There are a few things to look at, whenever it comes to cooling!
1) Was the cooling system clear full of fluid to begin with? With the original radiator, every inch of fins counts. Make sure it's all the way to the neck. It will probably burp a little out when it warms up, but that's just the way it is if you don't have a reservoir (and they didn't come with one from the factory.) Adding a catch can will reduce how often you have to check and see if your coolant is full, but it's not a big deal.
2) Is your timing set right? Too much timing will create higher-than-normal combustion temps, and can cause this. Oddly, running too retarded can also cause overheating, because of all the still-burning fuel going down your exhaust manifolds, and because your engine will be less efficient, requiring you to burn a lot more air and fuel to move down the road.
3) I can see that you have a shroud, but what kind of fan are you running? If it's a flex fan, those things do a lousy job. They are supposed to flatten out and conserve power at high RPMs. However, due to poor design, they mostly just make noise, thrash the air around, and steal power at all RPMs while doing a worse job than most stock fixed-blade fans. Doesn't sound true, but it really is.
4) How old is your radiator cap? Each pound of pressure in your cooling system will allow the temp to go higher without boiling. When they're old, sometimes they just won't hold any pressure at all, and your car will boil over at fairly low temps. This is a super cheap and easy fix, many times
5) Are your radiator hoses in good shape? It's common for the lower one to get so soft it collapses when you rev the engine. It goes without saying that this is detrimental to engine cooling.
and 6) Yep, could need to be flushed. Don't bother with a chemical flush at your local lube shop. Take it to a nearby actual radiator guy. They can hot tank your radiator, flush your block, and make sure it's going to be a lot more efficient.
A couple of other things to think about:
Don't put in a lower temp thermostat. Thermostats don't change the total amount of maximum cooling - they just determine what the max temp SHOULD be. It's your radiator and water pump that do all the work. If it's overheating significantly, whether your thermostat opens at 190 or 90 makes no difference except for how soon coolant starts to flow easily through the radiator. A low-temp thermostat will not let your oil get hot enough to boil away contaminants in the oil when weather gets cooler, and will cause sludge, rust, and bad things to happen in your engine.
If you added air conditioning, you may be overwhelming the factory radiator. Stock, the AC cars had a bigger radiator to begin with.
Depending on where you live, and the overall health of your old radiator, you might need a new one. The factory 18" radiator does pretty good, actually, and I drove mine (3 core brass) all over California in 117 degree heat. If yours needs to be replaced, I would recommend a factory 3 core. If you need to, you could go with an aluminum 2 core. As the tubes are wider on aluminum radiators, going to 3 core often introduces fitment problems. As air passes through so much surface area, it heats up to the point where the extra-thick radiators (4 core brass or 3 core aluminum) typically get very little benefit from the extra tubes. It's also harder for your fan to force air through them, which can cause problems while idling.
The very best radiator for these cars is probably the '69-70 24" big-block radiator, shroud, and thermo clutch fan, but as yours is an early Mustang, fitment would not be a simple matter! Hopefully you can just make your original one work fine.
Best wishes!
1) Was the cooling system clear full of fluid to begin with? With the original radiator, every inch of fins counts. Make sure it's all the way to the neck. It will probably burp a little out when it warms up, but that's just the way it is if you don't have a reservoir (and they didn't come with one from the factory.) Adding a catch can will reduce how often you have to check and see if your coolant is full, but it's not a big deal.
2) Is your timing set right? Too much timing will create higher-than-normal combustion temps, and can cause this. Oddly, running too retarded can also cause overheating, because of all the still-burning fuel going down your exhaust manifolds, and because your engine will be less efficient, requiring you to burn a lot more air and fuel to move down the road.
3) I can see that you have a shroud, but what kind of fan are you running? If it's a flex fan, those things do a lousy job. They are supposed to flatten out and conserve power at high RPMs. However, due to poor design, they mostly just make noise, thrash the air around, and steal power at all RPMs while doing a worse job than most stock fixed-blade fans. Doesn't sound true, but it really is.
4) How old is your radiator cap? Each pound of pressure in your cooling system will allow the temp to go higher without boiling. When they're old, sometimes they just won't hold any pressure at all, and your car will boil over at fairly low temps. This is a super cheap and easy fix, many times
5) Are your radiator hoses in good shape? It's common for the lower one to get so soft it collapses when you rev the engine. It goes without saying that this is detrimental to engine cooling.
and 6) Yep, could need to be flushed. Don't bother with a chemical flush at your local lube shop. Take it to a nearby actual radiator guy. They can hot tank your radiator, flush your block, and make sure it's going to be a lot more efficient.
A couple of other things to think about:
Don't put in a lower temp thermostat. Thermostats don't change the total amount of maximum cooling - they just determine what the max temp SHOULD be. It's your radiator and water pump that do all the work. If it's overheating significantly, whether your thermostat opens at 190 or 90 makes no difference except for how soon coolant starts to flow easily through the radiator. A low-temp thermostat will not let your oil get hot enough to boil away contaminants in the oil when weather gets cooler, and will cause sludge, rust, and bad things to happen in your engine.
If you added air conditioning, you may be overwhelming the factory radiator. Stock, the AC cars had a bigger radiator to begin with.
Depending on where you live, and the overall health of your old radiator, you might need a new one. The factory 18" radiator does pretty good, actually, and I drove mine (3 core brass) all over California in 117 degree heat. If yours needs to be replaced, I would recommend a factory 3 core. If you need to, you could go with an aluminum 2 core. As the tubes are wider on aluminum radiators, going to 3 core often introduces fitment problems. As air passes through so much surface area, it heats up to the point where the extra-thick radiators (4 core brass or 3 core aluminum) typically get very little benefit from the extra tubes. It's also harder for your fan to force air through them, which can cause problems while idling.
The very best radiator for these cars is probably the '69-70 24" big-block radiator, shroud, and thermo clutch fan, but as yours is an early Mustang, fitment would not be a simple matter! Hopefully you can just make your original one work fine.
Best wishes!
