On my recently acquired 1992 300D the upper radiator hose would be hard after the engine had cooled off over night. I had planned on pulling the head and installing a new head gasket. Anyway, in talking to a friend of mine he suggested trying a different coolant reservoir cap so I did. Seems that the upper hose is no longer hard after cooling overnight. The coolant reservoir cap was bad. So as a heads up keep in mind that just because this hose stays hard overnight may not mean there are head gasket/head issues.

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Jim

Don't you love it when it's an easy fix!

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

yea - lol

Now I can put the $$ on a front suspension rebuild.

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Jim

how was a bad cap keeping the pressure in the system?
the cap does not let pressure out... well I suppose it would hold too much pressure, then the cooling of the system would not allow the system to depressurize... but I still don't think a cap can keep pressure on a system overnight... where would the pressure come from?
I would put a pressure guage on a warm radiator, and start the car with the pressure guage still on there, and watch the guage... if it starts building pressure immediately, you got a bad gasket. it may only manifest itself after a lot of driving. and it may be a very small leak.
good luck. and I really hope it was just the cap.
John

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Actually a cap that is rated for a higher pressure than the system creates, can keep the system pressurized even when cold. In the course expansion and contraction (if the coolant level is low in the tank and the system needs more it will pull air instead), more can be pulled in than expelled. The pressure has to reach a predetermined level before the pressure is released. If the cap holds too much pressure, then the system will stay pressurized until something weaker gives out, like the seals on the end of the radiators, or a weakend hose.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

I have a question regarding the hard-hose test.

My '87 doesn't have the hard-hose cold, but I'm trying to figure out in my head how this could be created by a cracked head?

Regardless of whether the pressure is combustion gasses or expanding water/steam, the pressure will release at the cap rating. So, the max. pressure hot will be the same.

When the engine cools, it will contract, and the pressure will go down in the system and typically pull air in through the cap.

How does a cracked head cause the system to pressurize when cold? I know I've heard this test, but it doesn't make sense to me. Anybody have a theory?

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Gone to the dark side

- Jeff

No, the test doesn't make any sense if you understand thermodynamics.

But its an "old wives diesel" tale that gets reinforced and won't go away.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

The only way I can think of would be during a leak down test, or continued cranking on the engine without firing. Since Air compresses it would take a while for the pressure to build up.

__________________
RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Well, I've never experienced this condition, but there is a thermodynamic explanation for this "test":

In a normal cooling system (no leak) the pressure in the system is only due to condensable gas (steam). When the engine cools, the steam condenses and the pressure returns to the normal (approximately atmospheric) pressure. In a cooling system with a head (or gasket) leak, some of the cooling system pressure is due to exhaust gases which are not condensable. When the engine cools, these gases are "trapped" in the system by the pressure cap. Unlike steam, the exhaust gases do not condense and the cooling system will remain pressurized indefinitely (if it is leak tight). This assumes that the head (or gasket) leak is not severe enough to allow the cooling system to de-pressurize back to the cylinder(s) when the engine is stopped.

Take a closed system at 0 psi and 32F. and heat it to 180F. The pressure will climb to approx. 10 psi or so.

Now inject some additional combustion gas into the system and drive the pressure higher........say 15 psi or so.

Now, shut the engine down and let it cool all the way back to 32F.

What's the system pressure?

It's certainly not 0 psi.