Roger60

Have a 72 350 in a street rod. All stock except for a street rod style aluminum radiator. Was ok, now that I am running it more, it gets hot real fast and blows coolant out the overflow tube. Changed the thermostat, no help. Water pump seems ok, not loose or any noise. This is likely simple engine mechanics, but I just cant put my finger on the cause. Found that the temp gauge may not be reading right, and after shut off, it doesn't want to start until it cools. No other leaks. Oil is clean. Thanks.

oldcutlass

Define real fast, what temp is it reaching, and are you possibly over filling the radiator?

Vintage Chief

Replace the radiator cap.

Vintage Chief

The coolant system is designed to operate under pressure - you most likely have none. When the vehicle gets to normal operating temperature, under pressure, the coolant should be circulating. A box of donuts says yours is not circulating under pressure. To test: get engine to operating temperature, squeeze the upper radiator hose - it should be hard with coolant circulating. Yours is probably soft and mushy - no pressure. Instead, you get to operating temperature and the coolant begins to boil over and run out the overflow. Radiator cap pressure valve is shot (rusted, busted, wedged, etc.). Replace with 16lb radiator cap.

Roger60

Thanks. You are all thinking stock system. Plenty of pressure, can't remove the cap when hot. The radiator is more of what you would find in a late 30's early 40's car, tall, with the tanks on top and bottom. Aluminum aftermarket. All new, new cap. As far as how fast, within minutes. As I mentioned, I'm not sure about the temp gauge, its reading about 140. Seems hotter than that. Didn't know you could overfill a radiator. With cap at the top, I wouldn't think I would have an airlock problem. I know the only thing Olds here is the engine.

oldcutlass

If you are filling the radiator all the way to the top your not leaving any head space for the water to expand. When the water expands it muscles past the radiator cap on to the ground. Fill the radiator so the level is about 1-1.5 inches below the neck and see what happens. If you feel the temp gauge is not accurate, repair or replace it.

Vintage Chief

Roger - The fact you cannot remove the radiator cap when the engine reaches normal operating temperature does not necessarily demonstrate coolant is circulating optimally through the system at the correct PSI. I would still perform the simple test I suggested first.

With the radiator cap locked and secured:

(1) When the engine is cold, squeeze the UPPER radiator hose to get a feel for the firmness of the UPPER radiator hose. It should be at this point pretty soft & mushy - no coolant circulating under pressure.
(2) Start the engine. As the engine begins to warm squeeze the UPPER radiator hose. The UPPER radiator hose should begin to become firm under pressure. Continue to allow the engine to warm for ~10-15 minutes at idle. Squeeze the UPPER radiator hose. It should by now be under pressure, coolant circulating and the UPPER radiator hose should be very firm w/ circulating coolant.
(3) If the UPPER radiator hose does not become firm with coolant cycling through the system under pressure, at this point w/ some confidence we should be able to suggest the coolant is not circulating under pressure regardless if it's a bad radiator cap, bad seal at the radiator cap, air entering the coolant system, etc. At least you should be able to confirm if the system is under pressure and cycling coolant. This is a simple test but does provide some insight as to flow of coolant under pressure.

Fail the above simple test, the next step is to use a coolant pressure test hose and test the pressure of the coolant system.

Roger60

Thanks, I'll try that. Here are some pics of the radiator setup if it helps. My feeling is too many mismatched parts in revolt!

Vintage Chief

What is the stated PSI of the radiator cap? Remove the radiator cap after you perform or before you perform the simple test and ensure the radiator cap is constructed with what should appear to be a vacuum valve & pressure spring (at a minimum).

Bubba68CS

Does anyone know if the stock water pump can overcome that much pressure head? That is quite a lot higher of a radiator entry point than normal, so figured I’d toss it out there...

Vintage Chief

That's a good point. I do believe the first order of business should be determining if the system is, in fact, operating under any pressure via the simple test. Which might lead to the more definitive test of determining (with a gauge) what the true operating PSI is of the current system.

Bubba68CS

Completely agree. Start with the easy stuff. Eliminate that before making drastic changes.

dragline

Sometimes the inlet hose to the water pump sucks shut while revving the engine, sometimes there is no spring inside to keep it from collapsing.

olds 307 and 403

Are those billet pulleys, under driven, very small crank pulley where round head fasteners are used because normal bolts are too big to fit? I used them, changed to a more normal size March pulley, CVF pulley, available in 2 and 3 groove. https://www.cvfracing.com/oldsmobile...aft-pulley-2v/ https://www.cvfracing.com/oldsmobile...aft-pulley-3v/If so that is definitely part of your problem. Basic replacement thermostat? How many cores and what diameter in the rad? How much air does the fan pull? It may not be enough and a full shroud with rubber strips to direct air may be a good idea. I am adding a better shape under crossmember deflector and Grand National radiator surrounding flaps on my 88 Cutlass has always ran hotter than it should.