Pressure in cooling system after 12+ hours?

[RangerGress]

Guys

Pv=nrt

Pressure and/or volume will change with temp changes. Basic physics.

That's the "ideal gas law", not the "ideal gas and liquid law".

By definition it does mean the head gasket is not sealing all combustion from the cooling system, so yes, I'd say that means he has a problem if he has pressure in the system after it has completely cooled down.

At the same temperature as the day before, you should be able to run the crap out of the car, then let it cool for many hours and get no pressure change when opening the cap(assuming local barometric pressure is equal).

I disagree. The fact that the OP's coolant system retains pressure well doesn't tell us anything about too much pressure (combustion products) getting into the system.

When you pressure test a coolant system you pressurize it to 20psi or so and see how long the pressure holds. If the pressure holds up pretty well, say 30min to an hour, then the coolant system probably doesn't have any leaks. It's not a big leap from a pressure system that holds 20psi for an hour to holding 20psi for 24hrs.

The ability to hold pressure that additional 23hrs certainly doesn't indicate that the HG is bad. On the contrary, it indicates that the HG's water channels are sealed tight.

You want to know about combustion products in the coolant system? Pop the coolant reservior off, start the car, and then look in the reservior. You'll see little pop's of gas coming out of the coolant each time the cylinder with a leak fires.

[Michael9218]

I disagree. The fact that the OP's coolant system retains pressure well doesn't tell us anything about too much pressure (combustion products) getting into the system.

When you pressure test a coolant system you pressurize it to 20psi or so and see how long the pressure holds. If the pressure holds up pretty well, say 30min to an hour, then the coolant system probably doesn't have any leaks. It's not a big leap from a pressure system that holds 20psi for an hour to holding 20psi for 24hrs.

The ability to hold pressure that additional 23hrs certainly doesn't indicate that the HG is bad. On the contrary, it indicates that the HG's water channels are sealed tight.

Actually, a freind's car had a similar problem with pressure build up in the cooling system. Would only do it on track, not on the street. The pressure would stay in the system for hours, but he never had coolant in the cylinders. He would bleed it repeatably, thinking this was the issue. He eventually replaced the HG and the problem was solved.

Your theory of the pressure holding at 20psi sounds good, but the combustion chamber is pushing 200 psi. The leak may be small enough to allow flow at 200 psi and not at 20 psi.

[Brent 930]

I'm also wondering if a HG leak can be directional. That is, a coolant pressure checks out OK but combustion can leak into the coolant.

[osborni]

That's the "ideal gas law", not the "ideal gas and liquid law".

Yes, but water expands as well.

About 2-3% for a ~80*C delta.

But my point is that if you close the system at ~50*F, do a run, then open it at 80*F, the "at rest" pressure in the system is going change due to the fixed volume but the higher ambient.

[thejlevie]

I'm also wondering if a HG leak can be directional. That is, a coolant pressure checks out OK but combustion can leak into the coolant.

It can and frequently is "directional" in that the gasket may leak at high pressure but not a low pressure. That means that gas can be forced through the leak on the compression/power stroke, but coolant (being at a lower pressure) won't leak back.

[Def]

Actually, a freind's car had a similar problem with pressure build up in the cooling system. Would only do it on track, not on the street. The pressure would stay in the system for hours, but he never had coolant in the cylinders. He would bleed it repeatably, thinking this was the issue. He eventually replaced the HG and the problem was solved.

Your theory of the pressure holding at 20psi sounds good, but the combustion chamber is pushing 200 psi. The leak may be small enough to allow flow at 200 psi and not at 20 psi.

The combustion chamber "pushes" well over 2000 PSI at peak cylinder pressures.

Some people seem to be missing the fact that as temperatures come back down to their original levels, so will the cooling system pressure. If there is pressure in a cooling system that is cold, you are letting combustion enter the cooling system - END OF STORY.

And yes, it will typically only happen at WOT and high engine temperatures if there is a problem, so idling or even street driving are not good tests.

[fatboycowen]

This is exactly what i have been telling Nick all along. In a closed system under the identical conditions (atm pressure and temp), the cooling system will have the same (or zero) pressure each time, unless fluid or air is entering or releasing between checking it.

so, if you open it cold one day, you are at zero. Open it cold (same exact conditions) the next day, or next week the conditions are the same, and the pressure should be the same - zero. If not, something was added or taken away.

In nick's case, the system is pressurized, as the hose is hard, so its not creating a vacuum.

I know this isn't a fluke, because he has been obsessed with it lately. He checks the pressure in the system almost daily for months under the same conditions (cold). He texts me almost every time.

Here is the question - if the car runs great now (i think its been the same for 5 or more track days), should he hurry and fix it, or run it until it gets worse? Will he be causing engine damage to any other parts of the motor (bottom end) with this problem?

