Guide To Diagnosing A Damaged Radiator

Cars have come a long way from their roots as steam-powered horse carriages, but the internal combustion engine will always have an Achilles heel; overheating. It doesn’t matter how old your car is, all cars are susceptible to overheating, especially out on the race track where they can run at wide-open throttle for a long, long time. Nobody likes to see that temperature gauge start creeping up, as it could indicate any number of engine issues from a leaky radiator to a blown head gasket.

For such a simple-yet-integral part of an engine, there seems to be a million and one things that can go wrong with a radiator.

A good mechanic always starts by checking the simple stuff first, so for the purposes of this article we will focus on diagnosing a busted radiator. Much of what we discuss here can be applied to a broad spectrum of motorsports, as well as non-racing endeavors. When it comes to pushing an engine to its absolute limit though, there are few events as demanding or brutal as circle track racing.

The Common Cause Of Corrosion Is You

The obvious first step to diagnosing a busted radiator is a visual check, inspecting for leaky hoses, loose fittings, and so on. Sometimes finding a leak requires running the engine; other times it can be done by just looking under the car. Checking and tightening all the connections is a great first step, but a leaky radiator is usually just a symptom of a deeper problem, and diagnosing why your radiator is leaking is a job best left to the experts.

That’s why we turned to Jason Danley of Speedway Motors to fill us in on some of the most common issues when it comes to diagnosing a damaged radiator. “The biggest things with radiators are internal corrosion and leaks, damaged fins and a clogged radiator,” says Jason.

Radiator damage can take on many forms, though the two most obvious issues are corrosion and bent fins.

“On passenger cars the biggest issue with radiators is that people don’t change their coolant enough, or at all. What it will do is actually corrode the inside of the radiator, blocking passages and restricting the water, preventing the radiator from passing water through the passages to cool it. I’ve actually seen that corrosion break loose from the radiator and spread to the water pump, intakes, and even the engine block,” says Jason.

If the corrosion goes unchecked, it can even eat away at parts of the radiator and cause small holes that will only get bigger over time. These small holes can be difficult to find via visual inspection, so Jason recommends using a pressure checker. “Auto parts usually have these, and you just put it in the place of a radiator cap and make sure your cooling system is holding the pressure it is designed for,” says Jason. “The pressure depends on the car and the radiator manufacturer, though it is common for radiators to hold between 10 and 12 PSI.” Again though, that number depends on the specific radiator.

Depending on the age of your car and the condition of the radiator, corrosion may or may not necessitate a replacement radiator. If you want to keep your radiator in good condition though, the simple thing to do is replace the coolant every now and then. “The general rule of thumb is to change it every two years at least,” explains Jason. “If you’re putting a lot of miles on whatever you’re driving, you’ll want to do it more often.”

If you do find yourself with a leaky radiator caused by corrosion, you may consider using one of the many magical fluids claiming to stop leaks. “Those will work on pinholes on a radiator, or something like that,” says Jason. “The big thing with that is every time you change the fluid, you have are going to lose that stop leak, so you have to reapply it. And if you use too much, it cold plug up the system, preventing water from flowing through the radiator.” As we’ll see later, it is also important to ensure that you’re not causing a chemical reaction with whatever additive you put into your radiator.

Bent and busted fins like these can causing serious cooling issues.

The Funny Thing About Bent Fins…

Another common problem, especially among home mechanics (yours truly included) is the issue of bent fins. Radiator design hasn’t changed much in the past 100 years, save for switching the tanks from a top-to-bottom setup to a crossflow (side-to-side) setup. In all radiators, hot water from the engine flows into the radiator, and then passes through a number of narrow tubes with fins on either side. Air flows over these fins (either from forward motion or pulled through by the fan) which dramatically increases the ability of a radiator to shed heat, allowing the water inside to drop as much as 30 degrees before going back into the engine block.

As important as these fins are, they are also extremely delicate, and sometimes us home mechanics have the bad habit of banging and shoving parts like the radiator into place. This can bend and damage the fins, which is never a good thing. “Any time you bend that fin, it will prevent the air from going through the radiator,” says Jason. “This will in turn effect how it can cool. Small rocks can bend fins, or if you hit a bird or debris on the road. If you’re into driving off-road, mud or dirt can get into those fins too, blocking the air the same as a bent fin would.” Jason recommends giving your radiator the occasional blast of water to blow out debris that collects over time.

A close-up of what bent fins look like. While a few bent fins are nothing to worry about, anything larger than the size of a baseball could dramatically reduce the ability of a radiator to shed heat.


AFCO Dual-Pass Radiator

To compliment our crate engine in our project car, we ripped out the stock radiator that was consistently failing us and outfitted it with a brand new dual-stage AFCO radiator. Beyond cooling though, what are the advantages of upgrading a radiator?

  • 100% TIG welded with no epoxy.
  • All Aluminum design allows us to take a little unneeded weight off the nose of the car.
  • The dual-pass allows water to stay in the radiator longer for additional cooling.
  • 100% pressure tested before it leaves the factory.

















If you have bent fins though, don’t get too worried just yet; a few bent fins here or there isn’t the end of the world, or even your radiator. “If you have some decent-sized areas, like baseball-sized or larger, that could greatly affect the way the radiator cools,” says Jason. Even if you have a large area of bent fins, chances are your radiator can still be saved so long as the cooling tubes themselves aren’t pinched. “Most auto parts stores sell or loan what they call a radiator comb, which is used to straighten the fins back out. Unless the fins are broken, you should be able to straighten them out.

