I have had a slow leak form the coolant system for a few months, if I drive the car every day I have to top up every few weeks.

The car has 200K on the clock and is a VW Passat TDI, I only drive less then 5K most years, I brought the car a few months ago, ex lease car with full main dealer service history.

The garage has pressure tested the cooling system and can’t find the leak and is saying it is best to put in a “Radiator Stop Leak” product. He cannot get to the water pump to check if for leaks without a lot of expense and the CAM belt is not due to be replaced for anther 3 years.

3 Answers 3


Cooling system additives such as stop leaks should only be used as an emergency measure if at all. If you picture the position of the heater radiator, you will realise that they are always higher or indeed lower then the main body of the engine. This usually means you will get a mass of the stop leak blocking the heater radiator. Volkswagens have a mapped cooling system with a main and a minor cooling circuit. Radiator stop leak can easily block the minor circuit and effect the electrical thermostat passages, blocking the minor circuits. If you cannot see an external leak, look for antifreeze stains, it is a simple matter to loosen the cam belt cover and ease away it from the engine. You could then use a mirror and pencil beam LED torch to see if the tell-tale hole on the water pump is leaking. If the pump is leaking it should be changed along with the cambelt. You can also test for an internal engine water leak by using a fluid test checking for HC gases in the cooling system expansion tank.


If you're losing coolant and the mechanic can't find the source, you need to find another mechanic that will properly diagnose the issue and offer a solution instead of a bandaid that will make the symptoms go away for a little while. Cooling systems are under quite a bit of pressure and will push out any seal that a stopleak product will create.

Diesel engines get hot and it maybe hard to find the leak. One solution you can try yourself is to run the car pretty hard for a few minutes on your way home. When you get to your driveway, pop the hood and listen for any hissing or sizzling liquid sounds.

You may not have a leak in your cooling system at all actually. Other problems like a leaking head gasket or, depending on the year of your car, leaking seals on a water cooled turbo, can also cause you to lose coolant. Neither of them would be helped by 'stop leak' and would just be throwing money away.


As long as you follow the steps exactly and don't rush it. If you don't follow them exactly, you may clog up your cooling system.

I had a leak recently and it turned out to be a cracked coolant reservoir. I patched it and also found a hose clamp that was not tightened properly.

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