Coolant Leak I have a 1997 Pontiac Transsport that is leaking coolant. The leak occurs when the vehicle is sitting overnight, and appears to be coming off the rear of the transmission pan. The coolant is not leaking into the transmission, just the drip is from around the pan. What could be leaking from the rear of the engine or in this area to cause this?

Are you loosing this coolant overnight when the engine is cold or after you've been driving around and the engine is hot? If this overnight and a cold engine it has to be leaking out somewhere. If you have access to a garage, or somewhere that will stay dry, park there overnight and put a sheets of cardboard under the front of the vehicle. Make sure some sheets are far enough back to cover the area level with the driver's pedals. Check the cardboard the next morning at it will give you a rough idea where the leak is. After driving the vehicle and the coolant pipes are cool enough to touch use a rag or paper towels to make sure they are dry. Next morning with the engine COLD run your hand along the underside of all the radiator and coolant pipes you can reach. Your feeling for where they've gotten wet again overnight.
If it's happening when you've been driving around for a while it could be a 'pressure' leak. A coolant system is pressurised when it's running. Sometimes a small hole in a coolant hose can open and leak when the engine is hot and the hole will then close stopping the leak when the engine cools down. This often leaves 'stains' where the coolant has landed and evaporated. The stains will be a light colour the same as the colour of the coolant your using.
Another place where you can get bigger leaks is the water pump itself. These have a 'Weep' hole built into bottom of the pump which is designed as an escape valve and is a sign the water pump seal or the pump itself is failing.

If seal/tape goes bad it can leak..I suppose if something is cracked sensor can appear to leak..if temp gauge seems to work and u think sensor threads leak reapply some thread tape and put in..
If gaugedoesn't work then maybe sensor is bad/crackedt

Did you have a leak before? If so and thats the reason you replaced those parts, it could be leaking from another spot in the coolant system sometimes the system with leak from the weakest points then when you fix them the system will leak from the next weakest point due to the pressure in the system. I would try to spot the leak, your best bet is going to be to turn the car off, and wipe everything clean....start up the car and then watch for any changes in appearances for leaks and so forth. It can be tricky, but maybe you'll get lucky!

One of two places the Coolant will escape from! Check all of ur hoses . For soft or swollen ! These engines are notorious for head problems!

if it is still leaking check the hose from the coolant reservoir , check radiator hoses, then check radiator.

WHERE COOLANT LEAKS OCCUR
Coolant leaks can occur anywhere in the cooling system. Nine out of ten times, coolant leaks are easy to find because the coolant can be seen dripping, spraying, seeping or bubbling from the leaky component. Open the hood and visually inspect the engine and cooling system for any sign of liquid leaking from the engine, radiator or hoses. The color of the coolant may be green, orange or yellow depending on the type of antifreeze in the system. The most common places where coolant may be leaking are:
Water pump -- A bad shaft seal will allow coolant to dribble out of the vent hole just under the water pump pulley shaft. If the water pump is a two-piece unit with a backing plate, the gasket between the housing and back cover may be leaking. The gasket or o-ring that seals the pump to the engine front cover on cover-mounted water pumps can also leak coolant. Look for stains, discoloration or liquid coolant on the outside of the water pump or engine.

Radiator -- Radiators can develop leaks around upper or loser hose connections as a result of vibration. The seams where the core is mated to the end tanks is another place where leaks frequently develop, especially on aluminum radiators with plastic end tanks. On copper/brass radiators, leaks typically occur where the cooling tubes in the core are connected or soldered to the core headers. The core itself is also vulnerable to stone damage. Internal corrosion caused by old coolant that has never been changed can also eat through the metal in the radiator, causing it to leak.

Most cooling systems today are designed to operate at 8 to 14 psi. If the radiator can't hold pressure, your engine will overheat and lose coolant.

Hoses -- Cracks, pinholes or splits in a radiator hose or heater hose will leak coolant. A hose leak will usually send a stream of hot coolant spraying out of the hose. A corroded hose connection or a loose or damaged hose clamp may also allow coolant to leak from the end of a hose. Sometimes the leak may only occur once the hose gets hot and the pinhole or crack opens up.

Freeze plugs -- These are the casting plugs or expansion plugs in the sides of the engine block and/or cylinder head. The flat steel plugs corroded from the inside out, and may develop leaks that are hard to see because of the plug's location behind the exhaust manifold, engine mount or other engine accessories. On V6 and V8 blocks, the plugs are most easily inspected from underneath the vehicle.

