasn't the point... the cap rating only tells you the maximum pressure possible in the system. Jeremy (and others, myself included) wanted to know the normal pressure in the system at typical operating temps (~90°C).

Quote:

Originally Posted by funola

Would have been easier and cheaper to tee the gauge in at the neck of the expansion tank. no cutting of hoses necessary.

There are issues with plumbing the gauge at the tank. I don't believe there is room to enable a leak-free connection at the tank neck.

Jeremy, thanks for posting the results of your research!!

ah-kay · #5 01-27-2014, 12:14 PM

Quote:

Originally Posted by gsxr

That wasn't the point... the cap rating only tells you the maximum pressure possible in the system. Jeremy (and others, myself included) wanted to know the normal pressure in the system at typical operating temps (~90°C).

MBZ would have put something in there if it WAS necessary. The point is that you will never get a pressure higher than the relief pressure of the cap. When the engine is at op temp the pressure will be at or very close to the cap specification. Much to do about nothing.

OTOH, what does the pressure tell you?

funola · #6 01-27-2014, 12:14 PM

Quote:

Originally Posted by gsxr

That wasn't the point... the cap rating only tells you the maximum pressure possible in the system. Jeremy (and others, myself included) wanted to know the normal pressure in the system at typical operating temps (~90°C).

There are issues with plumbing the gauge at the tank. I don't believe there is room to enable a leak-free connection at the tank neck.

Jeremy, thanks for posting the results of your research!!

Huh? The gauge does not have to be on the tank/ neck. Add a 1 foot hose on the neck, add a Tee and done.

funola · #7 01-27-2014, 12:17 PM

Quote:

Originally Posted by ah-kay

Much ado about nothing. You will get as high as the cap pressure specification.

Not true. It is very useful to diagnose coolant system issues.

gsxr · #8 01-27-2014, 12:21 PM

Quote:

Originally Posted by ah-kay

When the engine is at op temp the pressure will be at or very close to the cap specification.

Jeremy's data proves this is incorrect. At normal operating temps (80-100°C), the pressure much, much lower than the cap rating.

Quote:

Originally Posted by ah-kay

OTOH, what does the pressure tell you?

The data proves wrong the common misconception that the system pressure will be at cap rating (1.2-1.4 bar) all the time, which it obviously is not.

Quote:

Originally Posted by funola

Huh? The gauge does not have to be on the tank/ neck. Add a 1 foot hose on the neck, add a Tee and done.

That won't work. The hose attached to the neck will only be connected to the coolant/air inside when the cap pressure rating is exceeded. It's designed to capture liquid overflow during extreme temperatures, the liquid is routed to a plastic reservoir inside the fender (on most 124 models except some late build models, such as the 124.131).

funola · #9 01-27-2014, 12:23 PM

Quote:

Originally Posted by gsxr

Jeremy's data proves this is incorrect. At normal operating temps (80-100°C), the pressure much, much lower than the cap rating.

The data proves wrong the common misconception that the system pressure will be at cap rating (1.2-1.4 bar) all the time, which it obviously is not.

That won't work. The hose attached to the neck will only be connected to the coolant/air inside when the cap pressure rating is exceeded. It's designed to capture liquid overflow during extreme temperatures, the liquid is routed to a plastic reservoir inside the fender (on most 124 models except some late build models, such as the 124.131).

OK. you're right. I was getting mixed up with my VW expansion tank, and my 617.952 where it would work.

funola · #10 01-27-2014, 12:28 PM

What is the heat soak theory? The gauges show what a properly operating engine should do. Coolant pressure is due to expanding coolant in a closed system. Once engine is shut off, engine cools off, coolant stops expanding and starts contracting and the gauge drops.

Quote:

Originally Posted by Jeremy5848

Last summer while I was diagnosing a coolant system pressure leak (turned out to be a bad pressure cap), I decided to put a pressure gauge in the cooling system of my '95 E300D. I was interested in how high the pressure got, did it get higher on hot days, did it continue to rise after the engine was shut down (due to "heat soak), etc.

