Question on Radiator Pressure Caps

[stopngo]

Took Rad in for Repair Yesterday. Don't know the verdict yet.

Told the Rad repair guy about recent blown head gasket. He tells me the Exhaust gases leaking into cooling system can damage the Rad due to much higher combustion pressures than the 15 PSI the system operates under.

My Question: A......... What is the purpose of the 15 lb pressure cap if it isn't there to prevent the above situation??

B.......Is it not also true that if the plastic overflow tank rises to the upper level at operating temp and then siphons back into the radiator as it is supposed to
do as the system cools and creates a vacuum also an indication that the pressure cap is operating as it should??

C.........If in fact the Rad pressure is excessive due to combustion pressure would the coolant not overflow somewhere at some point?? Has never happened.

[87Itasca]

The purpose of the cap is to vent any excessive pressure above 15 PSI into the overflow bottle. When most caps fail, it is due to the spring slowly weakening, resulting in it opening at lower pressures. The lower the pressure, the lower the boiling point. In these RV's, that's critical. It's the different between making the grade, and being stuck on the side of the road.

That's a sign of an engine running hotter than normal. Generally, if the radiator is full, when the vehicle is shut off, the pressure will not exceed 15 PSI and trigger the cap. If it is running warmer than usual, then yes, it may. Once it cools a bit (usually a few minutes), it will suck it back into the radiator. That's been my own experience with all my cars.

Generally if you had a decent head gasket leak, it would overflow. If it was small, then it likely wouldn't. I would get a shop to perform a combustion gas test to the cooling system. Then you will know for certain if you have a problem or not.

[stopngo]

Thanks for the input.

So if I am understanding you correctly then you are suggesting that I must be running too hot if my overflow bottle is fluctuating on a regular basis?

I should add to clarify that the minute I start it up from cold there is coolant moving into the overflow bottle yet it never rises above a certain constant level
and never overflows.

Also once at full operating temp I have no problem holding the top rad hose firmly without getting burnt which I cannot do on some of my other more modern vehicles with catalytic converters and higher designed operating temperatures.

[87Itasca]

That sounds to me like either the cap is faulty and opening well before 15 PSI, or you are getting gases from the head gasket into the cooling system.

Without seeing any of this, what it sounds like to me is you are blowing exhaust gas into the cooling system, causing it to push coolant into the overflow until there is a big enough air gap within the radiator for simply air/pressure to be vented into the overflow, vs. air/pressure and coolant. When it is shut off, there is a vacuum that the coolant in the overflow tries to fill until the pressure in the coolant system equalizes the vacuum.

I would do a combustion test. If it shows positive, that is exactly what is happening. Coolant should not be flowing into the overflow on a stone cold engine.

[stopngo]

Will check into it. Maybe its the cap........at least I hope.......because I just had the Head Gaskets replaced.

All Spark plugs are a nice light brown and none shows any signs of misfire......if that means anything.

[Rickf1985]

As soon as you start the engine the coolant is going to start to expand and in doing so will start to migrate into the plastic bottle. You should have a "cold" and a "hot" mark" on the bottle to show the respective positions of the coolant. Once the engine is up to temperature the coolant will stop rising in the bottle and when the thermostat opens it will actually drop just a little. It will continue to do this dance as long as things are working as they should and it is cool out. If it is hot out you will not see much of a drop in the bottle. Your first two statements were correct, the cap should release the excess pressure. Depending on how bad the head gasket leak is determines whether it is going to push coolant out. As was mentioned, if it is just a small leak it may not push coolant out but the cap will vent the gases that rise out of the coolant. I have seen catastrophic head gasket failures on race cars that literally blew the radiator cap off the radiator, bent the ears on the cap and still did not destroy the radiator. Id did balloon some of the tubes but not many. This was a 9,000 rpm blow-out, not something you are going to experience in you motor home. It is possible the radiator was just corroded from poor maintenance. Antifreeze should be changed ever few years or it breaks down and will wreak havoc on the cooling system. My radiator was like that, took it out and replaced it with an aluminum one and cool much better than before. As far as the upper hose, it should be warm and then get very hot and then cool down after the thermostat closes and then when the stat opens it will get very hot again. I don't know iif you have a radiator where you can see the fluid from the cap but if you can and it is flowing from the top hose as soon as you start it then the thermostat is bad, replace it with a 180 degree if you have a carburetor or a 195 if you have injection.

If you suspect a head gasket do as was mentioned and take to a shop that has an emissions tester and have them sniff the overflow bottle, if it shows hydrocarbons then you have a head gasket leak. If you start it cold and you do not see bubbles constantly coming out of the hose from the radiator into the bottle then you do not have a head gasket leak.

[stopngo]

As soon as you start the engine the coolant is going to start to expand and in doing so will start to migrate into the plastic bottle

And so I thought all this time.......glad I was on the right track

bent the ears on the cap and still did not destroy the radiator

Bent the ears on the cap?? Not sure I see how that can happen unless the Owner was using a 100 lb pressure cap where it should have been a 15 or 16 lb.

As far as the upper hose, it should be warm and then get very hot and then cool down after the thermostat closes and then when the stat opens it will get very hot again.

Yup.....Held my hand on Top Rad hose and the sudden surge of hot coolant from luke warm tells me thermostat OK. Took it out anyway and checked it in a pot of hot water on the stove with a Thermometer.

The small amount of coolant I have been losing lately is fully explained by the size of leak in the radiator so once the new rad is in and Im still losing Ill be watching for bubbles or Hydrocarbons.. Hopefully those 1800 bucks I spent on Water Pump and Head Gaskets last Nov were not in vain.

[stopngo]

Rick...........How much info do sparkplugs provide in terms of Head Gasket Condition?? Mine are all nice evenly light brown.

[Rickf1985]

The plugs would say it is running fine. Sometimes plugs do not show a small head gasket leak as long as it is still making decent compression. From your descriptions I think you are alright. Radiators just don't last forever. The OLD ones from the 40's and 50's seem to but have you ever picked one of them up? WOW! They are heavy!

[Rickf1985]

It bent the ears on the cap because it put so much pressure into the cooling system so fast that the cap could not pass it through. Luckily the cap let go before the radiator but it goes to show how tough a radiator is. To understand how that much pressure happens remember this was a race engine. 12.5 to 1 compression, 9,000 rpm at the time it blew. That is 4,500 compression strokes a second blowing through a head gasket that had half blown away into the coolant for the most part. A lot was also blowing out the side of the engine. All that pressure is more than the system can pass through the cap. I will say it was rather spectacular to see! In the end it didn't harm anything but the gasket, the radiator cap and the radiator neck, and the heads had to be milled since it cut a groove in the aluminum head where it blew.

[stopngo]

The Awesome effects and surprise elements of velocity and momentum where the cap blows before the rad even knows what's happening.

There is a lot of truth to that. I recall from my house framing days the science behind why the lumber doesn't bounce when using the Air Nailer as opposed to a manual hammer.......................By the time the 2x4 knew what hit it the nail was in