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W124 Coolant System Pressure Gauge
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) Total. . . . . . . . . . . . 627,762 "Oh lord won't you buy me a Mercedes Benz." -- Janis Joplin, October 1, 1970 |
#2
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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. |
#3
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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 |
#4
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Jeremy, thanks for posting the results of your research!! |
#5
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OTOH, what does the pressure tell you?
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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. |
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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 |
#7
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Not true. It is very useful to diagnose coolant system issues.
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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 |
#8
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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:
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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 Last edited by funola; 01-27-2014 at 01:26 PM. |
#10
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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.
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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 |
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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. |
#12
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Be my guest.
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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. |
#13
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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
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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) Total. . . . . . . . . . . . 627,762 "Oh lord won't you buy me a Mercedes Benz." -- Janis Joplin, October 1, 1970 |
#14
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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. |
#15
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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 |
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