Since I didn't need to see the gauge while driving, I decided to install a simple mechanical gauge under the hood. My first location was the expansion tank.

That worked and it was easy to see but I had a hard time getting the joint to not leak, it was obviously not "stock" in appearance, and I was afraid I'd accidentally "whack" the gauge with a tool and break the tank. So, I replaced the tank with a new one (it was old and yellow anyway) and moved the gauge to the rubber hose running between the expansion tank and the radiator. (For safety, I used a brand-new OE Mercedes hose.)

Now the gauge was down out of sight but still easy to see if you knew where to look. It was leak-tight and worked great. To test the "heat soak" theory I took the car for a drive and photographed the gauge immediately after stopping and then once a minute. The results show that there was no heat soak effect. (I need to repeat this on a hotter day.)

Jeremy

ah-kay · #11 01-27-2014, 12:39 PM

Quote:

Originally Posted by gsxr

Jeremy's data proves this is incorrect. At normal operating temps (80-100°C), the pressure much, much lower than the cap rating.

The data proves wrong the common misconception that the system pressure will be at cap rating (1.2-1.4 bar) all the time, which it obviously is not.

That won't work. The hose attached to the neck will only be connected to the coolant/air inside when the cap pressure rating is exceeded. It's designed to capture liquid overflow during extreme temperatures, the liquid is routed to a plastic reservoir inside the fender (on most 124 models except some late build models, such as the 124.131).

If you want to go into the ivory tower. Be my guest. There is another holding tank in the fender to capture overflow coolant when the cap opens. That means the cap will open at some point in time when engine is at op temp to allow the coolant to flow into the holding tank. When the engine cools, a vacuum will be created and sucks the coolant back into the expansion tank. That is the way most, if not all, cooling system works.

ah-kay · #12 01-27-2014, 12:57 PM

Quote:

Originally Posted by funola

Not true. It is very useful to diagnose coolant system issues.

Be my guest.

Jeremy5848 · #13 01-27-2014, 01:06 PM

Quote:

Originally Posted by ah-kay

If you want to go into the ivory tower. Be my guest. There is another holding tank in the fender to capture overflow coolant when the cap opens. That means the cap will open at some point in time when engine is at op temp to allow the coolant to flow into the holding tank. When the engine cools, a vacuum will be created and sucks the coolant back into the expansion tank. That is the way most, if not all, cooling system works.

Only true at extremely high temperatures/pressures. Under normal conditions, provided the expansion tank is at its correct (marked) level when cold, the coolant in the expansion tank never comes close to overflowing into the capture tank. As GSXR noted, later 124s such as my 124.131 didn't even get an overflow tank.

The pressure gauge is a great diagnostic tool. It tells you if there's pressure in the cooling system -- do you have a leak? -- is it safe to loosen the pressure cap? (amazing how long a tight system will hold pressure) -- do different pressure caps hold pressure better than others (yes they do, I haven't figured out why yet). It tells you if something has changed ("the pressure always used to be at X after my daily drive, now it's at Y under identical conditions").

Jeremy

ah-kay · #14 01-27-2014, 01:15 PM

Quote:

Originally Posted by Jeremy5848

Only true at extremely high temperatures/pressures. Under normal conditions, provided the expansion tank is at its correct (marked) level when cold, the coolant in the expansion tank never comes close to overflowing into the capture tank. As GSXR noted, later 124s such as my 124.131 didn't even get an overflow tank.

The pressure gauge is a great diagnostic tool. It tells you if there's pressure in the cooling system -- do you have a leak? -- is it safe to loosen the pressure cap? (amazing how long a tight system will hold pressure) -- do different pressure caps hold pressure better than others (yes they do, I haven't figured out why yet). It tells you if something has changed ("the pressure always used to be at X after my daily drive, now it's at Y under identical conditions").

Jeremy

It does not alter the fact that it is much ado about nothing. I am done with this thread.

tjts1 · #15 01-27-2014, 01:20 PM

That's a good idea. This could be especially useful to those of using driving an OM603 + #14 head.