Engine Cooling System

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The Ultimate Engine Cooling Troubleshooting Guide for Performance Cars and Trucks

So you're having some trouble with engine overheating. It's one of the primary problems you can have when you start modifying an engine to produce more horsepower, but it might be easier and cheaper to fix than you think.

Designed for the do-it-yourselfer or novice/shadetree mechanic, this guide, based on real world experience, will help you troubleshoot the following problems and more:

  • Vehicle seems fine until you drive it fast such as on a track, but then overheats
  • Vehicle overheats at idle
  • Vehicle will idle for hours and make short trips, but overheats on longer trips
  • Vehicle blows off coolant pipes, heater hoses or couplers, seemingly at random
  • Vehicle seems fine for a few minutes, but then the gauge just climbs up slowly until it overheats
  • Vehicle eats coolant

Tools

If you're expecting to see a long list of expensive tools that you need listed here, prepare to be disappointed.

There are dozens of tools on the market designed to troubleshoot overheating issues, ranging from sniffers that seat in your coolant filler to detect exhaust gas, to radiator cap replacements, to radiator flush and fill kits, to pressure pumps that come with 10 types of different radiator caps with test outlets. Well, we have a $5 tool that makes all of them obsolete.

This Coolant System and Head Gasket Pressure Test Valve plugs in to any 1/8" NPT hole in your cooling system. Many cooling systems already have holes this size, sometimes for temperature senders. If yours does not, all you need to do is drill and tap the hole anywhere in the hard pipe of your cooling system where there's sufficient space and wall thickness for the threads (the thermostat housing is a good place to start). You can leave the valve installed long-term, and it will not rust or create a leak in your cooling system, so if you ever need to test in the future, you're ready to go. Even better, if you install it at all of the peaks in your cooling system (the places where there is a line higher than other lines), you can install one of these tools at the high point in the line and bleed out all the air. To make it even easier, check out our Ultimate Cooling System Filler, Tester, and Bleeder. Using this along with the proper adaptors, you can just cut your heater hose and install this, so you don't even have to do any drilling.

Once installed, our test valves work by attaching any air compressor charged to 30 PSI. By pressurizing your cooling system with the car off, you can test for all of the following:

  • Leaks in radiator hoses and heater hoses
  • Leaks in radiator core
  • Problems with the radiator cap either holding too much pressure, or not enough
  • Head gasket problems
  • Air bubbles in cooling system

You'd be surprised what this simple test can turn up. We were having a problem with our shop car for a while where it would be fine on shorter trips, but would overheat on longer trips. When we applied pressure to the cooling system, it shot right past the radiator cap into the overflow tank! The cooling system wouldn't hold any pressure at all, and the cap was only a year old. We replaced the cap, and then the car started blowing off radiator hoses. We pressure tested again, and found that the system held 30 PSI when it was only supposed to hold 13 PSI. The brand new radiator cap we purchased was wrong for the application and would not open at all.

Please check the Coolant System and Head Gasket Pressure Test Valve product description for more details on performing this test.

Vehicle seems fine until you drive it fast such as on a track, but then overheats

Boy have you come to the right place. Once you have determined that your engine cooling system is indeed insufficient for your level of horsepower, there are several ways to solve the problem. The most commonly suggested is to buy a $600+ aftermarket radiator that's time consuming to install, and this might solve the problem. However, we have a couple suggestions that might save you about $550, and add a few horsepower while you're at it.

Think about upgrading all your radiator hoses to our stainless steel radiator hoses. This will add some metal surface area, conducting heat out of your coolant when it's on its way to and from the radiator as well as in the core. They look better, and are less likely to develop leaks. Please check the Stainless Steel Radiator Hoses product description for more details.

Also consider using header wrap, exhaust wrap, and intake thermal wrap. The benefits of these products for engine cooling are huge, because they decrease engine bay temperatures. When you have something that is 2000 degrees about two inches away from something you're trying to keep at 180 degrees, it's just common sense to use some kind of insulation. As an added bonus, you get better exhaust scavenging which will add some HP. Reduced engine bay temperatures combined with our intake thermal wrap can also lead to lower intake temperatures, aiding further in engine cooling and providing still more HP. Exhaust wrap is a controversial product, with some people claiming it decreases the longevity of headers and exhaust parts. This is true in some instances, however we have some suggestions in our exhaust wrap product description that will minimize the issue. If you have a turbo car, also consider adding a turbo blanket for more of the same benefits.

Finally, do you have an oil cooler? Don't underestimate the importance of engine oil coolers. Engine oil coolers can not only prevent your oil from overheating, but because the engine oil system has different passages through the engine than the coolant, using an oil cooler can also promote a more even temperature throughout your engine resulting in less chance of breakage and more power. Because engine oil directly coats many of the moving parts in your engine, experts say that engine oil temperature is even more important than coolant temperature. Engine oil coolers also provide a margin of safety for your oil temperature both by cooling your oil and by increasing your oil capacity and decrease the burden on your coolant system. Think of it this way: If you're going to increase your radiator size and fluid capacity, why not do it with a supplemental oil cooler instead of a replacement radiator?

One side note here is that adding or replacing your radiator fans will not help you in this situation. Unless your car overheats at idle, your radiator fans are fine. You'd be surprised how many people ask this question.

Moving along...

Now we'll get into some other, more mundane, common scenarios of engine overheating in performance cars and trucks.

Before you start looking at this list, you should consider just slapping in a new thermostat and radiator cap. Depending on your vehicle, it might cost as little as $20 and take less than ten minutes, and it will resolve almost half of the common cooling issues below. It means a lot that we're saying this in here, because thermostats and radiator caps are some of the only things in this guide that we don't sell.

