LS cooling... internal bypass?

My Express Van (5.3L) has a tiny vacuum leak in the HVAC.  Any time I go over about 1/4 throttle, the blend door shifts to the floor, and I believe the heat blend door partially opens allowing hot air to mix with the A/C.

I have tried multiple times to find where the leak is and failed, and getting to all the tentacles of the HVAC hoses is, uh... daunting in a van.

I know on old-school SBCs, the heater core WAS the coolant bypass.  My thought to get through this summer in the van was to splice in a ball valve in the heater hose to keep the core cool and worry about the vacuum leak later.

As a side note, the rear A/C (which is on the same system) blows ice cold all the time, which is why I'm leaning toward the problem being in the front air box.

As I understand it, the LS must use the heater as a bypass.  Or looped hoses.  You can't just block them off.

You can certainly loop the hoses for summer.

In reply to Curtis73 (Forum Supporter) :

Something's really screwy - the HVAC should go to defrost when the system loses vacuum.  Engineered in failsafe, driver safety is top priority so it fails to keeping the windshield clear.

My guess is that the checkvalve has failed or gone missing.

They sell specific LS heater bypass hoses for those unable to bend a piece of hose themselves. I suppose you can also tee in a 2nd valve to use as a bypass for the summer only. 

 

Yes it is a bypass.  I once got home with a suck thermostat with the heater on high.

These little vacuum tees/ check valve goes bad. I think they are on the right side of the dash.

Re: heater bypass 

You don't need the fancy hose loop if you pipe plug the hose outlets and drill a hole in the wall separating them in the water pump behind the thermostat or through the outlets.

Pete. (l33t FS) said:

In reply to Curtis73 (Forum Supporter) :

Something's really screwy - the HVAC should go to defrost when the system loses vacuum.  Engineered in failsafe, driver safety is top priority so it fails to keeping the windshield clear.

 

My guess is that the checkvalve has failed or gone missing.

 

I guess I'm not completely sure it goes to the floor.  I can reach down and feel some flow down there, but that could just be the normal flow that sneaks past the door.  I can't reach the defrost vents while driving, so I guess I should figure that out.

In reply to garaithon :

Ok, thanks.  I'll try to locate it.  I suppose I can suck on it to see if it's bad.

You could install a shunt between the hoses, just a couple tees with a ball valve plumbed between. Open it in the summer to reduce hot water going to the heater core, and close in the winter to put all flow through it he heater core.

Curtis73 (Forum Supporter) said:

In reply to garaithon :

I suppose I can suck on it to see if it's bad.

Let us know how it tastes

Ok, have to revisit this.

I was digging under the dash to find the vacuum lines.  I found one severed, so I fixed it.  Same problem.  I replaced the check valve.  Same problem.  I checked the vacuum hoses from the vac reservoir to the HVAC.  All good.  I even traced them with a hose to my ear to listen for vacuum while the engine was running.

Am I correct in assuming that it almost has to be one of the vacuum actuators?  That's the only thing that makes sense.  At low throttle, there is enough vacuum to operate things, but when you go heavy throttle (when manifold vac drops and you're reliant on the reservoir/check valve) it give up because there is a leak somewhere between atmosphere and the check valve.  That almost has to be an actuator.

Put it this way... if it isn't an actuator, I'm not keen on tracing a half mile of vacuum hoses to potentially find a pinhole.