72 455 EVERYTHING NEW STILL OVERHEATING ON START UP GOING CRAZY
 

I finished the build on my 72 455 GP can't seem to figure this one out everything is new and it gets hot on start up only first 5 minutes run time then will run cool without any issues .Start up will go to 240 to 260 can hear the coolant popping gets that hot hoses get rock hard then everything goes to normal never gets over 200 while driving or idling .
Block bored 30 over.
Heads completely done 6x4
New heavy duty fan clutch
New radiator core with oversize core
Second thermostat 185 degrees
Water pump original done by Kellog with separator plate and spacing
every hose changed
coolant and purified water correct mixture
Long branch manifolds with 3 inch exhaust
As soon as the t stat opens all the way my temp drops and everything is normal makes no sense great flow through radiator getting frustrated and hate to even ask but at a dead end !

My first inclination would be to say the thermostat is sticking, or installed upside down.

In my experience the situation is usually caused by engine coolant not getting to the thermostat ... either not enough coolant in the engine, or air trapped underneath the stat. Engine heat soaks, it gradually warms up the t-stat ... then it opens.

You sound pretty thorough ... so these causes would seem unlikely to me.

Did you drill a bleed hole in the Tstat? It bleeds air and keeps water running past the spring.

The key part of the T-stat is the brass colored can that faces down into the intake. It contains a paraffin that swells as it heats up, pushes a plunger that overcomes spring pressure and opens the valve.

If the brass can is facing up (upside down) it's in contact with cooler water in the radiator hose and won't open till things get really hot.

Or if for some reason, like 455HOG suggests, you have an air pocket under the T-stat it will "insulate" that paraffin can and then it will have to wait until that air bubble gets hot enough to trigger it ... which takes a lot longer than hot water.

Once the water is flowing .... you are all good.

You didn't by chance block off the bypass between intake and timing cover? Some people like to put a pipe plug in the intake side. Matter of fact Edelbrock high end manifolds are tapped for it.

Thanks for the reply I have installed 2 thermostats second one drilled bleed hole raised frt end when filling so as to not get any air pockets but you are correct it sounds like heat soak is causing it to open. It is installed correctly have done way to many to install incorrectly what would cause the hot coolant not to get to the thermostat ? I ran a long brush through the intake to ck the coolant ports they are not blocked does it get it's flow from the water pump bypass hole ?

No but I did have to reseal it maybe to much silicone ?

Thanks been doing this way to long to know the difference thought it might be bad so I replaced it and drilled a bleed hole in it .

I did also raised frt end when refilling coolant !

Any chance that you have compression getting into the cooling system? Cracked head, leaking head gasket, cracked block. This scenario will make an engine OH in short order depending on how bad the leak is.

The rock hard hose comment could be a symptom of this problem.

With a pressure tester the cooling system should hold pressure for 5-10 minutes without a significant drop.

One sure way to find out if it is t-stat related ... is of course to remove the t-stat.

And you could probably rule out any rpm or air movement issue if it does the "overheat then cool down" behavior while just sitting in a driveway idling.

Have you noticed if the heater works when the engine gets hot on start up?

I see what you mean ... strange problem ... sounds like something crazy like the water flowing the wrong direction :)

No did not block it off I pulled intake this morning to ck for blockages everything looked fine ran a drift into the timing cover and the bypass hole is open for some reason the hot coolant is taking to long to get to the thermostat . Next pull water pump see whats up .

Yes heater is working I installed an electronic control valve as the 72 has none and it uses a blend door for a/c I am trying to see why ft of eng takes longer to get hot .

Heads were maged and decked block tested coolant and it was good. hose getting hard from heat.

That still leaves head gasket condition unknown, a head gasket is the most common source of gas entering the cooling system. Heat is a by product of a gas laden cooling system as gas will not cool any engine with a liquid cooled system. The hottest portion of the engine is where the gas is going to collect to keep the coolant off of the interior of the heads cooling jackets.

Hoses get hard from pressure, not heat. The pressure is controlled by the cap, and can be either from expanding coolant, or compression getting into the cooling system.

After 45 years as a mechanic the first thing I'd be doing is a pressure test to first see if (1) the system holds pressure, and (2) if it builds pressure with the engine running.

You're asking for opinions because you can't find the problem, that's one opinion, discount it if already know the answer.

Possible trapped air pockets working their way out?
Make sure lower hose to timing cover not collapsing.

I have changed many head gaskets on stock engines with just the problem you are speaking of. A small amount of water leaks into a cylinder while the engine is cold and leaves an air pocket under the thermostat causing a long opening time. Once the thermo opens air pocket is gone until the engine sits for another long period of time. Check to see if you are losing a small amount of water out of the engine when it is cold.

Thanks for the reply I had prev done a block gas test and the results were neg also pressurized the system at 13lbs and it held overnight .I am sure the hoses are getting hard fron heat build up pressure once the thermostat finally opens they release and everything is normal takes about 5 minutes of run time . I pulled the manifold yesterday to see if the bypass hole was obstructed it was not . So I guess I will pull the water pump and recheck the plate and tubes only thing left.

Where is your sender located?

As mentioned earlier, replace the thermostat with a gutted unit as an easy way to rule out that issue. It's a few minutes of work.