Coolant leak My c230 has been pressure tested, but after driving it and turning off the engine, about a gallon of coolant leaks onto the ground. At this point, adding additional coolant just pours onto the ground. It then can be pressure tested again and tests ok. What is it?

LEARN TO DO THE TESTS.
use the tool, called radiator leak down test. (has hand pump)
find the like that way safely cold engine or
OR , the classical why
hood up
best when cold a new radiator cap, that does not leak.
run engine until fully hot , takes 15min in summer.
now at this moment, fully hot the RAD and the whole loop is at 15psi pressure
and the magic pressure lets you as the tech find the actual leak.
to the ground? or engine oil pan or to the front passenger toes (foot well flooding
with antifreeze leaking. anywhere
look every where, engine all sides, not just causal glancing,
hose leak, or even squirting and the landing point fools you,before.
all hoses, not just those 2 big ones
heater core leaking or again its 2 hoses only.
water pump leaks
see head bottom edge leak is bad head gasket (or warped head )
freeze (core) plugs leaking,
some intake manifold can leak coolant.
look every where there, make no assumptions find the leak.
yes a cracked head can push combustion gasses to the RAD
and land in the side coolant recovery bottle (jug)
we use the next tool, a combustion leak gas checking tool to top of RAd CAP
A GREEN/BLUE DYE TESTER.


RAD CAP OFF IN THE MORNING DEAD COLD
NO OIL DROPS THERE, NO BLACK CARBON DUST SEEN FLOATING THERE.

DO A COMPRESSION TEST YET? OR CYLINDER LEAK DOWN TEST?

It's unlikely that there is a freeze plug in that area. The most common sources of coolant leaks on BMW E46 3-Series models are the expansion tank and the radiator. However, there are a few other possibilities that could cause a leak in that area:

1. Heater Core: The heater core is located inside the dashboard and circulates hot coolant to provide heat to the interior of the car. If the heater core is leaking, coolant can drip down onto the motor mount.
2. Coolant Pipe: There is a coolant pipe that runs along the left side of the engine and connects the engine block to the heater core and radiator. If this pipe is corroded or damaged, it can leak coolant onto the motor mount.
3. Head Gasket: A failing head gasket can cause coolant to leak externally from the engine. While this is less common, it's worth checking to see if there are any signs of coolant leaking from the head gasket.

It's recommended to have the coolant system inspected by a qualified mechanic to determine the source of the leak and to make the necessary repairs.

Finding a small mysterious coolant leak can take time. If your engine is loosing coolant it can only leak three different ways. Externally, on to the ground. Internally, into the engine oil. Lastly the head gasket, head or block could be to blame. When this type of failure occurs you may see coolant build up on a spark plug, coolant system over pressurizes, pushing coolant into the over flow tank or cotton white smoke out the tail pipe. Regardless of what the pressure test yielded, there is a leak. Look around the radiator and engine for white dry coolant trails/stains. If none are found check the oil pan. Water will sink to the bottom. If that passes I suggest purchasing a "Compression in Coolant Test Kit". I the leak is very small you may need to run this test twice. One when the engine is cold and one when the engine is hot. Be very care of hot antifreeze. You could get badly/terribly burned removing a radiator cap from a hot pressurized coolant system. Any questions please write back. Good Luck.

You need to have your cooling system pressure tested. This will let you examine the engine area without the hazards of the engine running. You may have head gasket issues. Depending on the area of the leak, your cylinder compression can actually pressurize the cooling system. When the cooling system pressure exceeds the rating of the cap, the cap seal opens and can blow coolant back into the overflow bottle where it eventually leaks while driving. Check to see if your overflow bottle is full.

2 tests
1-- compression test for gasket and cracks
2--- coolant system pressure test to check for split tanks , welsh plugs , leaking through corrosion holes behind pump impeller

TO FIX COOLANT LEAK.YOU HAVE TO LOCATE IT.BEST TIME FIND COOLANT LEAK WAIT UNTIL MORNING.ADD COOLANT UNTIL COOLANT LEVEL IN RADIATOR CORRECT. START THE ENGINE LET IDLE UNTIL START GETTING HOT.THEN TURN OFF ENGINE.COOLANT PRESSURE WILL PUSH COOLANT OUT WHERE LEAKS ARE.CHECK TOP RADIATOR HOSE AND BOTTOM RADIATOR HOSE.CHECK AROUND RADIATOR LOOK DOWN BELOW COOLANT FAN AREA BEHIND RADIATOR.LOOK FOR LEAKS AT RADIATOR CORES AND SIDE CONTAINERS.CHECK FOR COOLANT LEAKS AROUND WATER PUMP GASKET OR WEEP HOLE.CHECK FOR COOLANT LEAKS AROUND HEATER HOSES AND HEATER CORE.CHECK ENGINE CRANKCASE FOR COOLANT LEAKS.

You have done everything right so far. Sometimes water leaks out when the engine is hot and evaporates almost instantaneously, a classic point is the seal on the water pump pulley shaft. To find such a leak you need to pressure test your coolant system. The normal operational pressure of a coolant system is 7lbs/sqinch at running temperature. The pressure test will put in 11lbs/sqinch and then monitor this over some time looking for a bleed down due to the leak. If a leak exists to the exterior of the engine you should be able to spot the seepage. The next test will be to run the engine up to normal temperatures and to monitor the pressure, if the pressure jumps to 15lbs/sqinch when still cold then this indicates an internal gasket leak as exhaust gases are pushing out of the cylinder in to the coolant. One check to make is to remove the plugs one at a time and inspect them. Sometimes if water leaks into cylinder it has the effect of 'washing' the specific spark plug clean.

If it is a four cylinder, your coolant capacity is two gallons. If it is a 2.7 six cylinder it is just over two gallons. The waterpump on the four cylinder has a history of leaking in which you would also replace the timing belt while in there since it drives the waterpump. Some radiators have been known to leak also. Have the system pressure tested to find out for sure.

The water (coolant) in the expansion tank will rise and fall with the engine temperature. What the expansion tank does is collect and return coolant to and from the engine. When the engine warms up the coolant gets hot, builds up pressure and opens up the radiator cap. The coolant then goes into the expansion tank. Now when the engine cools down the pressure drops in the cooling system and the pressure drop (vacuum) pulls the coolant back into the engine via the radiator cap. The radiator cap allows the cooling system to build up pressure and by doing so increases the boiling point of the coolant, but when that pressure exceeds the caps rating the cap opens and the coolant goes to the expansion valve. The cap has another part to it that when the engine cools down and a partial vacuum is created in the cooling system a "valve" in the radiator cap opens and allows the coolant to be drawn back into the engine. I would look at your radiator cap to see if any gunk or build up is on it, and check the rubber gaskets for cracks. It's easiest just to replace the cap because they are inexpensive and easy to replace (2-10 dollars). The expansion tank should have two hoses on it. The one on the bottom comes from the radiator and the one on the top (possibly part of filler cap) runs down and is open to the ground. That way if it is overfilled or becomes overfilled it will slowly leak onto the ground. When and if you change your radiator cap, make sure the engine is cooled down, remove cap and start engine and turn heater to full blast, full heat. Leave the cap off and let it run until engine warmed up. This should burp out any air pockets that may have happened when coolant was changed. Also top off the coolant in the radiator while it is running. Hope this helps and good luck

Well this is going to be like finding a needle in a haystack! You can get a test done where they put a flourescent ink in your coolant, run the engine and then go over your engine with a black light. With a leak this small, that might be your best bet!