While you're at it, it's always a good idea to replace your tired old rubber hoses with our special products. Radiator hoses 1" and larger can be replaced with our Flexible Stainless Steel Radiator Hose for better cooling, looks and longevity. Heater hoses can be replaced with our Reinforced Silicone Heater Hose. This hose is more resistant to nicks and cuts than factory hose, and won't crack as quickly with age. A common scenario with rubber heater hose is that they rip around where the hose clamp tightens, which these are also more resistant to. They also look better and are easier to clean. If your vehicle has over 100,000 miles on stock hoses, you really should replace them even if you go with cheaper rubber hoses. Otherwise you'll be re-visiting this cooling system troubleshooting guide within a year. I hope you installed our Coolant System and Head Gasket Pressure Test Valve!

Vehicle overheats at idle

  • First, check your fans for proper operation. If your vehicle is starting to overheat while idling and the fans don't come on, there's your problem. What you need to do will vary by car. You could get a new $200 fan assembly, clutch, belt, blades, shroud, or some other crazy thing an auto maker came up with years ago. Or you could just take a shortcut which is often better and almost always less expensive than repairing your stock setup. Spend $20-$30 on one of our electric cooling fans and wire it up, and ditch the factory setup. You'll almost always find that this is sufficient for keeping the vehicle cool at idle, while also being simpler and lighter than the stock setup, easier to install, and you might even save a little rotating mass if the engine doesn't have to turn the fan.
  • Fans working? Good. Still having problems? If you haven't already, try replacing your thermostat.
  • Check your coolant level. If it's low, add coolant and see if the problem persists. This is where most people call it quits and wonder why they have the same problem in a month - you're not done yet! After putting a few miles on the car, check the coolant level again. If it's low again, go to "My car eats coolant" below.

Vehicle will idle for hours and make short trips, but overheats on longer trips

  • Your car is almost definitely eating coolant. Check your coolant level. If it's low, add coolant and see if the problem persists. This is where most people call it quits and wonder why they have the same problem in a month - you're not done yet! After putting a few miles on the car, check the coolant level again. If it's low again, go to "My car eats coolant" below.

Vehicle blows off coolant pipes, heater hoses or couplers, seemingly at random

  • If you've got this problem, please do not just keep tightening your hose clamps down. The weakest link in a system is the one that will fail, and if you tighten down all your hoses until they are invincible, your radiator will explode (maybe).
  • You've almost certainly got a problem with your radiator cap. Your radiator cap is designed to relieve cooling system pressure above a certain level. The level is different from vehicle to vehicle, but it's typically 4-18 PSI. Using our Coolant System and Head Gasket Pressure Test Valve, test your cap by pressurizing the system. If it holds 30 PSI, you've got a problem. Replace your radiator cap.
  • Note that all cars have slightly different requirements for what pressure your radiator cap should hold. You should find out what pressure cap your vehicle is supposed to have, because there's always the possibility that the previous owner used the wrong cap and now its your problem. Check out the Coolant System and Head Gasket Pressure Test Valve product description for more details, but here's what should happen when you use the valve to pressurize your cooling system. For example, let's say your system is supposed to hold 13 PSI. If you pressurize the system slowly, the pressure will rise to 13 PSI, and then you'll hear air escaping through the radiator cap (it might go into the reservoir if equipped or into the atmosphere otherwise.) Then the cooling system should hold 13 PSI for at least a minute. If it holds substantially more or less than 13 PSI, you've got a problem. Note that we have found some variation in the radiator caps you buy at auto parts stores. Using their application guide we have purchased 13 PSI caps that hold 16 PSI, and even one that the spring got fully compressed during normal installation and would hold over 30 PSI.

Vehicle seems fine for a few minutes, but then the gauge just climbs up slowly until it overheats

  • If this happens under all circumstances - on the freeway and while parked, and if your vehicle does not eat coolant, it's most likely your thermostat. Your thermostat exists to help your engine warm up faster and maintain a higher operating temperature. It basically bypasses the radiator until the vehicle reaches operating temperature, and then opens by way of magical spring, letting the coolant go through your radiator. The failure mode of a thermostat is to just always stay closed. This of course always bypasses your radiator and leads to overheating. If your symptoms started gradually and got worse and worse, +1 for replace your thermostat, as they tend to fail gradually.
    • What happens if you just take out your thermostat instead of replacing it? It's awesome, that's what. Your engine will run much cooler and produce surprisingly more horsepower. No joke! Unfortunately, doing this causes high emissions, reduced gas mileage, a potential decrease in engine longevity, possibly a check engine light, and reduced ignition timing or other electronic handicaps depending on the vehicle. Some people have experimented with using custom ECU tuning to avoid those handicaps and check engine lights. Only you can decide if it's worth the other issues and risks.
  • It's possible that this overheating issue could be caused by a faulty water pump. However, typically a faulty water pump will cause a coolant leak (see "My Car Eats Coolant" below.) Your water pump circulates water through the cooling system, and exchanges the heat between the engine and the radiator. If your water pump is dead or dying, less heat will be exchanged, and it will build up in the engine causing overheating. Very unfortunately, the only cure here is to replace your water pump, which is expensive and time consuming, without any opportunity to add horsepower in the process.

FAQ

Do I need new fans?

Do I need a larger radiator?

Is my headgasket blown?

Why does my coolant level keep going down?

Do I need to flush/fill my radiator?

Do I need a new water pump?

Do I have air bubbles in my coolant?

Incidentally...

Do not drink coolant. It's worth mentioning, because coolant is delicious. That's why so many cars like to eat it. Once a pet or even a child gets a taste, there's a danger they will want more. And, any amount of coolant can kill you. So don't drink coolant.