Why Do Tractors Blow Water Up and Out of Radiators? [How to Fix It Easily]

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Coolant blowing out of your tractor’s reservoir is usually not ideal! So we are here to teach why tractors blow water up out of the radiator. And better yet, we make some helpful and beginner-friendly repair suggestions.

Two farmers working on a broken tractor.

Don’t let a loss of coolant damage your tractor’s engine!

Instead, follow these tips.

What Causes Coolant to Come Out of the Reservoir?

Generally, coolant gets forced out of the reservoir by adding something. One of the most frequent culprits is exhaust gas creeping into the radiator by combustion pressure. Combustion pressure usually comes from one of the two following sources.

1. Cracked Head

A crack in the head allows combustion pressure to be forced into the water jacket, adding pressure to the system and pushing the coolant out of the reservoir.

2. Blown Head Gasket

A blown head gasket acts pretty much the same as a cracked head. It just costs much less to repair.

A relatively inexpensive block tester will tell you if exhaust gasses are present in your radiator coolant. Knowing for sure helps you know where to look.

Both a cracked head and blown head gasket are usually caused by overheating.

Ford Tractor Coolant Blowout | Tractor Leaking Coolant
Why do tractors blow water up out of the radiator? It depends. And working with old farm equipment can surprise you! For example – we studied this video from Yanasa TV about a tractor coolant blowout. The culprit? It was a loose fan that kept hitting the radiator hose regularly! The result impacted the coolant flow and made it look like the overflow tube broke. The broken fan was a surprisingly easy fix. But it revealed a host of other water pump and coolant tube issues! (Keeping the radiator clean was the least of their worries.)

Will the Radiator Pull Coolant From the Reservoir?

Radiator cap designs allow coolant to escape into the overflow reservoir under higher pressures. They will also allow coolant to be sucked into the radiator when needed. If you are unsure of the cause of your problem, change the radiator cap first. Easy and inexpensive!

Make sure you also check the hose for holes and cracks. Doing so keeps your radiator from sucking air.

This only works if your tractor has an overflow bottle. A lot of older systems don’t have an overflow bottle – and as a result, they don’t return. They do not pull coolant from the reservoir.

What Causes Excessive Pressure In a Radiator?

Cracked heads, blown head gaskets, and overheating can cause excessive pressure in the cooling system. The wrong radiator cap can also cause an overheating issue by not holding pressure, letting excessive coolant out. Also, not holding pressure lowers the boiling temperature.

(Check your owner’s manual to ensure you have the correct part.)

What Happens If There Is Too Much Coolant In the Reservoir?

Nothing much. Except for a puddle on the ground or floor. All reservoirs get built with an overflow. It’s a low-tech safety system.

What Causes Reservoir Coolant to Boil and Overflow?

If you have excess heat and your cooling system can’t keep up – there are a number of causes:

  • Cracked head
  • blocked radiator
  • Loose fan belt
  • No radiator shroud
  • Stuck thermostat
  • Non-sealing radiator cap
  • Etc.

When the water gets too hot in the cooling system, it expands. It has to go somewhere – it goes out through the overflow.

A radiator cap that is not sealing or holding pressure can cause water to boil at a lower temperature. Instead of holding water in under pressure, it lets it get pushed out of the overflow. You need the right amount of pressure in the radiator to keep the coolant in, and to prevent it from boiling too easily.

removing and checking the radiator cap on a garden tractor for annual maintenance
If your tractor blows water out of the radiator, the first thing to check is the radiator cap. Coolant leaks are also alarming and potential tractor head gasket symptoms. A new radiator cap could fix the issue if you’re lucky. But if your tractor engine still spits water through the overflow pipe when cooled and with a reliable radiator cap, then you may suspect a broken or cracked head gasket. (Yikes.)

What Happens When Air Gets Into the Cooling System?

Air in your cooling system can be the cause of your engine overheating. Generally, you only get air when you’re changing the coolant.

Here’s a tip to keep air out of your system when changing coolant or topping it off. Run the engine without the cap. As the coolant warms up, the thermostat will open up, allowing the coolant level to drop.

When that happens, fill your radiator to the top and replace the cap. Make sure that the overflow reservoir gets filled to the line.

Some engines have an air bleeding valve. It’s usually somewhere near the top of the cooling system. You may need to look in the owner’s manual to see if there is one on your tractor–and where you can find it.

Personal Tip About Cold Coolants – and Hot Radiators

Use caution when adding cold coolant to a hot radiator! If you pour it in really cold into a hot engine, you can get too big a temperature difference, which can potentially crack the head.

If you are doing this, pour it in slowly.

What Are the Signs of a Blown Head Gasket?

One of the sure signs of a blown head gasket is oil in the radiator or overflow reservoir. It usually appears as a brown mayonnaise-type substance. This goop develops via the high-speed blending of oil and water.