Heater Core -- The heater core is located inside the heating ventilation and air conditioning (HVAC) unit under the dash. It is out of sight so you cannot see a leak directly. But if the heater core is leaking (or a hose connection to the heater core is leaking), coolant will be seeping out of the bottom of the HVAC unit and dripping on the floor inside the passenger compartment. Look for stains or wet spots on the bottom of the plastic HVAC case, or on the passenger side floor.

Intake Manifold gasket -- The gasket that seals the intake manifold to the cylinder heads may leak and allow coolant to enter the intake port, crankcase or dribble down the outside of the engine. Some engines such as General Motors 3.1L and 3.4L V6 engines as well as 4.3L, 5.0L and 5.7L V8s are notorious for leaky intake manifold gaskets. The intake manifold gaskets on these engines are plastic and often fail at 50,000 to 80,000 miles. Other troublesome applications include the intake manifold gaskets on Buick 3800 V6 and Ford 4.0L V6 engines.

INTERNAL COOLANT LEAKS
There are the worst kind of coolant leaks for two reasons. One is that they are impossible to see because they are hidden inside the engine. The other is that internal coolant leaks can be very expensive to repair.

Bad head gasket --Internal coolant leaks are most often due to a bad head gasket. The head gasket may leak coolant into a cylinder, or into the crankcase. Coolant leaks into the crankcase dilute the oil and can damage the bearings in your engine. A head gasket leaking coolant into a cylinder can foul the spark plug, and create a lot of white smoke in the exhaust. Adding sealer to the cooling system may plug the leak if it is not too bad, but eventually the head gasket will have to be replaced.

If you suspect a head gasket leak, have the cooling system pressure tested. If it fails to hold pressure, there is an internal leak. A "block tester" can also be used to diagnose a leaky head gasket. This device draws air from the cooling system into a chamber that contains a special blue colored leak detection liquid. Combustion gases will react with the liquid and cause it to change color from blue to green if the head gasket is leaking.

Head gasket failures are often the result of engine overheating (which may have occurred because of a coolant leak elsewhere in the cooling system, a bad thermostat, or an electric cooling fan not working). When the engine overheats, thermal expansion can crush and damage portions of the head gasket. This damaged areas may then start to leak combustion pressure and/or coolant.

Cracked Head or Block -- Internal coolant leaks can also occur if the cylinder head or engine block has a crack in a cooling jacket. A combustion chamber leak in the cylinder head or block will leak coolant into the cylinder. This dilutes the oil on the cylinder walls and can damage the piston and rings. If the coolant contains silicates (conventional green antifreeze), it can also foul the oxygen sensor and catalytic converter. If enough coolant leaks into the cylinder (as when the engine is sitting overnight), it may even hydro-lock the engine and prevent it from cranking when you try to start it. Internal leaks such as these can be diagnosed by pressure testing the cooling system or using a block checker.

A coolant leak into the crankcase is also bad news because it can damage the bearings. Coolant leaking into the crankcase will make the oil level on the dipstick appear to be higher than normal. The oil may also appear frothy, muddy or discolored because of the coolant contamination.

Leaky ATF oil cooler -- Internal coolant leakage can also occur in the automatic transmission fluid oil cooler inside the radiator. On most vehicles with automatic transmissions, ATF is routed through an oil cooler inside the radiator. If the tubing leaks, coolant can enter the transmission lines, contaminate the fluid and ruin the transmission. Red or brown drops of oil in the coolant would be a symptom of such a leak. Because the oil cooler is inside the radiator, the radiator must be replaced to eliminate the problem. The transmission fluid should also be changed.

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do you know if they replaced the O ring rubber that connects the pipe under the intake Mark?
One of its trademarks is it only leaks after the vehicle sets overnight.

It is definitely your heater core. There may be a leak in it which is why there are vapors coming out

this happens alot on older vehicles most often times because the coolant hose shrink and expand around a fitting. try and tighten all coolant clamps. this often is all it takes to solve the problem. you should also look to see that all seals around heads of engine show no seepage

sounds like a freeze plug, when the motor is warm the metal expands and its seals itself, when it cools over night it leaks..

let them redo the test on a cold motor