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LTX and LSXPutting LT1s, LS1s, and their variants into Third Gens is becoming more popular. This board is for those who are doing and have done the swaps so they can discuss all of their technical aspects including repairs, swap info, and performance upgrades.
I decided it was time to figure out why my vents won't switch between defrost and vent. Apparently I have something hooked up wrong because its stuck on defrost.
Where do the two vacuum lines that come from the firewall go? I got rid of the vacuum canister, thinking it wasn't needed but now I'm not sure. I have both of the lines hooked into a T, which is routed to the rear of my LS1 intake manifold. Are both of these lines supposed to have vacuum or does just one?
This has me stumped... as I haven't been able to find any schematics on how this vent system operates.
Hopefully someone can point me in the right direction...
The tee is actually a check valve that opens/closes the hose to the intake manifold. The check valve works in conjunction with the vacuum canister to hold vacuum on the HVAC controls temporarily when there is not adequate vacuum in the intake manifold. If you don't have the vacuum canister then your HVAC will seem demon possessed while driving.
The HVAC doors are operated by vacuum. Sounds like your vacuum lines aren't hooked up in the correct sequence. Here is a copy of the HVAC doors schematic,
Thanks for shooting me in the right direction QwkTrip. My vacuum lines definitely aren't hooked up right. I should have thought this through before removing the original L98, but I didn't. Ordered a new vacuum canister and check valve. I'm debating on what to do with the heater valve though. Should I install it or leave it out? Hawks told me they don't use it with their swaps. But I wonder how bad the heater core would heat soak the car in the summer?
I'm going to at least install the canister and check valve, so the blend doors work as they should. Thanks for the help!
The heater valve on the intake is just there to shut off the coolant flow to the heater core when AC is in the max position.
My car didn't come with that valve from the factory. I've been running through the heater core without any problems.
It only bypasses coolant when the temp lever is ALL the way on cold, not outlet lever dependent, it took me a while to figure that out. It's almost not even necessary since the blend doors cover the heater core from air flow when in the A/C position.
As others have said... one line is hooked to vacuum to supply the HVAC controls with vac power. The vac canister/reservoir is good to have, because when you stomp the gas or anytime the vac goes to zero, you're vents will go to defrost. It doesnt hurt anything to have it in the system, and you just need to plumb it with a check valve.
The other line supplies vac to the heater control valve when you put the controls into the hot position. There is a mechanical vac switch inside your controls that handles the distribution.
I fully support the idea of running a heater control valve. It has to be the correct one for the LS1 swap though. It cant just shut off coolant flow to the heater core and thats it. It has to be connected to both heater hoses, and when clsoed it has to loop the coolant back to the water pump.
Here are two threads I made that might help. One shows the controls and kind of explains the vac lines. Not very good....but might help. The other is a thread on the heater control valve myself and others have used. Its cheap, splices right into you 5/8" and 3/4" heater lines, and will accept and work off the vac line.
For what its worth, I noticed a HUGE difference in A/C performance when I installed the valve. Regardless of where the blend doors are, the cold air has to pass through the outer HVAC box, then inside through the rest of the HVAC box. If the heater core is in there, sitting at 180-200 degrees, it makes the entire box and area hot. EVen if a blend door covers it, the blend door is hot. That all nips away at the a/c air coming in little by little and then you get mediocre performance. Just my
I looked at a bunch of threads about vacuum lines in a LS swap and this one kinda fits my issue. (I know this is from 2016 but I didn’t want to start a new thread)
1987 ls1 swap. I’m trying to hook up my HVAC vacuum lines. I have two lines but then a T fitting (may be a check Valve but I’m not sure). In total there is 3 fittings which is throwing me off. My car had cruise control but I ripped that out. I have the ball vacuum reservoir in the original place.
issues:
1. I’m not sure which line goes where.
2. do I have the check valve already? Do I need to buy anything else?
3. how do I clean up the lines, length is too long.
please see picture
Thanks in advance and any help is appreciated.
Last edited by Urocmyiroc; 02-18-2022 at 07:46 PM.
11-28-2016, 09:09 PM
