Troubleshooting coolant vacuum fill failure

[Rendak]

Alright, so the air compressor is not the problem. It really is a vacuum leak in the system. I removed the expansion tank from the vehicle, put vacuum plugs in the outlets, and ran the vacuum on the expansion tank and it held -24ish inHg, so I’ve ruled that out as the cause. I also hooked it back up and ran the vacuum again on the system and went around spraying all the visible and even somewhat hard to reach junctions with soapy water and saw no bubbles forming. The big radiator hose that connects to the top half of the front crossover sucks inwards under vacuum, as does the lower radiator hose that goes to the thermostat, and the hose from the transmission cooler to the radiator that serves as the primary port for draining coolant. Next up is the hose that goes under the supercharger and connects to the rear heater manifold.

I’m really hoping it’s that one, because I’m not excited about taking the whole damn thing apart again just to re-check the seating on the lower half of the front crossover or the junction between the rear heater manifold and the engine block.

 

[Rendak]

Side note, but has anybody ever used a mechanic’s stethoscope to find vacuum leaks by sound in hard to reach places? I tried it today to get at the junction between the lower front crossover and the engine block and didn’t seem to hear any appreciable difference between what that junction sounded like and what another junction that I knew wasn’t leaking sounded like.

 

[scapistron]

Soapy water only works for a system under pressure, not vacuum.

 

[Rendak]

Ah, thanks for the clarification!

 

[Rendak]

Well I took the whole damn thing apart this weekend. Checked the water pump (both O rings installed correctly), reseated the lower half of the front crossover pipe, and made sure the rear heater manifold was sitting right on the engine block and that the O rings weren’t pinched. Put it all back together and alas, the vacuum leak is still there! So gonna refill with coolant and pressure test. I just can’t imagine where the leak is. I’m worried about it being the head gaskets but haven’t seen any of the common symptoms associated with those kind of leaks.