Since I recently decided to put a/c back on my car I needed to put a bypass valve inline on the heater hoses to keep the conditioned air from blowing across a hot heater core. You'll notice that the stock 3rd gen bypass valve has only 3 outlets and you need 4 for an LSx swap (2 to heater core, 2 to water pump).

The part number for the valve is 74781.

I plan on painting the two 90* brass fittings black so they don't stick out like sore thumbs, but I literally just finished this today and wanted to get it all mocked up. I also wish the vacuum port was facing rearwards but it is what it is.

 

Great find, I would be interested in the part number. Vintage air has a one inlet one outlet valve. I could never get an answer supported by facts on whether or not the ls water pump needed constant circulation (heater lines) or could simply be turned off.

 

I've looked for the box it came in and don't believe I have it anymore. I wish I knew what the number was too in case I ever need to replace it.

 

On second thought it looks like it may be from a Ford Ranger, based on a Google image search.

 

Ghettocruiser did a similar swap about 4 yrs ago or so.

https://www.thirdgen.org/forums/ltx-...-valve-ls.html

I followed his lead and used some premade hoses that allowed for the hoses to follow the ac box rather than going over the top of the motor. The part numbers and pics are in that thread as well.

it works great and I am very happy with it.

Enjoy!

 

It does need constant circulation. If you're plugging the HC lines, swap the LS1 t-stat for a SBC version. This way coolant continuously circulates through the WP and the t-stat still regulates in/out of the engine

 

Looks like we're using the same valve, part number 74781. I'm using the same hoses as well it's just that I cut the 90* elbows off and used them on the valve side instead of at the heater core.

 

Thanks, that helps me a lot.

Still did a good job on your own car though.

Thanks, that helps me a lot.

 

Lol it ended up being the part number in the link above, I edited the original post so others can look it up at the parts store.

 

Same part number as mine . Not sure if it's just mine but I had about 3 defective units since I did my swap .

 

Used the same valve, but bought some formed hoses to clear the engine.



I forget the numbers but Napa has 5/8" & 3/4" hose sections with tight formed 90's on them that you can use to clear the engine.

 

Holy crap that was 4 years ago?!?!?

Time is going by WAY too fast...

My valve is still in and working great! When I rebuilt the 86, I considered using metal tubing for the heater hoses. Similar to the 4th gen setup. In fact, I still have the 4th gen heater hoses, and I looked at modifying them again. My main reason for just using the hoses..is that they are available locally for super cheap, and it makes for easy mounting of the heater control valve.

I too used the hoses with pre-bent 90* angles. Works great to get them out from behind the motor.

J.

 

My AC wiring has 2 vacuum lines coming out does it matter which one goes to the intake and which one goes to the valve?

 

Yes, it does matter. Short tube goes to manifold vacuum, and long tube goes to heater coolant bypass valve or is capped.

The short tube doesn't actually connect directly to the manifold though. It goes to a vacuum check valve. The other two legs of the check valve go to the manifold and the vacuum ball (reservoir). The purpose of the check valve is to close off the line to manifold when manifold loses vacuum. The vacuum ball will allow the system to temporarily hold static pressure so the HVAC controls don't do funky things.