Pressure under the coolant cap

GreenDieselEngineering · **Joined:** Sun Jun 07, 2009 5:10 pm **Posts:** 1111

As several have mentioned, if the coolant bottle overflow portion is clean, then there is no head gasket leak...yet.

The system pressure can vary based on speed and load from 0-7ish psi, average driving should be on the lower end. When pulling a trailer up a pass the pressure increases substantially. Is the t-stat factory OEM? There are several possibilities for variability in coolant operating pressures (t-stat, airflow, rpm, air temp, A/C,heater core...)

geordi · **Posted:** Sat Apr 12, 2014 12:39 am

Racer, stop thinking in absolutes too. Some pressure on a hot engine is normal for every vehicle. The real indication is the gas test.

Auberon · **Posted:** Sat Apr 12, 2014 2:34 am

Mate if the pressure was not elevated it would boil under load even idling in traffic. I'm afraid your mechanic is wrong. It was in the 1950's that zero pressure cooling systems were discontinued. My 1952 British tractor has one - a pretty monstrous brass screw-on cap.

Simple physics of cooling systems.
As an example with Glycol coolants
At sea level:
33% boils @ 212 F
44% boils @ 224 F
50% boils @ 226 F

@ 8 psi above
33% boils @ 240F
44% boils @ 245 F
50% boils @ 248 F

@ 16 psi
33% boils @ 2600F
44% boils @ 265 F
50% boils @ 267 F

As a rule of thumb:
For every pound of pressure exerted on the coolant in the system the static boiling point of the coolant is raised by approximately 3 deg F.

Racer at what temperature is your engine operating when you take the cap off and do this?
Need to look at numbers associated.
Even the early cap-off test you do - I'd be wondering what the temp is.
In any case there would be the evidence GDE cited as well as bubbling and changes in level- it could go up and down under different conditions.
Have you pressure tested your cap? or has the vent valve been left open inadvertently meaning no pressure build-up.
If the mechanic has left this open and it has just been closed - then you could have a change in behaviour.
I'd think if you have no pressure you are really lucky not to have boiled her dry.

Is it positive or negative pressure on the 1 minute test you did?
Perhaps you need to P test the system if you are sure there is something going wrong.

Mountainman · **Posted:** Sat Apr 12, 2014 9:59 am

And your "cap off and check after a minute test" doesn't tell you much. When you pop the top of a hot container, heat air rises out, and colder air drops in. So, as soon as you put the lid back on, that colder air expands a lot, and voila, you have pressure almost immediately even though the hot fluid hasn't changed temp significantly.

:2cents:

jlgail4309 · **Posted:** Sat Apr 12, 2014 11:37 pm

Did you do the arb bolts on the last head gasket?

racertracer · **Posted:** Sun Apr 13, 2014 8:32 am

There is pressure under the cap after I run the engine for one minute at idle.

flman · **Posted:** Sun Apr 13, 2014 2:17 pm

So any coolant in the over flow, is it losing coolant, I thought you were going to do the combustion gas test? :POPCORN:

pwrwagn · **Joined:** Mon Jul 22, 2013 2:40 am **Posts:** 192

Even after mine reaches full operating temperature, is driven 20 miles, and then parked in front of the house, whether you let it heat soak or not, there is NEVER more than a slight puff. And never any other time than when it is at full operating temperature. The only thing that comes out is a slight steam vapor.

This was not true when I had a failing head gasket. It would not develop pressure under the cap until it had been driven on the road, and put some throttle into it.

grywlfbg · **Posted:** Mon Apr 14, 2014 1:30 am

flman wrote:

So any coolant in the over flow, is it losing coolant, I thought you were going to do the combustion gas test? :POPCORN:

I've read that folks have had varying degrees of success w/ the exhaust gas test but the losing coolant and overflow checks are pretty critical to this diagnosis. What's the word racer?

racertracer · **Posted:** Mon Apr 14, 2014 2:43 am

The exhaust gas leak test indicated a slight change in color at idle.

Highway driving for an extended period of time produces considerable amount of pressure under the cap.... explosive pressure.

The overflow reservoir is empty

This vehicle has never experienced pressure under the cap before. Never.

I am pretty sure that I am at the beginning stages of a HG leak that will increase in gravity the more I drive it.

I will soon prepare for a TB replacement and cylinder head leak repair.

I will perform the work myself.

grywlfbg · **Posted:** Mon Apr 14, 2014 1:48 pm

Keep us posted but until you start losing coolant via the overflow I would keep driving it.

jlgail4309 · **Posted:** Mon Apr 14, 2014 9:17 pm

took mine like a year before it started pumping water.

racertracer · **Posted:** Tue Jun 03, 2014 3:21 am

Update.

TB parts, studs and HG are on the way.... but something puzzling is happening.

