I had a thermostat leak fixed this week, and am now monitoring my coolant level very closely to make sure that I have no further leaks. The engine never overheated.

I now notice that when I release the overflow coolant cap, even after 12 hours of not having used the car there is still pressure being released. I am assuming that I did not notice this pressure before because of the original leak.

Is it normal however for the pressure to remain in the cooling system for 12 Hours or more? I should add that the hoses themselves are not under pressure when the engine is cold.

I am assuming that this is a good sign. Is that right?


Thanks,

Phillip

We need to know the year and model of the car. Is it the 1995 E300 in your profile?

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

Sorry I forgot. Yes it's the '95 E300 Diesel

Pressure or vacuum?

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Good question! I thought it was pressure, but I suppose that it could be vacuum. Is there an easy way to tell? What is the significance of either one?

Vacuum is normal, a result of the coolant and any air shrinking as it gets cooler. Pressure has to come from somewhere, and pressure on a cold engine is usually a bad head gasket or cracked head (aluminum heads are the worst for both).

If the hoses are soft, and it makes a puffing or sucking sound when you take the cap off, it's vac. If the upper radiator hose is collapsed, get a new cap, it's not relieving vac like it's supposed to (this will draw coolant back out of the overflow tank, by the way.)

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I am still not clear whether the sound I hear when I release the radiator cap with the engine stone cold is pressure, or vacuum. Which one is normal? (if any) is vacuum normal, or is pressure normal?

How do I tell the difference?

Vacuum is normal. As the engine cools the air that is in the system contracts and you get a slight vacuum. The radiator cap is designed to allow the system to draw some air back in as it cools but it will still allow a little vacuum to remain. If you listen carefully you can usually hear the air being sucked in. Another way to tell is to spray some water around the cap before you remove it. If there is pressure you'll see water blown away from the cap as you remove it (you'll need to watch carefully). Usually if you have pressure the hoses are hard to squeeze but if you have a vacuum they are easy to squeeze. Since you don't have hard hoses my guess is that you just have a normal vacuum. If however the vacuum is strong enough to make the hoses start to collapse and/or the cap hard to remove, you nee a new cap.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold

If it is vacuum that I hear when releasing the cap, can I then safely assume that I don't have any more coolant leaks?

Not necessarily. Most leaks occur when the system is hot and under pressure. It's a good sign that it is holding a slight vacuum but I'm afraid that won't automatically translate to no leaks under pressure. Just monitor the coolant level and you should know if there is a leak. By the way, if you top up the resevoir it is normal for the system to overflow slightly. That's the whole point of the expansion tank, to hold a certain amount of air (room) to allow for expansion. Once you've cycled the coolant temp a few times the level in the tank should remain pretty constant at about half or two thirds full.

__________________
LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold

I think you are right. I was trying to maintain my coolant level at slightly above the maximum point (1/4" above the max.).

Every day I would notice that I would be loosing about 1/4" of coolant. I would then top it off again (above the maximum), thinking that I still have a leak. I was in a vicious circle. I now believe that the excess coolant was going out the overflow, and that there is no leak at all.

I will adjust the fluid level to just slightly below maximum and will monitor it for the next few days. Hopefully no more missing fluid.