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

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
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Much ado about nothing. You will get as high as the cap pressure specification.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

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

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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!!

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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?

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

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.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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).

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

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Be my guest.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

I have used a tee-ed in pressure gauge to diagnose a leaking head gasket in my VW before tearing into it. All fixed now.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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

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CENSORED due to not family friendly words

You could have saved yourself some time and refrained from posting five replies to a thread that is "nothing".

*shrug*


EXACTLY. The primary symptom of a failed #14 head crack is excess pressure in the cooling system. Jeremy has helped answer the question, "what is NORMAL pressure".

I know my #14 head is leaking combustion gasses into the cooling system, has been for at least the 20k miles I've owned it. I put a colder 71c thermostat in it and and lower pressure coolant reservoir cap. I can hear it hissing away even after a short drive with the engine still cold. So long as it doesn't consume any coolant or oil I'll keep driving like this. The #17 head is stashed away in the garage. One of these days I'll get around to swapping it in. If I was still running at 90c all the time with a high pressure coolant cap I would definitely add a pressure gauge.

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CENSORED due to not family friendly words

You can drive it with the cap loose, but you must watch the temp gauge closely. Don't let it go over 100C with the cap loose. This reduces the strain on the hoses and water pump seals, which are otherwise getting full rated pressure of the cap (1.2/1.4 bar) due to the abnormal pressure entering the system.