Bad Ass Cars

My early Mustang runs hot, Why?

My early Mustang runs hot, Why?

Everyone seems to think that 289s and 302s run hot for some reason, or that somehow early Mustang engines have a cooling issue in the engine, and that's simply not the case. The fact is; early Mustangs (64 1/2 - 68), as well as Falcons, etc, have a design flaw with the radiators until 1968. In 1969 Ford figured it out and fixed it, which is why you see every Mustang parts catalog offer 3 and 4 row radiators for 65 - 68 Mustangs but none really for later model cars from 69 and up, or for 5.0L cars, because they don't need it.

The 65-68 radiators have the inlet and the outlet right over the top of each other (on the passenger side), which causes the radiator to not use most of it on the driver's side. Water simply follows the path of least resistance into the inlet, and then straight down the tubes to the outlet which only utilizes about 1/3rd of the cooling rows of the radiator. I outlined in red about how much of the radiator gets used in the early designs to show why there is an issue.

early mustang overheating issues

Ever see a 5.0 Mustang have overheating problems? Nope. It's because they used the same design as the 1969 and up radiators with staggered outlets on the radiator and NOT the 65 - 68 models where the inlet is right over the top of the outlet. You pretty much never see 3 or 4 row radiators advertised for 5.0L Mustangs or '69 and up cars because they fixed the design flaw by making the water pump with the hose on the driver's side and a radiator with the outlets staggered, which forces water to go through most of the radiator, rather than just through a narrow portion of it.

It isn't about how many "rows" the radiator has. You can make it 10 rows if you wanted, which would actually cause it to run hot because the air picks-up heat as it makes its way through the rows and fins, and by the time it gets through all of those rows, it's hot and isn't able to wick away any more heat. This is exactly why lots of guys who install 3 or 4 row radiators in their 65-68 Mustangs still have overheating issues on hot days, but again, do you ever see any later model Mustangs having that problem? Nope, and they are ALL the same blocks when talking about 289 or 5.0L equipped Mustangs. In other words; all of the cylinder wall thicknesses, water passages, etc are the same from a 289 to the 5.0L. It's NOT the engine's fault it's running hot.

It's also not about the width of the radiator. Not when you are talking about the flawed design the early radiators have. It can be 10 feet wide and it still won't cool the engine any better because again, water takes the path of least resistance. There's no reason for the water to go into the radiator and then make a sudden 90 degree left hand turn and flow over to the other side when it wants to flow straight down the first 8 or 10 inches of rows. In fact, if you had a radiator several feet wide and you took an infra red gun to it at running temperature, the passenger side (where the inlet and outlet are on top of each other) will be hot, but the driver's side (away from those outlets, where the water isn't flowing) will be cool because it isn't receiving any of the hot water TO cool it. So guys who try to install wider 67 and 68 radiators in their 65 & 66 Mustangs aren't doing anything to cool the car any better if they didn't go with a radiator that has staggered outlets. It's also why guys with 67 & 68 Mustangs also have cooling issues even though their radiators are wider. Same exact engine... bigger radiator... still has cooling issues. It ain't the engine... it's the flawed design radiator.

Installing a staggered outlet radiator and switching water pumps to a 69 and up with the hose outlet on the driver's side moves your lower radiator hose over to the driver's side which forces the water to go into the radiator on the passenger side, and then flow diagonally across the radiator to the lower driver's side outlet which utilizes the whole radiator and in turn cools the engine MUCH better. Keep in mind, you'll also need the 69 and up pulleys and brackets to complete this job. 

65 mustang overheating problems


The image to the right shows a typical 68 and earlier water pump with the hose inlet on the passenger side.

66 mustang overheating problems


This image shows what a 69 and up water pump looks like with the hose inlet on the driver's side. By simply swapping your early pump to a 69 and up pump with the 69 and up pulleys and brackets, and then installing a radiator with staggered outlets, you'll eliminate the bad design of the early systems and will eliminate your overheating problems once and for all.

These pumps bolt right onto your existing timing cover (unless you have the early 65 and older cover where the cover is actually the back side of the pump. It's easy to tell. If your pump has a backing plate on it and you cannot see the impeller just sitting out in the open, then you have the correct timing cover to use a 69 and up pump with. If yours is the early type, then you will need to swap-out your cover to a 66 and up which you can buy pretty much anywhere, from Summit to Jeg's, or even your local parts store if they carry Dorman Products.

67 mustang overheating problems

Be warned though;

1) You don't have much room over there on the driver's side with the fuel pump being right there, but everything will fit. We do it all the time, plus keep in mind; ALL 69 and up cars have the fuel pumps over there as well as the lower radiator hose.  

2) You will have a hard time seeing your original timing mark, so what I would recommend BEFORE doing this swap is to bring your timing mark to TDC and then make a mark on the passenger side of the harmonic balancer using something like fingernail polish (which is real durable and sticks REAL good) where you can see it better. Now, some early covers may need some modifying, such as possibly having to cut the cast-in timing pointer down to allow clearance for the pump, but again, you can relocate one to the other side of the cover pretty easily. 

3) If you are adding aftermarket pulleys to your engine, this is a perfect time to do it because the pulley depth is slightly different on 69 and up engines than the earlier ones, but this is a simple fix with either factory pulleys & brackets from a 69 and later engine, OR by adding aftermarket pulleys & brackets for 69 and up engines. This is all minor stuff for getting rid of your overheating issues. It's not brain surgery and if you want to be able to drive your car on hot days and not be afraid of overheating, this IS the absolute remedy for it.

Another way to do this for those who have a 65 or earlier engine who want to keep their early type water pump and timing cover can still use a staggered outlet radiator by simply using a 180 degree bent crossover pipe that routes the passenger side lower hose over to the driver's side of the radiator. Any muffler shop can make this pipe for you for a few bucks, and / or some of the bigger Mustang supply houses used to offer a kit for doping this, like what you see in the image to the right. Your overheating and running-hot woes will be over once and for all.

** Note: Evidently the 180 degree crossover pipe kits are as hard as chicken teeth to find these days but not to worry because any guy at any decent muffler shop can make you the pipe you need for a few measly bucks. Again, this ain't brain surgery, and if you want the end-all fix to your early Mustang overheating issues, this is what you need to do. 

For lots more free tips on why your engine may be over heating or running hot, and what to look for and how to fix it, whether it's a Ford, Chevy, Chrysler, or whatever, click HERE

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