You may also see the same mayonnaise-type stuff in your engine oil. Quite often, you will not see the mayonnaise-type stuff on your dipstick. Pulling off the valve cover will expose some of the remnants of the goop – letting you know that you have a head gasket problem.

A blown head gasket allows a build-up of exhaust pressure in the radiator, which may cause the top to blow. The excess pressure can also crack and break plastic reservoirs. Too much pressure and old, weak hoses or fittings don’t go well together.

Another possible problem caused by a blown head gasket is water getting drawn into one or more combustion chambers. Small amounts of water will cause whiter exhaust smoke – often lots of it. Too much water in one or more cylinders can cause a hydraulic lock.

What If Your Tractor Engine Hydro-Locks?

Normally, in your cylinder, you have air and fuel. These get compressed by the piston coming up on the compression stroke.

When you have water in your cylinder, water does not compress. But the piston keeps coming up regardless. It tries to compress it, but cannot. Something’s gotta give. Usually, the outcome is a bent conrod – the weakest link.

Hydro-lock is bad.

old farmyard tractor kept in immaculate condition
Even an old garden tractor or diesel tractor can run like a champ if you perform routine maintenance. We found an excellent daily tractor inspection guide on the Michigan State University website with an easy daily maintenance routine. It’s a must-read if you want your tractor to continue working for years! They advise starting each day by checking your tractor’s fluid – including engine oil, coolant, fuel, and hydraulic fluid. (We also love this guide entitled Keeping Your Tractor Running from the Cornell Small Farms program. They advise checking fluid levels daily – and remind you that regular maintenance can help prevent costly repairs!)

What Can Happen If Radiator Cap Pressure Is Too High?

Radiator caps, like gas caps, get no respect. Something goes wrong, and they are usually one of the last items considered. Yet a radiator cap that creates or allows high pressure can be the root cause of many problems.

  • The high pressure can blow plastic tanks apart.
  • It can cause hose leaks.
  • High pressure can burst old, weak, or rotten pipes.
  • High cooling system pressure can also cause leaks in the heater core under your dash.

Read More!

What Causes My Radiator Coolant to Get Pushed Out When I Start The Engine Without a Radiator Cap?

Some engines will push some coolant out when you run the engine without a radiator cap. The cap’s there to keep it in, and hold a small amount of pressure to raise the boiling temperature.

If your motor pushes coolant out when you start the engine without a radiator cap, it’s possible you’re looking at a head gasket problem.

Try putting the radiator cap back on and see if the water stays in. If you can drive it without problems (you don’t have to top up the coolant) – there’s no problem.

Keep in mind that engine designs differ. Some will push coolant out when revved up. Some will make the coolant level drop when revved. These slight nuances make diagnosing the problem more interesting.

What Are the Signs of a Clogged Radiator?

The most obvious sign of a clogged radiator is an overheating engine. Your radiator can get blocked on the outside or inside.

1. External Blockages

These are easily detected by shining a flashlight through the radiator fins from one side and looking through the fins from the other. If they are blocked, you should be able to clean them out with either an air compressor or a pressure washer. 

(Note to self. And to others: Do not drive around in a dusty field with wet radiator fins!)

2. Internal Blockages

Checking the coolant temperature on both the outlet and inlet side of the radiator will give you a good idea of how significant your blockages are. You’re checking for a temperature difference between the water going in vs the water coming out of the radiator.

The difference between the two is how much your radiator is cooling. This will tell you how efficiently your radiator is working.

With a blocked radiator, you won’t have much difference between the two. It’s not really cooling.

It’s possible to see blockages in the tubes down through the radiator cap hole. The water level will have to be below the tops of the tubes to see this. You may have to let some water out to be able to see.

If you can see blockages there, chances are there’s a whole lot more you can’t see.

Sometimes, backflushing the radiator with high-flow water can clear blockages. This can help. However, if it’s badly blocked, it won’t work.

repairing broken tractor in a field during harvest season
Here you see two farmers repairing a tractor in the middle of their harvest. We’ve seen too many cases like this. A half-hour work session leads to a seized or overheated tractor! The moral of the story? Don’t wait until the last minute to double-check your water hose and tractor radiator fluid! And if you notice your tractors blowing water up out of the radiator when you start working? Then check your radiator cap. And pray it’s not your gasket!


As you can see, many of these symptoms can point to more than one cause. If in doubt, start with cheap repairs, like blowing out the radiator.

A new radiator cap will cost under $20.00. Test kits cost around $40.00.

Head gaskets can cost around $100.00. And you can easily blow $500.00 for cylinder heads. Not to mention the labor involved. 

(Make sure that you cover all of the inexpensive options first!)

Take things slowly.

If you have questions about getting your farm or garden tractor running – feel free to fire away! We are more than happy to help troubleshoot.

And good luck!

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