Coolant bleed system question
 

The Boxster has a bunch of vent lines that run from the front of the car to the rear. In the middle of the car there is a single line running from the passenger side of the engine up to the middle of the car. There, a small, rubber "Y" splits that line into two. Two hard lines continue to the front corners of the car.

My question is, if that little, rubber "Y" piece was leaking, would that allow air into the cooling system and coolant out?

The background: I replace my engine with a used one a year and a half ago. Since that time I've had nothing but problems with the cooling system. The car keeps losing coolant. I can smell the coolant when I get out of the car, but can't see any anywhere. I've done all the standard stuff; replace the AOS, water pump, main hoses (the two big ones from the engine) and the coolant tank. Although the water pump was leaking, I'm still losing a small amount of coolant. This last time since I filled it, it ran for about two and a half months before the low coolant light came on again (I had actually, until that time, though I fixed the problem by replacing the water pump). About a mile after the low coolant light came on, I parked the car at work. After getting my bags out of the car, I looked down to see a steadily growing puddle of coolant coming out from the area of the left-front wheel/radiator. The temperature was half-way, to 2/3 of the way from 180 to the next mark (215). So it should not have overheated.

After work - two days later - I drove my car that mile again and parked it, then had it towed home. When I initially moved the car (without starting it), I looked at where the puddle originated. It looked like it came from right under the shifter area.

Today, I put it up on jackstands and removed the undertrays. Everything in the middle part of the car is wet. However, beyond the passenger compartment, everything is dry. I see no evidence of any leaks at any hoses, or hose ends. While laying under the car, I discovered that vent line. Looking along it, I saw that "Y" piece. It would be right at the center of where everything is wet. Looking at it from the bottom, it appears fine. However, when I felt the top of it, I could feel that the top was completely deteriorated. In fact, I could put my finger into the hose.

So I know that piece is bad. I thought, "This could be the thing that has slowly been leaking all this time, and now it finally blew." However, when I read that it's a vent line, I worried that a leak in the vent line might not cause what happened.

Does anybody know?

Replace the line, report back.

I plan to. I was hoping somebody would have an answer to my question. That would allow me either keep searching for another leak, or stop searching.

Year, model, transmission type, part number of the y hose would help you get assistance.

99 Boxster, 5-speed, 999-106-546-55

Yes those hoses run to the top of both radiators and will leak coolant. Their purpose is to vent any air out of the top of the radiators.

I believe the part number begins with 996 instead of 999.