What used to be evident at first with bursts of pressure escaping via the cap, is now no longer evident. No more pressure is felt.

I have been continually checking the pressure under the cap after every long or short trip to get a better feel for how much pressure is building up.

I now am experiencing absolutely no more pressure under the cap, none, it was a sudden stop. This is after running both short and long drives.

I am in the northeast, our area is coming out of the colder winter months where we were experiencing 0 to 20 degree weather before.

This week the ambient temperature is higher, we are experiencing 75 to 85 degrees.

The pressure under the cap is no longer there, I have been observing this after 6 long trips and short ones also.

3 to 4 weeks ago the amount of pressure under the cap was explosive. I just needed to run the engine for a minute before the slightest bit of pressure was felt. But now nothing .... not even after allowing the engine to idle for 15 minutes and none even after an hour long drive.

This weekend I took a long 2 hour drive and when I stopped to check the pressure there was none, absolutely none, not even a tiny hiss.

Today during my 90 minute commute to and from work, no pressure under the cap.

I have not done anything to the engine or the cooling system to correct what I think the problem could be, a HG leak.

What the heck is happening here? Anybody? Is the aluminum head expanding and retracting?

74,800 miles on the clock.

Posting my observations for posterity and Data....hoping for solutions.

flman · **Posted:** Tue Jun 03, 2014 6:23 am

Your cap is leaking? Because you are supposed to have pressure, both of mine with good head gaskets have pressure. It is physics, water is heated and expands in a sealed vessel and builds up pressure, a blown head gasket is not required to achieve this, but a seal is.

racertracer · **Posted:** Tue Jun 03, 2014 6:35 am

Thank you for your response and I understand what you are saying, but do you have another valid explanation for what is happening?pwrwagn

If you read pwrwagn post above, he also indicates that his CRD has never had slightest bit of pressure under the cap, NEVER.

flman · **Posted:** Tue Jun 03, 2014 6:39 am

racertracer wrote:

Thank you for your response and I understand what you are saying, but do you have another valid explanation for what is happening?

If you have no pressure you have a leak, since you have no fluid on the ground, it could be the cap, or some where in the recovery bottle? But you should have pressure with a hot engine. Did you ever test the cap?

racertracer · **Posted:** Tue Jun 03, 2014 6:45 am

The cap going bad is not the case...... I have gone through the cap test many times before on two other CRD with many more miles on the clock more so than this one and they turned out to be good..... I still have three brand new caps on the shelf that i could test. That route is futile.

If you read pwrwagn's post above, he also indicates that his CRD has never had slightest bit of pressure under the cap

tjkj2002 · **Posted:** Tue Jun 03, 2014 9:18 am

racertracer wrote:

The cap going bad is not the case...... I have gone through the cap test many times before on two other CRD with many more miles on the clock more so than this one and they turned out to be good..... I still have three brand new caps on the shelf that i could test. That route is futile.

If you read pwrwagn's post above, he also indicates that his CRD has never had slightest bit of pressure under the cap

If you don't have pressure when hot you don't have a sealed system,it does not matter if another does not have pressure as his system is not sealed also.

racertracer · **Posted:** Tue Jun 03, 2014 9:43 am

tjkj2002 wrote:

racertracer wrote:

The cap going bad is not the case...... I have gone through the cap test many times before on two other CRD with many more miles on the clock more so than this one and they turned out to be good..... I still have three brand new caps on the shelf that i could test. That route is futile.

If you read pwrwagn's post above, he also indicates that his CRD has never had slightest bit of pressure under the cap

If you don't have pressure when hot you don't have a sealed system,it does not matter if another does not have pressure as his system is not sealed also.

Are you thinking that the cap is leaking and that is the reason why the system is not sealed?

tjkj2002 · **Posted:** Tue Jun 03, 2014 9:50 am

racertracer wrote:

tjkj2002 wrote:

racertracer wrote:

The cap going bad is not the case...... I have gone through the cap test many times before on two other CRD with many more miles on the clock more so than this one and they turned out to be good..... I still have three brand new caps on the shelf that i could test. That route is futile.

If you read pwrwagn's post above, he also indicates that his CRD has never had slightest bit of pressure under the cap

If you don't have pressure when hot you don't have a sealed system,it does not matter if another does not have pressure as his system is not sealed also.

Are you thinking that the cap is leaking and that is the reason why the system is not sealed?

Well something is preventing the system from building pressure.Just test the cap with a pressure tester.If it's a stant cap it's most likely bad.2 years ago I cleared the shelves from 3 parts stores,that would be 15 caps, from being bad out of the box.

I personally prefer Motorad,never had a issue with there stuff(besides there failsafe t-stats but all fail safe t-stats are junk).

Pressure under the coolant cap

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