[S.Lang]

If there is pressure in a cooling system that is cold, you are letting combustion enter the cooling system - END OF STORY.

Not always - I found years ago that running straight distilled water in my cooling system was causing some kind of chemical reaction in the system that was resulting in a slight buildup of pressure (audible upon cap removal) after periods of non-use. Switched to normal (softened) tap water and water wetter and the issue went away.

It was wierd, but it's another data point.

[RangerGress]

Your theory of the pressure holding at 20psi sounds good, but the combustion chamber is pushing 200 psi. The leak may be small enough to allow flow at 200 psi and not at 20 psi.

Sure, but that's not my point. I'm saying that a discussion of too much pressure in the coolant system because of exhaust gases, is a different discussion then the coolant system being able to retain appropriate pressure overnight.

Yes, but water expands as well.

About 2-3% for a ~80*C delta.

But my point is that if you close the system at ~50*F, do a run, then open it at 80*F, the "at rest" pressure in the system is going change due to the fixed volume but the higher ambient.

And my point is that the change in water volume due to the change in temp is irrelevant. 2-3% isn't enough to go for 0psi to 1psi.

The rest pressure is going to change because the water pump has been running. The 20-30psi that you had when the car stopped gradually bleeds away, but not because the car cools. Gas pressure increases linearly with temp, but fluid pressure increases so little it can be discounted.

An engine overheating is a problem.
Water in the oil is a problem.
Oil in the water is a problem.

Pressure in the in the cooling system after 24hrs is a little surprising, but all by itself, is not a problem. In the absence of a "real" problem, I wouldn't worry about it. If and when there is a real problem, the pressure issue might be an interesting symptom that helps one understand what's going on.

[Michael9218]

[quote=Def;20487692]The combustion chamber "pushes" well over 2000 PSI at peak cylinder pressures.[QUOTE]

Interesting. I'd always thought that the pressure as measured in a compression test was the peak pressure in the combustion chamber.

Sure, but that's not my point. I'm saying that a discussion of too much pressure in the coolant system because of exhaust gases, is a different discussion then the coolant system being able to retain appropriate pressure overnight.

I don't see how the two are not related. There shouldn't be residual pressure in the system unless the pressure is being added. In this case by exhaust gases from the compression chamber.

An engine overheating is a problem.
Water in the oil is a problem.
Oil in the water is a problem.

Pressure in the in the cooling system after 24hrs is a little surprising, but all by itself, is not a problem. In the absence of a "real" problem, I wouldn't worry about it. If and when there is a real problem, the pressure issue might be an interesting symptom that helps one understand what's going on.

Well, it's only a problem if you're in the school of thought that you should fix a problem before catastrophic failure.

[das borgen]

[quote=Michael9218;20488722][QUOTE=Def;20487692]The combustion chamber "pushes" well over 2000 PSI at peak cylinder pressures.

Interesting. I'd always thought that the pressure as measured in a compression test was the peak pressure in the combustion chamber.

then you weren t factoring in the added pressure spike caused by the actual combustion (step 3-4 below), which is where power comes from


P is pressure and V is volume

[Michael9218]

then you weren t factoring in the added pressure spike caused by the actual combustion

Oh yeah, combustion...yep, I'd neglected that part.

[savage217]

Here is a video I took this morning so you guys can sort of see what I am talking about. I tried to get the hiss from the pressure on video. The hose was pretty darn firm. I think their is a problem but I don't plan on doing anything to it until something happens because temps are perfect and there is no oil in water or vice versa.

[ame]http://www.youtube.com/watch?v=VxuAY5ixQls[/ame]

[S.Lang]

So what happens if you don't drive it at all after you've relieved the pressure and put the cap back on? Say you come back 2 days later.....what does it do?

[das borgen]

yep, that a leaky but not YET blown HG or a head with some cracks in it

[savage217]

So what happens if you don't drive it at all after you've relieved the pressure and put the cap back on? Say you come back 2 days later.....what does it do?

iirc it will be at 0 pressure. (no hissing out of the cooling system or hard hose)

[PHARMDVIPER]

that was a lot of built up pressure in the video

that video was taken after it sat for a while? if I let my car sit over night and open the expansion tank I have no hissing at all

[94is]

Voted not normal. But as you know the job is cheap if you do it yourself. I had to replace mine a few weeks ago and it cost $180 in parts and labor (cylinder head pressure test and warp test). Took me about a day to do the whole job alone. Good luck!

[das borgen]

Voted not normal. But as you know the job is cheap if you do it yourself. I had to replace mine a few weeks ago and it cost $180 in parts and labor (cylinder head pressure test and warp test). Took me about a day to do the whole job alone. Good luck!

how was it that cheap?!....$180

[94is]

how was it that cheap?!....$180

$80 for a headgasket, head bolts, vanos o-rings, and paper gasket on the hard coolant line at the back of the head. Intake manifold gaskets, exhaust manifold gaskets, exhaust hardware, etc were all almost new so I didn't need to replace them.

