Hi Jordan,

There are several places where engine coolant can leak and not be detected.

1) The coolant pump is the worst place for a coolant leak. The problem is the seal around the pump shaft start to leak or the freeze plugs can leak if the coolant pump is equipped with freeze plugs.

2) Hoses with bad clamps. I found 2 crimp type clamps that were leaking on my 99 XC. I replaced them with the screw type clamp. On newer Volvos (1999 and on) the turbo has coolant lines.

3) Cracks in plastic. Like cracks in the coolant reservoir, cracks in the radiator end caps and the most common place is cracks in the radiator end caps where the hoses connect to the radiator.

4) Head gasket leak. This is one place where you do not want a leak to occur. This is not an easy or cheep fix.

Now how to find the leak? First my experience with pressure testing the coolant system is that it only detect the obvious. If you have a small slow leak pressure testing will not reveal the problem. This is due to the point that when pressure testing is done it is done on a cold engine. Cold parts behave differently then hot parts. Most notably is that hoses become soft when hot (prone to leak) and hard when cold (not prone to leak). The same is true with rubber type O-Ring gaskets.

To find the type of leak I describe in #1 is to look for a white film on the bottom of the engine directly below the coolant pump. Or remove the plastic housing surround the timing belt/water pump and look for a white film directly below the pump on the engine block. What is happening here is when the engine is hot the gasket around the water pump shaft gets soft and can start to leak due to pressure in the coolant system. The coolant leaks out from the shaft on to the hot engine and evaporates leaving a white film. As the engine cools off the coolant pump gasket gets hard, the pressure in the coolant system goes down and the leak stops. If you have this condition I would just keep toping of the coolant until the leak gets more severe.

To find the leak I describe in #2 is to inspect all hose clamps for either coolant or the white film left when the coolant evaporates. Also I would do a slight tug on the hose to make sure the crimp type clamps are on tight. If you can pull the hose off then I would replace the clamp.

To find cracks in plastic I would look for coolant and or the white film. Check closely the plastic pipes that are used to connect the hoses to the radiator. This is a common place for cracks to occur.

And last to determine if you have a head gasket problem is to heat up the engine with the coolant reservoir cap off. Once the engine is hot insert a Smog Emission probe into the coolant reservoir and see if you can detect engine exhaust gases in the coolant. If you can detect carbon monoxide in the coolant reservoir then there is a good change you have a head gasket problem.

A note on after market anti leak additives; If you have an old engine I would recommend the use some of these additives. Some work quite well in stopping leaks. However, if you have a new engine I would not recommend the use of after-market anti leak additives. These additives can damage the coolant pump and over time can plug up a radiator. But if you have an old car and want to get another few thousand miles then the additives are a cheep way to accomplish this.

Good Luck,

Steven---