Digging Deeper Into The Cooling System

Dirty and corrosive coolant, loose hoses, and bent fins are common causes of an overheating engine, but sometimes the problem is deeper than the radiator itself. In maintaining our “Fix the simple stuff first” theme, it’s time to start looking outside of the radiator for overheating issues. “You have to make sure that the water is flowing through the engine,” explains Jason. “Stuck thermostats will impede that flow, as well as a blockage a radiator hose. It could also be a sign of a broken water pump.”

“A lot of cars these days are equipped with electric fans. A pretty good sign that you have a fan problem is if your car overheats while you’re sitting on the grid or under caution, but while you’re moving the temperature stays down,” says Jason.

If you’ve gotten this far, but are still having cooling problems, the issue could be one of the most dreaded engine malfunctions known to mechanics the world over; a blown headgasket. For the average passenger car, a blown headgasket is a problem that can be ignored for a quite a while. For serious racers though a blown headgasket could mean a whole lot of damage to the engine, including the radiator.

Replacing a head gasket is a lot more involved than replacing a radiator, or thermostat, and so you want to be sure that that is really the problem. “If you do a pressure test on the radiator, and it doesn’t hold pressure, that could be a sign of a blown head gasket,” explains Jason. “If you’re losing coolant but don’t know where its going, you could have a blown head gasket. But you’re going to want to run further tests, like a cylinder leak down test first. If you’ve got coolant in your oil, it is a pretty good sign you have a head gasket.”

The Football Blowout Issue

But what if you check to ensure the radiator isn’t leaking, the thermostat and water pump is working, and you fix the broken head gasket, but the system is still not cooling right? That blown head gasket may have damaged your radiator more than you know. As Eric Saffell over at AFCO Racing explains to us, excessive internal pressure can be a very big problem. “The case I want to talk about is a racer who sent us back his radiator due to a cooling issue,” says Eric. “Naturally you want to make sure that the thermostat and water pump are working, and that the radiator itself isn’t leaking. This case that all checked out.”

The left side of this radiator is what a core should like; the right side has been “blown up” by excessive internal pressure caused by a leaking head gasket.

However, if an engine loses a head gasket, all of that compression (in the cylinders) has to go somewhere,” Eric explains. “If the engine is leaking on the compression stroke, that compression is leaking into the water cooling system. The capillary tubes, which are wide and very flat, can be deformed by this pressure, taking on an almost football shape, pointed on the ends by rather thick in the middle.”

“What happens,” says Eric. “Is that the pressure essentially crushes the fins, restricting airflow through the radiator. In that instance, you’ll have a particular section of the radiator, or in a worst-case scenario, almost the entire core of the radiator, and you’ll see all the fins look like they’ve been smashed. A quick check for this is to drain the radiator of all the fluids, take it out of the car, and hold it up to the light. If you can’t see look through the radiator, you want to do a more thorough visual inspection.”

“In this case though, you could see that the whole core itself was swollen in a football pattern. The common thinking is if a radiator isn’t leaking, it’s fine,” says Eric. “We’ve probably seen this more in circle track applications, but with power adders becoming more commonplace in drag racing we’re likely to see this happen more often there as well. You know when guys lift the head, they don’t think they’re damaging the radiator or cooling system, but in this case a lot of damage was done, and the only solution was to replace the radiator completely. If you had a pressure checker in the cooling system, you’d definitely see a spike in pressure in the cooling system in cars where this happened.”

Chemistry In Your Coolant

As of late, Eric has also seen the increasing use of cooling additives in the radiator. Back in the day, a lot of radiators were made from brass or copper, but today radiators are more and more made from aluminum. These metals all have different properties, and mixing and matching can be corrosive. “Sometimes you’ll have guys who find an old bottle of radiator additive and they’ll add it without thinking, causing a chemical reaction within the radiator that can eat away at the internals. It will eventually find a place to create a pinhole or a series of pinholes.” To avoid this, just make sure whatever stop leak or cooling additive you use is designed for an aluminum radiator (if that is what you’re using…which you probably are.)

Corrosion is a silent killer, and can be caused by anything from old, dirty coolant to improper additives, or even a poorly-grounded ignition system. Bent fins don’t help much either.

Another problem? Electrolysis. “When the ignition system isn’t properly grounded, the electrical current can run through the chassis and through the radiator,” says Eric. “Electrolysis can cause rapid corrosion through an electrochemical reaction in aluminum radiators. You can actually measure the electrical current in the chassis and the radiator.” The easiest way to check for electrolysis is to hook up a test light to your radiator while turning on the car and making sure you ground your ignition system properly.

For such a simple-yet-integral part of an engine, there seems to be a million and one things that can go wrong with them. But if you start off your diagnosis with the simple stuff, chances are you can save yourself a whole lot of time and frustration. Sometimes though a leaky radiator is a symptom, rather than a cause. This guide should help you get through some of the agony and irritation of fixing an overheating problem, and hopefully next time your radiator springs a leak you’ll be better prepared get it back to working order. And big thanks to Jason Danley and and Eric Saffell for helping us put this article together.

About the author

Chris Demorro

Christopher DeMorro is a freelance writer and journalist from Connecticut with two passions in life; writing and anything with an engine.
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