$100 for the machine shop to test the cylinder head. Usually I can get that done for ~$65, but I needed it done that day since the car had to drive onto the trailer for a race the following day.

[das borgen]

oh...carry on

[RangerGress]

I don't see how the two are not related. There shouldn't be residual pressure in the system unless the pressure is being added. In this case by exhaust gases from the compression chamber.


Well, it's only a problem if you're in the school of thought that you should fix a problem before catastrophic failure.

Re. residual pressure. You have to account for the 1.4bar cap on the reservior cap.

Any time the E36 has more then 1.4bar (1.2bar for E30), the reservior cap should allow some pressure to escape.

Scenario 1. No unusual pressure inputs. Depending on where you measure in the coolant system the pressure is around 5-20psi. When you turn off the engine there is residual pressure that gradually leaks out. Entropy always wins.

Scenario 2. A HG leak is introducing combustion gases. Any impulse of gases causes over pressure and the reservior cap allows some pressure out. So 1.4 bar, more or less, is still maintained. When you turn the engine off, you have the same residual pressure of ~1.4 bar, which then gradually bleeds out.

Re. catastrophic failure. Some things are "interesting" and some things are "problems". If a person chases the merely "interesting" they can spend a lot of time and money chasing. My father used to say "Scott, if it ain't broke, don't fix it". And every time I violated that I regretted it.

If a person thinks that they might have a problem they need to test for it hard enough that they become certain they have a problem, and pretty sure what the problem is.

The OP might indeed have a HG problem, but this "I still have pressure in my coolant system the next day" symptom is not conclusive. I'm just saying don't pull the head until the problem is certain.

[fatboycowen]

Scenario 1. No unusual pressure inputs. Depending on where you measure in the coolant system the pressure is around 5-20psi. When you turn off the engine there is residual pressure that gradually leaks out. Entropy always wins.

I don't buy this. In order for the pressure to build and stay high over night, something would need to be added to the system after the cap was popped originally.

It's a closed system. whatever air and coolant inside never changes. The volume of that air, and of that coolant should be equal before and after the car is run, and let cool.

Sure, if more then 1.4 bars is present, the cap lets it out. But, i don't see this happening on this motor.

[RangerGress]

I don't buy this. In order for the pressure to build and stay high over night, something would need to be added to the system after the cap was popped originally.

It's a closed system. whatever air and coolant inside never changes. The volume of that air, and of that coolant should be equal before and after the car is run, and let cool.

Sure, if more then 1.4 bars is present, the cap lets it out. But, i don't see this happening on this motor.

Sure, it's supposed to be a closed system, but who hasn't seen a little coolant end up in their catch tank, or have to add a little water once or twice a year. But that's not really relevent. All I'm suggesting is that it's entirely reasonable to assume that the reason the coolant system has pressure the next day is that it's just holding pressure well.

Re. "in order for pressure to build and stay present..." How is static pressure going to build above 1.4bar? Are you suggesting that something is pressurizing the coolant system after the engine is off?

Re. "sure, if more then 1.4bars is present, the cap lets out..." Explain pls. The idea that additional pressure from combustion is the cause of holding pressure over night is the key to the whole thread. The pump alone can do >1.4bar. The only reason the pump doesn't blow out the cap is that the reservior is at the pump inlet, not outlet, so dynamic pressure is at it's lowest ebb.

[Def]

And my point is that the change in water volume due to the change in temp is irrelevant. 2-3% isn't enough to go for 0psi to 1psi.

The rest pressure is going to change because the water pump has been running. The 20-30psi that you had when the car stopped gradually bleeds away, but not because the car cools. Gas pressure increases linearly with temp, but fluid pressure increases so little it can be discounted.

You don't really understand the pressure/volume relationship between compressible and incompressible fluids as well as you think...

2-3% volume increase on a ~3 gallon system is a relatively small amount... when not in a closed system... but large for a car's cooling system. Now this volume increase is fixed since it is a fluid. You've got a relatively small amount of air in the closed system, and it will also increase in pressure due to temperature. Then additionally compress the air by the change in fluid volume. I think you'll find the volume change of the fluid dominates the pressure gain in the system, not the temperature increase(which increases by a relatively small amount due to the temperature being on the absolute scale).

An engine overheating is a problem.
Water in the oil is a problem.
Oil in the water is a problem.

Pressure in the in the cooling system after 24hrs is a little surprising, but all by itself, is not a problem. In the absence of a "real" problem, I wouldn't worry about it. If and when there is a real problem, the pressure issue might be an interesting symptom that helps one understand what's going on.

Letting it go for long periods of time under track usage is a good way to warp a head beyond usefulness. Don't run your engine into the ground, fix it when you have the first opportunity to do so when you notice something like this(which is about every year for most M5x/S5x based motors ).