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Water Pump Testing

Got overheating problems? Here's how to accurately test your water pump in a few hours for under $15.

By Graham Pring, pix by Julian Edgar and Graham Pring

Click on pics to view larger images

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One of the reasons an engine can overheat is because of poor design of the water pump and its impeller. Some impellers cavitate badly, effectively causing boiling at temperatures way below 100 degrees C. In fact, as you'll soon see, many water pumps are pretty bad! And if the water-pump is not very efficient, a large improvement in cooling can be had by changing the water-pump to a better design, or even just adjusting its internal tolerances. But before you can do that, you need to see how good the pump in your engine actually is.

This is All That You Need:

  • A 1.25 or 2 litre soft drink bottle with a hole cut in the bottom.
  • Some heavy-duty wire to tie the bottle up and hang it above the engine.
  • Two metres of heater hose to make a bypass system.
  • A thermometer capable of displaying temperatures up to 100 degrees C / 212 degrees F.
  • Hose clamps as required.
  • Extra coolant or water.
  • Protective glasses and clothing.
  • Rags to allow you to handle very hot pipes and materials.
  • A container in which you can store excess, hot coolant.

Some Optional Extras:

  • Clear glass Pyrex-type tube or perspex tube about 150 - 200 mm long, capable of taking fluid at 100 degrees C and up to 45 psi pressure.
  • Some radiator hose clamps

Getting Started

Cut a hole in the base of the soft drink bottle big enough to allow a heater hose to be easily fed into it. The hose may need to be removed and replaced in a hurry, so don't make it too tight a fit.

Next, make a wire cage around the soft drink bottle so that the bottle can be suspended upside down (ie inlet neck facing downwards). Push a metre of heater hose into the neck of the soft drink bottle, making sure that this hose is a tight fit.

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Find the heater inlet and outlet on the engine. If you do not have a coolant type heater, then locate a bypass of some sort, which means looking for hoses that have a pressure difference across them. If there is still no bypass hose available, take the "optional" route, where a piece of clear tube is inserted in a radiator hose to allow you to actually view the coolant as it passes around the system.

Attach the short length of heater hose (with the soft drink bottle attached) to the heater hose input nipple on the suction side of the water pump. Hang the soft drink bottle from the bonnet or from something higher, like a beam in your shed or from a tree or clothesline. This bottle must be securely suspended - soon it will be full of hot or even boiling liquid!

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Extend the original heater hose that you removed from the suction side of the pump. This needs to be able to reach into the top of the soft drink bottle, and also at times needs to be held even higher to check water pump or system pressure. An easy way of extending this hose is to have a piece of glass tube or suitable plastic tube cut to use as a joiner, connecting an extension piece of hose to the original. Doing this also allows you to see if the bypass hose has coolant in it - you don't need to wait to see if the coolant rushes or dribbles out the end when you start the motor up. This makes it much easier to see what is happening!

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Put the lengthened bypass hose into the top (ie through the hole in the original bottom) of the soft drink bottle, making sure it will not fall out or be ejected when it acts like a hose under pressure.

Finally, block off the coolant system overflow pipe that normally goes to the overflow bottle near the radiator or coolant system pressure cap. If you have a remote reservoir with a pressure cap on the reservoir, plug the small diameter tube exiting the remote reservoir to the ground. If you have a remote reservoir without a pressure cap in it, temporarily crimp or close off the small tube from the radiator to the remote reservoir.

(The reason for closing this small tube is that if you have a good pump and there is a large pressure differential between the inlet side of the pump and the outlet side, the pump may draw coolant and/or air from the overflow system rather than through the radiator or rest of the system. This has happened to me with a cross-flow radiator and remote reservoir with attached pressure cap.)

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If you wish to install the optional viewing pipe in the top radiator hose, now is the time to do it. However, rather than chop one of your car's radiator hoses in half to insert the clear viewing section, source some hoses from your local radiator repairer's rubbish bin. Just remember that there are quite a few variations in radiator hose internal diameters, so make sure you get some that will seal to your viewing pipe, the radiator and the engine plumbing connection.

Using this viewing pipe can be a real eye-opener (even those people who have been in the industry for a long time!), so if you can get one to fit then it will be worthwhile.

