The other day about 6 km from home on flat ground at a red light steam started coming from under the hood. I pulled over shut it down opened the hood. The steam was coming from under the engine cover directly below the power steering pump return line. After about 40 seconds it quit. I had it towed home removed the engine cover and only see a coolant line connected to the oil cooler. I pressured the system to 17 psi and no leaks.

Would anyone know where the pressure relief valve is the reservoir cap is a solid cap.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Well looks like there was too much coolant in the system and something released the excess. This is a closed system without the overflow tank and no release hose.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Sounds like you may have popped the self bleed hose that works back to the upper part of the reservoir. Look for a hairline crack in the hose near the clamp.

The leak may only open up when the engine is hot.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

I had pressure tested the system for an hour at 17 psi no leaks. The steam was definitely originating from around the oil cooler area but of course the engine cover blocked my view. I have been driving it since without issue.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10