Hi my name is Ed I'm new to this form and Pontiacs however not new to other GM products, I recently purchased a 68 Firebird 350 350 2 Barrel fairly stock and honest car. since purchased I was concerned about the coolant level in the radiator fluctuating this may be normal just wanted some opinions.

with the vehicle running (idling) at temperature with the cap off the level will fluctuate from the filler neck to about 5 to 6 inches below the filler neck. At this condition there is approximately another three quarts of capacity that could be filled in the system.

Vehicle does not overheat in traffic or in the driveway when performing this test it operates at roughly 190° F I have replaced the thermostat and flushed the whole cooling system confirming that there are no blockages in the radiator, heater core or engine block.

When the radiator cap is on (16 lb) it will run and maintain the 190 ish temperature under all conditions.
My concern here is the radiators feel firmer than I would expect them to be The radiator hoses are firmer than I feel that they should be. You can squeeze them in approximately an inch to an inch and a half but you cannot pinch the two walls together.

Any feedback opinion suggestions other tests that I should try are very welcome thank you very much

Hello Ed and welcome to the group.

What you described is normal. You need 3" below the neck to allow for expansion so antifreeze does not puke on the ground. With the cap on and the car warmed up, there will be pressure on the hoses. That is normal. What you have described is all normal and nothing to worry about.

You also might want consider adding an overflow tank.

X 2

Fellas thanks for the feedback, not to be a nuisance I'm trying to understand is this unique to the engine or Pontiac engines? The reason I ask is I have not had this type of situation with Chevy or Mopar engines in mid to late 60's muscle cars. Typically i would expect with cap off engine running at steady state coolant level would settle down and stay at a consistent level.

With the cap on the pressure I'm feeling in the hoses seems excessive to the point I'm a little concerned there might be a burst condition.

I'm struggling to understand what is making the coolant continually raise and fall 4-5 inches in the rad (once warmed up viewing through filler neck with cap off). Could there be a flow restriction, air bubble captured in the system, etc? I would like to pin point where the elasticity in the system is coming from, while the cap is off.

Any feedback is greatly appreciated.

Thanks,

Ed

Expansion/contraction. Look at newer car overflow tank. Cool level lower than hot level. Get the overflow tank.

Having the water level a few inches low is normal, if you don't have an overflow tank.The radiator starts full then when the thing heats up the fluid expands and some dumps out the overflow. When it cools the fluid contracts and the level is low. While the engine and hot the level should be near the top of the rad. What you are seeing may be air pockets in the system expanding and pushing the fluid level up, then as the air is expelled the fluid level drops once again.Or it could be the thermostat shutting off flow, the coolant in the rad cools and contracts until the thermostat opens again and the hot fluid is pumped into the rad.

Like the guys said, get an overflow tank and a proper rad cap for an overflow tank system. When the fluid expands it will go into the tank when it cools it will go back into the rad. Having a rad 100% full is better for cooling than having a rad with air pockets in it.
imo

Welcome...
When you say it "fluctuates", is this within a few seconds or minutes..?
If this is happens [at steady idle] within a few seconds there may be an issue with the themostat. If it happens every few minutes it is normal themostat cycling.

I have an original radiator and there's a fill mark below the filler neck. If I fill to that mark it runs & cools fine. Even if there's extra coolant, the system will push out the excess and self correct the level. Since I live in the South-West I installed an over-flow tank just to have a little extra coolant.

If your hose were NOT firm, I'd be concerned. The benefit of a closed/pressurized cooling system is it raises the boiling point of the water/coolant.

Also, your your upper hose can collapse when the engine cools(with the cap on, of course). The cooling (contraction) of the coolant can cause a vacuum in a closed system. That's why there is often a "spring" in the lower hose...to keep it open.

Hi PeteJD,

To answer your question the fluctuation cycle of the coolant raising to the top of the rad, falling several inches and then raising again to the top of the rad is roughly 15-20 seconds. Bear in mind this happens with and without a thermostat.

