That line bypasses the T-stat. Yes, it goes to the housing, but there is a passage right into the block. I forgot to post this pic.

 

In, right there, (photo) with the lower radiator hose off. Never put cold water in a hot engine. Let it cool after whatever "product" has had its "specified" time to work, then only there (in photo) so you are not pushing back into the hearer core.

Those clamps on the heater hose you see in the photo are where the Prestone flush deal (the PO used), failed and made it puke it's coolant and overheat.

 

Are you saying Preston's kit isn't good? Also how do you by pass the heater core? Did you just buy some tube to connect them? When I put the product in do I disconnect the heater core cables then?

 

And this method will flush everything out? And I can just run a hose to the core to flush that out?

 

Buddy of mine is saying to take out the tstat and put the housing back on then flush it all through the upper rad hose with the heater full and engine on. But I'll have to get a new tstat and gasket?

 

Prestones kit is "almost" useless on a closed system, the only thing I believe you could use is the water hose adapter and even that is not necessary having both heater "hoses" unhooked. Now if you are talking about the radiator flush stuff it's okay... just bypass the heater core and flush that with fresh water "both" directions till clean.
Yes you can pull your T-stat, but don't have to.If your T-stat was fine then run it if you trust it, you don't always have to get a new one. The new gasket if you pull the T-stat I would get some gasket shelac, this stuff when applied right will allow you gasket to be re-used within reason.
If your engine is full of crud the upper hose flush may not be sufficient.

 

Sounds good, are you saying to unhook the lower rad hose and hook the garden hose to one of the heater core hoses and flush it out through?

 

X-2 on rad drain. Had problem with it on ex's Grand. On hers and son's XJ I just pulled lower hose from engine to drain.

As to flushing, I used a 3/4" double barb nipple and 12" of 3/4 hose to attach smaller (5/8, lower? on water pump) hose to thermostat hosing, and hose from hosing is long enough to go to water pump tube. This allows reverse feed to heater core.

There are cooling system flushes available, some one part, some two. I have used two part in past with satisfaction, which are some kind of acid and neutralizer. I actually used about a cup of muratic acid, clear water flush, then baking soda in flush, then more clear water flush.

BLA, bla time consuming. Got the job done

 

Drain the radiator then reconnect and refill with water.
use the heater hose(s) for the flush, both disconnected so water will flow out for flush. Do the heater core by itself, this seems to garner better results.