My 740T overheated due to a leaking radiator. We replaced the rad and since then have had varying levels of coolant in the overflow tank. I will add coolant and it may stabilize or afetr driving,it could fill the tank. Today, driving on the freeway, the gauge pegged and then returned to normal. We checked and the tank was empty. When the tank was opend, the pressure expended everything. The tank required a lot of water to fill and when we returned home, the tank level was higher and rose as the car set. What is wrong?

I have considered warped head, head gasket etc. Could it be a bad radiator? water pump??

Knowing how hard it is to find a leak on these vehicles (took 8 months on my 85) it turned out to be a pin hole leak on the bottom of the heater hose at the left rear of the block. It only leaked when the motor flexed, was under pressure and sprayed directly on the hot block which left no trace or puddle. I found it by hearing it hiss for about 5 seconds one day after the wife had shut the car off. Burned my fingers flexing the hoses.

Duane

As other poster stated water level in tank goes up and down with temperature. If engine gets too hot it can blow out coolant with cap on or off. Check for leaks as the other poster suggests but you could have nothing more than a sticking thermostat. You did not say if it was replaced with the rad, if not it should have been.

Don't consider replacing anything until the problem is diagnosed. Bad water pump will not cause overheating directly but it can leak and thereby cause overheating. Rad should not be bad if you replaced it. If you overheated at a constant freeway speed of say over 50 then fan would not be the cause.

If no leaking detected and thermostat replaced then have coolant system pressure tested and check for gases in exhaust as suggested. It is possible that because of the original overheat episode(s) that other damage could have occured such as head gasket or worse cracked head (we hope not).

Good Luck

1. Check and compare coolant levels only when cold, since glycol expands with heat.

2. If you have overpressure, have a garage check the coolant chemically for the presence of exhaust gases. If the head gasket is leaking gases into the coolant, they will overpressure the system but will also show up in the coolant as dissolved gases. Easy to check. If so, you need a new headgasket.

/// The water must go some where quickly. Usually a head gasket has other symptoms, oil in radiator, missing cylinder, but not always. Be sure you do not have water in the transmission if automatic.

Fill the car, crank and run until hot, park over a dry level spot, turn off the engine, leave for fifteen min. to half hour. Go back and look for the wet spot. There are several possible locations for a leak.

The heater lines. They are rubber, at the rear of the engine, after initial leak only steam will exit so there may not be much to see when driving on the highway. A small hole may leak under pressure but be almost invisible after stopping. The metal line from the waterpump is next in that circuit. The heater core would pool water inside on floor.

Depending on your engine there are usually temp sensors on Volvo, any one could leak at the pipe thread or through the sensor.

The water pump has a weep hole on top of the shaft that leaks when the seal fails. It is hidden by the pulleys. Water will drip from the rear of the WP pulley if the engine is off. It will be spun and blown away if runnning. The seal at the block/WP should not just start to leak. The O-ring at the top might, not first choice.

Did you replace hoses, if so check clamps. Check older hoses at the overflow tank, the tank, and the cap.

If you cannot find the leak use a can of block seal and see if that stops it. You can some times drive for years. This is useful if the leak is unusual, (rusted freeze plugs) and you need a few weeks to get around to it.

First of all, replace your expansion tank cap. They are famous for developing cracks and leaking. Then replace your thermostat, this is an easy $10/10 min. job. Then check all of your cooling system hoses for hard/very squishy/leaky behavior. Verify hose clamp tightness (don't over tighten upper rad hose to rad junction).

Also check your transmission fluid for the presence of coolant. If you have a new rad, this is unlikely, but worth a quick check. BTW, this is probably the biggest disadvantage of the "orange" coolants (DexCool, et al), becuase it is not immediately obvious to see the orange coolant mingling with the red ATF.