Audi 2.4 v6 coolant leak from behind engine bay, only when engine was running. is there any hoses on the back of the engine bay?

I'm assuming for now that these are two separate problems:
1. Overheating and emptying expansion tank.
2. Leak into front passenger footwell.

Solution to 1. is likely to be a crack in the expansion tank. This happens often with these tanks. Look under the cap and you'll find two necks. The narrow one is the pressure tube that the lid seals in to. Check the narrow neck for any cracks and check the pressure cap o-ring for condition. Replace both if suspect. They're cheap from FCP Euro or ECS tuning.
The wider neck is the overflow. This drains into a small tube under the lid area of the tank. You could connect a hose to this if you were so inclined, but it normally it just drains into the engine bay (onto the left hand exhaust manifold normally on V6's!). If yours is losing the coolant from here its a classic sign of a cracked filler neck.
Make sure the cooling system gets bled properly as well - top bleeder is the right hand heater hose (factory hose if still on the car will have a hole marked by a white circle - pull the hose off the heater core connection tube to expose this hole to the coolant, undo and raise the expansion tank above the heater hose level to bleed).
On V6's the lower bleed is on the engine cross flow pipe on the rear of the engine on the opposite end to the engine temp sensor - 6mm allen key undoes from memory. Not sure where lower bleed is on 4 cyl models - possibly the coolant pipe in the vicinity of the inlet manifold.

The other possibility for 1. is a failed cylinder head gasket, but I'd be suspecting the expansion tnk ahead of this. Seek professional help if the expansion tank isn't the issue.

2. I would suggest is a blocked plenum chamber drain. You can check by pouring some water down the windscreen with the bonnet (hood) open and seeing if the plenum area fills up without draining or draining slowly. Main drain is in the middle I think and It gets clogged by leaves and debris. Cleaning is difficult, but you can try a shop vac with some heater hose taped to the nozzle to try and get in there, or some compressed air with an air gun or something to try and blow it through.
Search something like "Audi B5 plenum drains" on Google and you may find some other suggestions.
2. may also be blocked HVAC drains for the airconditioning...

The pipe that runs on top of motor from the front to then back connects inyto the heads at the back and the heater hoses connect at the back there as well.

If there is coolant hoses running back there then its either the coolant flange that distributes coolant threw the engine. or a ruptured hose, if there is no such hoses running behind then it could be a crack in the block or a bad head gasket. Just my guess, Does the engine over heat ? Hope it helps

Look at drivers side front,near ECT and also on driverside. On upper intake(plastic) GM designed it to fail internally (where u can't see it) to drink your coolant through a bypass. Replace the intake with DORMAN. Around 150 or so.

You didn't mention the model, year and engine. If this is a V6 or V8 then suspect a leaking intake manifold gasket.

If your coolant level dropped you have a leak in the system. Just adding coolant will not solve the problem, as it will continue to overheat. You should have a pressure test done to determine where the leak is. To bleed the system you have to slide back the hose going onto the heater core. At the back of the engine you will see two hoses going thru the fire wall. The hose on the left has a hole in it. Loosen the clamp ,slide the hose back . Add coolant to the system until it flows freely from the hole.Push hose back onto pipe ,tighten the clamp. Start the engine and turn on heater controls to high and run engine until normal operating temp is reached. Stop engine and check coolant level in the expansion tank and add if necessary.

find out where the leak is.. then fix it.

with my experiance on these engines it's usually the upper outlet hoses running from the heads into a t then into the upper radiator hose they are pain to replace but 3 out of 3 of my v6 isuzu's have had the same problem you'll have to pull off the theromstat and the coil pack the hoses are very small and run from the end of the heads one on each into a t then into the radiator hose just under the housing

head gasket!

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. So 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 bead 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, as is the area where the cooling tubes in the core are connected or soldered to the core headers. The core itself is also vulnerable to stone damage. But a major factor in many radiator leaks is internal corrosion that eats away from the inside out. That's why regular coolant flushes and replacing the antifreeze is so important.
oses. 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 (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 eventually eat through allowing coolant to leak from the engine. The plugs may be hard to see because they are 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 can�t 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 carpet. 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 30,000 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.


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