Pressure on the coolant system..... - PeachParts Mercedes-Benz Forum

Diesel911 · #1 06-16-2010, 08:17 PM

Quote:

Originally Posted by oldiesel

False signals from that little float switch is not all that unusual,I have had it on occasion on my 87 and my son has had it frequently on his 93.He was going to replace the float switch but recently it has been working OK or at least not giving false warnings.
Pressure in the system when the engine is running is normal from the heat/expansion in the coolant,thats why they put on a cap that is designed to hold a certain amount of pressure,for one reason to raise the boiling point of the coolant and also to prevent the build up of steam pockets in areas of the cylinder head. As for holding pressure overnight i dont understand how that could happen as the cooling of the engine would definitly decrease the pressure from what it was at operating temperature and how high the pressure is at normal operating temp is limited by the design of the cap.The cap even has a built in check valve so that if the cooling system gets cold enough and goes below atmospherec pressure it can allow air to enter to keep from sucking the hoses flat.It seems to me that an engine with a cracked head should loose pressure faster than a good one,but thats just me. Don

I do not remember who but one of our Members explained why the pressure gets retained.
Under normal cirumstances the hot Coolant expands and there is not too much air (gas) in the system so when the cold Coolant contracts enough that it decreases the pressure in the system and the Check Valve in the Cap opens.
But, when there is a Head Gasket leaking gas into the system the volume and pressure of compressed gas in the system is such that when the cold Coolant contracts and the cold gas contracts there is not enough contraction to relieve enough pressure to cause the check Valve in the Cap to open.

Fulcrum525 · #2 06-16-2010, 09:13 PM

Quote:

Originally Posted by Diesel911

I do not remember who but one of our Members explained why the pressure gets retained.
Under normal cirumstances the hot Coolant expands and there is not too much air (gas) in the system so when the cold Coolant contracts enough that it decreases the pressure in the system and the Check Valve in the Cap opens.
But, when there is a Head Gasket leaking gas into the system the volume and pressure of compressed gas in the system is such that when the cold Coolant contracts and the cold gas contracts there is not enough contraction to relieve enough pressure to cause the check Valve in the Cap to open.

Hmm so is what i'm describing normal operating conditions? (Pressure on the hoses but it goes down after shutdown?) A faulty sensor would be the best case scenario but I don't want to take any unnecessary chances.

Fulcrum525 · #3 07-14-2010, 08:26 PM

I talked to me indy today and I think i'm going to bite the bullet and get the head gasket done. No oil in coolant or anything but there is a lot of pressure on the system and while the pressure does subside overnight I don't want to risk blowing the head. (So about a grand for a new head gasket VS multiple thousands for a new head)

Thoughts and recomendations?

vstech · #4 07-14-2010, 09:28 PM

Quote:

Originally Posted by Fulcrum525

Hmm so is what i'm describing normal operating conditions? (Pressure on the hoses but it goes down after shutdown?) A faulty sensor would be the best case scenario but I don't want to take any unnecessary chances.

yes.
your pressure reporting is normal for any internal combustion engine with a water cooled motor.
if it's still got pressure in the morning, there is a problem,
and if it builds pressure in less than 1 minute at idle there is a problem.

Fulcrum525 · #5 07-14-2010, 11:20 PM

Quote:

Originally Posted by vstech

yes.
your pressure reporting is normal for any internal combustion engine with a water cooled motor.
if it's still got pressure in the morning, there is a problem,
and if it builds pressure in less than 1 minute at idle there is a problem.

How long should it take the pressure to build up at idle under normal operating circumstances?

Also I put coolant into the system a few weeks ago and the light wouldn't come on. Low and behold a few days ago it started flicking on again under acceleration. Now there is a possibility that my SECOND new radiator is actually leaking as there is a cream colored residue along its top edges. I'm probably going to have that one returned again in the near future.

aaa · #6 07-14-2010, 11:03 PM

Quote:

Originally Posted by Fulcrum525

Hmm so is what i'm describing normal operating conditions? (Pressure on the hoses but it goes down after shutdown?) A faulty sensor would be the best case scenario but I don't want to take any unnecessary chances.

Yes. And the sensor's got nothing to do with it, it just picks up coolant level (its a float). If the light goes on and you see plenty of coolant in the overflow tank (it has to get way low to turn the light on legitimately), then it's just a wonky sensor.