To bluebirds point it could be air pockets, however on other applications I have only experienced trapped air in a cooling system when the rad filler neck was not at the highest point of the cooling system. That's not to say that there's not air in the system but I think it would work it's self out fairly easy considering the configuration of the cooling system on this application.

These cooling system are very simple, I have never experienced this anomaly on any other application.

I would ask any member that has a similar application, Pontiac 350.
Do the following on thier own vehicle and report back the findings.

Remove the cap from the rad measure the approximate distance from neck of the rad to the fluid level, start the engine observe/record any significant changes from ambient to steady state. Record the steady state temp observe and record coolant level several for minutes. Place the cap on the rad squeeze the top rad hose record inches of squeeze to full pinch.
Let run for 10 min periodically squeezing the hose and recording.

Just to be clear I fully understand the expansion and contraction of fluids as they heat and cool, this is not the question of this thread, rather the the erratic fluctuating coolant level.

As reference on any vehicle I have that doesn't have a coolant overflow tank I do a simple calculation to provide the appropriate air compensation to prevent overflow dV(volume change)= V(volume of coolant system) B(volumetric temperature expansion coefficient)((T1(final temp)-T0(initial temp). So for this application dv= .016088M^3*.00057(104C-21C)=.0007m^3 or 26 ounces. So in this case with engine off I filled the system complete to the top of the rad then opened the petcock at the bottom of the rad and pulled off a little less than a quart, made a mark on the rad to note the proper level for the system once burped, started the engine and this is when I noticed the issue with the fluctuating.

"As reference on any vehicle I have that doesn't have a coolant overflow tank I do a simple calculation to provide the appropriate air compensation to prevent overflow dV(volume change)= V(volume of coolant system) B(volumetric temperature expansion coefficient)((T1(final temp)-T0(initial temp). So for this application dv= .016088M^3*.00057(104C-21C)=.0007m^3 or 26 ounces. So in this case with engine off I filled the system complete to the top of the rad then opened the petcock at the bottom of the rad and pulled off a little less than a quart, made a mark on the rad to note the proper level for the system once burped, started the engine and this is when I noticed the issue with the fluctuating."

...or...fill up the radiator, let engine run with cap off. Once thermostat opens add coolant until level is just below filler neck. Monitor temperature as engine continues to run with cap off to burp system, also monitoring coolant level. Shouldn't take more than about 20 minutes. Replace cap.

Your engine is operating at a perfect temp. Why overthink it?

I've had the very same condition on a couple of my daily drivers. Some of them had other motor issues at the time. Like head gasket issues. The coolant will start to rise and fall in the rad while warming and after some arm up time the coolant level would eventually rise high enough to bubble out of the neck. Seemed fine with the cap on. Wish I could tell you why. I don't know myself but I have to wonder if it isn't positive pressure being exerted on the coolant system by a leaking gasket.

X2!! Keep it simple- I agree with all the advice that folks have provided. If you have new hoses, they will obviously be stiffer than older ones, and if you are overwhelmed with curiosity, just pull both hoses off and look through them to see if you have any springs or other support in them making them so stiff.

But like Bronze Bird says- if it is at the right temp- dont worry so much. Just verify the integrity of the hoses, connections and that the juice is flowing when running.

There is a fill to line on the OEM radiator 3" from the top, The manual says fill to that level. That seems simple to enough to me. No need for a slide ruler.

I am struggling with why we all need to run a science experiments on our 50 year old cars. Either you don't have the level adjusted properly or there is something wrong with your motor. I follow the simple instructions GM puts out and guess what, I don't puke coolant. BTW Harold, Sprint back running from Terry Walters, great recommendation.

Good deal Larry. Glad it worked out for you. I need to get up with you sometime so I can check out the work.

OH my! BB, that was harsh. Friggin hilarious but harsh. Life lessons eh? LOL.