Now is the time to fill the system up with coolant, adding it through the suspended bottle.

Bleeding The System

You will always have some air in the system, air that you will be unable to get out at a static fill.

Make sure the cabin heater switch or lever is set to "hot" so that coolant will flow through the bypass hose if you are using the heater bypass system hoses. Jacking the front of the car up or parking up a hill will help get air out, although most people will need to start the engine and get the fluid circulating to get rid of most of the air bubbles/pockets. Running the engine for about one minute should have most of the air either out of the system or trapped in the highest part of the system and unable to get out.

If the coolant is not yet hot and you have a radiator pressure cap at the top of the radiator, you should be able to stop the motor and release the pressure cap to get rid of any air build-up. If there is any pressure in the system, then something is wrong and you will need to diagnose what the problem is. (Remember, your bottle bypass isn't totally sealed, so no pressure should build up.)

Coolant should be flowing into and out of the soft drink bottle while the pump is operating. If it isn't, it might be because of the following:

  • No hole or entry to the inlet side of your pump, the apparent opening being either blocked or un-drilled.
  • The heater bypass hose valve is stuck closed or the levers/switches aren't working correctly.
  • The soft drink bottle hoses are not connected to places that should have a pressure differential
  • The pump isn't very good.

Keep bleeding/running the system until you are happy with the fact that you have bled as much air out of the system as is possible. The optional top hose viewing tube makes this job easier - air bubbles are obvious as they move past.

Finally, disconnect/remove the electric radiator fan/s and/or the engine driven fan.


Insert the thermometer into the coolant in the soft drink bottle. As the fluid in the system heats up it will expand, so be ready to drain some coolant by having the soft drink bottle input hose (the top hose pushed through the large hole) briefly feed into another container at some stage while the engine is still running.

Remember: The coolant may be very hot! The hose may be very hot to touch! You may spill some or splash some coolant! Be prepared with insulating materials, protective gloves, glasses and clothing. You will be playing with very hot water and it can burn you!

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As the coolant heats up, there will be more and more boiling in the system, indicated by the air bubbles that will appear at the viewing locations (the bottle, and the clear tubes if they are being used). Initially these bubbles will probably be very small, and so may be seen as 'froth' in the soft drink bottle. If you enlarge this pic you will be able to se the boiling bubbles, happening at a temp of well below 100 degrees C!

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As the boiling increases, the level of the water in the soft drink bottle will rise and rise, necessitating that you frequently feed the top hose going into the bottle into another container to lower this level. The frothy bubbles appearing in the coolant can be clearly seen in this pic.

When boiling is well under way, measure the temperature of the coolant in the bottle.

The best water pump is the one that allows the coolant to boil at the highest temperature.

I have tested five different water pumps on my Holden V8 engine. The temperatures that the coolant had bubbles or boiling in the system ranged from mid eighty degrees up to one hundred degrees Celsius. That's an incredible range, and shows how widely pumps vary in their efficiency of flow.

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This picture shows testing taking place on a 460 cid Ford V8. The pump boiled the rusty-looking coolant at just 65 degrees C!

But does the boiling matter? Yes it sure does! Any air circulating through the system has a negative effect on the ability of the engine/heat exchanger to transfer heat. This is because air is very much worse than coolant at transferring heat.

You can also test the pressure differential of the 'in' and 'out' hoses to the soft drink bottle by raising the inlet hose above the fluid height in the bottle and measuring the difference in height when flow stops. This is another indicator of how well the pump is performing - 1 psi is 27.67 inches of water. A good Holden V8 pump will well exceed one and a half metres in height at idle. (I say "exceed", because I stopped measuring height when I ran out of heater hose and it was too hard to get the coolant into the soft drink bottle without an external aid such as a tube or funnel!)


If you are not having overheating problems, you can get away with using a bad pump. This is because there is so much excess cooling capacity in your system that it can cope. However, the water pump is the first area I would look at if I had overheating problems after doing any of the following:

  • modifying the motor;
  • fitting a new water pump;
  • changing engines.
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Doing the test described here will very clearly tell you how good or bad your water pump is. If it's bad, replace it with a better one! The test could save you an awful lots of dollars spent on new multi-core radiators and extra thermo fans....

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