As a passionate auto DIY mechanic, one of the things I’ve noticed, over and over again is that the biggest auto issues always start small – and only when ignored do they fly over and above board. I’ve also noticed that if you miss little things like bad coolant cap symptoms, you’ll sooner than later, pay the price – which is usually overheating.
Granted, this escaped me for a while when I shared my article on signs of a bad radiator to watch out for.
However, since then, I’ve been thinking: what happens when you have a great radiator but somehow, the cap is faulty?
The answer, curiously, stares us all straight in the face. First things first though:
Article Outline
What Does A Radiator Cap Do?
A vehicle’s cooling system is a pressurized unit – which is regulated mainly by the vehicle’s radiator cap.
Closed properly and fitted in the right manner, the pressure a vehicle’s cooling system can bear helps it starve off overheating and generally, increases the boiling point of the coolant (or water) in such a system.
But, how does this principle work, in reality? It all starts with the pressure rating of the radiator cap.
Radiator Cap Pressure Rating
On the face of every radiator cap, there’s a number, 0.9, 1.0, 1.1 in that fashion. That number, usually in bar, is the total pressure that the radiator cap can help build – and hold – in the cooling system of that vehicle.
The higher this number, the hotter the coolant (or water) in such a system can go, without boiling over.
Theoretically, you’d be forgiven if you imagine that you can easily slam on the biggest rated radiator cap on your radiator and ensure the benefits of a higher pressure system, including the ability to starve off overheating.
In reality, however, cooling systems are generally built to be able to withstand pressure up to a certain point and this differs from vehicle to vehicle; with smaller engines generally having lower points while larger systems with heavier requirements generally requiring larger pressures, denoted by larger radiator cap numbers.
Assuming you ignore this guide and decide to slam the largest radiator cap on the smallest cooling system you can find, one thing will happen: the pressure in the system will rise beyond what the system can generally tolerate and it will give up at its weakest point(s), which is usually the radiator hoses.
However, in my years as a DIY mechanic and automotive writer, I’ve seen high pressures also result in bad coolant cap symptoms such as a compromise in the relief valve of the cap itself, causing a pressure drop and eventually, overheating.
This takes everyone back to the very starting point; the point where the ultimate end was the avoidance of overheating in the first place.
Bad Radiator Cap Symptoms
There are many faulty radiator cap symptoms. However, most of the signs are simply a duplication of others.
Carefully outlined below are the four signs that you should watch out for and take very seriously when/if they occur.
Here, let me break things down for you.
1. Compromised (Physical) Integrity
A radiator cap is majorly made up of the top seal, pressure spring, bottom seal, and pressure relief valve. These components aren’t simply fixed on the unit for the fun of it: they all serve distinct and important functions.
Unfortunately, due to the taxing job the radiator cap performs, eventually, one of the components fails, sooner (or later). In my experience, the first part that usually gives way is the vacuum relief valve.
When this happens, the protruded extension from the radiator cap vanishes, and overheating logically takes center stage.
This brings you back to the very point you were eager to avoid.
2. Messed Up Coolant
Generally, messed up coolant is a sign of a bad radiator (or radiator hoses). However, it’s also a bad radiator cap symptom too.
Coolant, generally, has no business being outside of the cooling system: it travels through the radiator hoses and cools the engine block, returning to the radiator itself to be cooled after the cycle is complete.
However, when the radiator cap goes bad, coolant finds its way out of the conventional travel routes that it uses, to the fan shroud and general engine bay.
If you ever find a coolant leak outside of your cooling system (especially if the leak happens around the radiator cap), then, you most likely have a faulty radiator cap.
Note however that when this happens, sometimes, the issue may not be a bad radiator cap necessarily but rather, a unit that doesn’t fit (or isn’t designed for your vehicle).
If this is the case, kindly get the right cap and the issue will be resolved.
3. Vanishing Coolant (In the Reservoir)
The coolant reservoir is a simple but amazingly efficient unit of the cooling system. Because it isn’t pressurized directly, it can usually be safely opened even when the vehicle is overheating, and opening the radiator directly is unsafe.
Besides the above, it is where coolant is topped into the radiator, when/if the need arises. From the reservoir, coolant is drawn into the cooling system during normal engine operation when there’s a need for it.
During sustained RPMs too, the coolant reservoir takes the overflow that happens as a result of the heating and expansion of the coolant; this gets sucked back in when it cools and contracts, hence the alternative name, ‘overflow’.
This normal operation cycle does not reduce the coolant level permanently as when it cools, it returns back to normal levels.
However, if for any reason the coolant vanishes from the reservoir and fails to return back even when the engine cools down, you’re most likely dealing with a compromised radiator cap as this is a classic symptom (of such a failure).
4. General Overheating
The cooling system of an internal combustion engine works on the principle of a closed loop.
By closed loop, coolant is maintained in the unit primarily through the radiator cap, acting as a ‘stopper’ or ‘guide’.
This stopper or guide also has the important function of keeping the system pressurized, to avoid overheating.
If you’re experiencing general engine overheating and can’t figure out what could have caused it, great chances are that you’re dealing with a compromised radiator cap.
Or, you’re dealing with a radiator cap that is rated below the pressure level that your engine ought to operate.
Either way, the best-known solution is to change the radiator cap (to a known good unit or one properly rated).
Wrapping up On Bad Coolant Cap Symptoms
The radiator cap of an internal combustion engine is one of the most important components of the cooling system. It seals the system and ensures it remains pressurized – as it should be.
When it goes bad, however, overheating, coolant leaks and all things inconvenient happen; things that delay your movement and make your day bad.
Luckily, isolating a bad radiator cap is easy as it gives itself out to easy diagnosis by compromised physical integrity, leaky coolant, and general overheating, traceable to the radiator cap.
Love the Above? Subscribe for More
Throttle Lan
DIY Mechanic and Freelance Auto Writer