As the title asks - how long should the cooling system remain pressurized on a 617? I started my car up yesterday in approximately 25* (no block heater) and drove it enough to get fully warm. Today about 24 hours later I find that there is coolant seeping from a few different areas....so I take off the cap and POOF! A big wave of pressure, I would guess approximately 10-15 psi (I believe rad cap is rated at 14psi but still should not be pressurized after sitting alll night in the cold). I've already replaced the following:
- water pump, including housing
- WP gaskets
- Thermostat & gasket
- Expansion tank cap (equivalent to the radiator cap on the w123s)
- just about every rubber hose and clamp under the hood...all clamps are as tight as I can get them, or as tight as I feel comfortable getting them on the plastic radiator parts.
- radiator itself

I'm starting to go crazy now, I feel as though my engine is constantly overpressurizing...I've replaced all the above parts due to blowouts (due to excessive pressure). I figured it was just the rad cap not releasing when it should, but I've replaced that. I'm tired of losing coolant, and I can't just throw water in there because now its below freezing almost every night. Now I'm running with the cap loose to alleviate any pressure, not really a problem since its cool enough not to deal with overheating engines, but I would much rather get it under control. Any thoughts?

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TC
Current stable:
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- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

Could be a bad head gasket allowing combustion pressure into the cooling system. If it's holding pressure overnight, a cooling system pressure test might not give you a full story unless there's a way to test a hot engine. Is there indication of engine oil mixing with coolant?

Sixto
87 300D

Haven't seen any mixing, coolant doesn't have a sheen on the surface and the oil isn't milky - not sure what else to look for. I have noticed that the oil pressure at hot idle is lower than it used to be though...not sure when it started happening, used to be in between 1.5-2bar now it is a hair under 1.5 if that is any indication.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

Bump to top....

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

Any bubbles showing with the cap off and engine running? Since the whole system seems to be reacting. It is reasonable to assume it is being pressurized by a crack somewhere or leaky head gasket.

When you start the engine how long does it take the cooling hoses to really firm up? Releasing any retained presure before starting of course.Normally not much or any pressure should occur until you build a little coolant temperature.

I wonder if there is a diesel kit to test the coolant for the presence of combustion by products?

After the engine has fully warmed up, the coolant has expanded and raised the pressure in the closed system. Loosen the exp tank cap slowly (careful you don't burn yourself) to release the pressure. Tighten the cap and let it run a few minutes then loosen the cap again. There should be no pressure. If you have pressure there is likely combustion gas leaking into the cooling system.

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83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

This is a better test in my opinion. Make sure to use a rag over the cap especially the first time.

I don't think its the pressure that develops when the engine is hot that you are seeing in the morning after a cold night. When the coolant cools down to room temp it may well have no pressure in the radiator. When it then cools below 32F I have heard it expands a little, this would cause the pressure to build. This may be what you are seeing.

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My bet is on a head gasket.

Get an oil sample and send it off to a lab to confirm if there is glycol in the oil. That's a definitive test. The only way this test fails is if the head gasket fails in such a way that combustion gases get into the coolant and no coolant gets into the oil. It's possible, but not likely.

your post is a guess based on what you think was 15 pounds of pressure when you turned the rad cap to remove if i read it correctly < a normal cooling system will maintain its pressure for a few days easy and most systems run right at cap pressure so yes that is normal

if you have a 14 pound cap and it has over 15 as a guess by the amount of air blown out at removal than the 14 pound cap is junk and not doing its job, the level will move up and down in the expansion tank as it should cold to hot and back --

20 DEG and seapage might be a thing i would run the car for a 1/2 hour on the road than re tighten the hoses that are seaping while they are hot and soft the clamps will just bend on very cold hoses and make it worse THAN see what happens -- jz

This is the way I see it. Tell me if you think otherwise. In a normally functioning engine with a pressurized cooling system, when started cold with zero pressure and as the coolant heats and expands, pressure builds and is regulated and released by the cap. After engine cools (overnight ?) to the same temp before the engine was started the previous time, the coolant will have contracted and the pressure in the cooling system should be zero, unless the cap is not working or there are other other pressure source such as combustion gas.

I would suggest pressure testing the cap first, then check the leaky hoses by taking them off and cleaning the mating surfaces of corrosion and make sure the hose ends are not damamged. Do not over tighten hose clamps! That's one way to kill a good hose. They only have to hold 14 to 17 psi!

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I'll do this today to see what happens...my gut tells me its not a good outlook. I've been driving it around for a while now with this happening, probably almost a month and I attributed it to a bad radiator cap not venting when it should, hope I haven't damaged anything. I'm curious though--wouldn't I see some sort of coolant in the exhaust as white smoke, or oil in the coolant or something/anything if the head gasket were bad? I've worked on quite a few gas engines and all the head gasket failures were pretty easy to diagnose. I haven't been losing oil and the only time I've lost coolant is when it seeps out, which strangely occurs intermittently.

Thank you for all your input!

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

I agree with your reasoning--even with a combustion leak I can't figure out how it would retain pressure though, the cap should either vent it or it would leak back into the cylinder or wherever it is coming from. I haven't noticed any bubbles in the expansion tank but will double check again. All the mating surfaces are clean, I've replaced almost everything under the hood by now, cleaning and tightening as I go.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

You could have coolant in the exhaust or coolant in the oil. You won't see either at the initial stages of failure. The coolant in the exhaust is immediately dissipated as vapor. The coolant in the oil immediately boils off and leaves traces of glycol and sodium in the oil.

Wait awhile and drive it under these conditions and the symptoms will worsen. However, at that point, you'll have compromised the main bearings if glycol has accumulated in the oil.

Go and get a proper oil analysis and quit theorizing without any data.

Find a shop with a smog machine and have the expansion tank checked for hydro carbons.That should tell you if you have a blown head gasket.