Bypass hose

You wouldn't be referring to the hose down low behind the AC compressor? If so, that is one of the heater hoses. It attaches to the top of the water pump housing on one of two pressed-in stainless steel pipes, about an inch and a quarter long. That pipe corroded out on my 1994 Grand Caravan thanks to the total lack of cooling system maintenance by the previous owner. The coolant looked like dark brown mud! The corrosion inhibitor additives had to have been depleted many years ago. Replacing that pipe is actually easier than you would expect, but you need an acetylene torch to warm the housing, and you have to get the pipe from a salvage yard. It's not available separately. The alternative is to buy the complete housing through the dealer for around $600.00. Also, it's a real big job to replace the whole housing that way.

I've searched the online service manuals for a diagram of the thermostat bypass hose, but apparently that is top secret. It's listed as taking 0.8 hours to replace the hose, so we know it is somewhat difficult. I've never done one on a 3.3L / 3.8L so I don't know if you'll need to remove the intake manifold. The hardest part of replacing the bypass hose on the 3.0L engine is getting the right pliers in there to slide the hose clamps off.

Any chance it's the thermostat housing gasket that is leaking? That's in the same area as the bypass hose.

I wish I had taken photos of the job. One nipple comes out the top of the aluminum housing and one comes out the back, which would be pointing toward the driver's side. I used a hack saw, then a flat chisel and big hammer, in the salvage yard, to break off that part of the housing so we could work on it in my friend's shop. From there, by heating the housing around the nipple, it was real easy to slide it out with a pair of pliers. Same on my van. There was so little left of the old one, there was nothing left to remove. The AC compressor and mounting bracket were removed, which wasn't that hard to do, then, while warming the back of the housing with the torch, we had the nipple lined up and just started, then it was just a matter of tapping it in. Didn't take much effort at all. No sealant of any kind on the nipple.

The fear was we would be shoving the remaining part of the old nipple into the water pump impeller, but there was no noise or other symptoms. Even if the old nipple causes a problem, there's only five bolts to remove the water pump to get the old fitting out. The pump is sealed with a large rubber o-ring, which is reusable, but I always put some RTV gasket sealer on it too for good measure.

The two nipples are pressed into the aluminum water pump housing, but that housing also holds the oil pump on the back side. As such, it is actually the front cover of the engine. The oil pump is driven by the snout of the crankshaft.

The top nipple, which is the one that corroded out on mine, connects to a hose that's about a foot long. It runs straight up to the thermostat housing. That nipple is screwed in and sits right behind a stiffening bracket. That bracket makes access a little difficult, but not impossible to get the hose and clamp off.

The nipple that comes out of the rear of the water pump housing connects to another hose about a foot long, but that one connects to a steel pipe that runs to the driver's side, then over the top of the transmission and to the back of the engine. I didn't follow it all the way, but I assume that is one of the heater hoses.

If you disconnect the AC compressor hoses, the compressor will never get turned on when you switch to "AC" or "defrost". There's a low-pressure cutout switch for this purpose. That prevents the compressor from potentially pumping the low side into a vacuum when the refrigerant has leaked out. If it were able to do that, air could get sucked in along with the deadly humidity in it. With the hoses disconnected, all that will happen is the compressor clutch will free-wheel like it normally does when it isn't turned on.

Once the lines are reconnected and you want to charge the system, it is necessary to pump it into a vacuum first for a good 30 - 45 minutes. In a near-perfect vacuum, water boils at 77 degrees. Turning to a vapor makes it easy to draw out. If any moisture remains in the system, it combines with refrigerant to form an acid that will corrode the condenser and evaporator.