Where's the heat?

I have a 1998 GMC Jimmy SLT 4dr 4X4 SUV that I have owned since 2001. 1 previous owner only used it to tow a boat during the summer. I have maintained all necessary schedule mtce required in the owners manual as well as other repairs. My mechanic that I have used for 20 years on this vehicle and my other 17 yr old truck adv me approx 3 yrs ago about this Anti-freeze lawsuit issue that the anti-freeze reacts badly to metal and turns to rust (and yes I have flushed the radiator when required) it finally caught up to me by clogging my heater core, radiator, head gaskets. I started with flushing the heater core, fine for awhile, only to repeat that several times over couple months. My mechanic adv to replace the heater core which would cost $1000 but he didnt have time, so I contacted a speciality shop that only works on radiator/air conditioners… They adv me they were familiar with this rust issue and that a clean out of the radiator was the appropriate road. This was removal of the radiator, let it sit in some kind of bath overnight which removes all rust etc particals. I inquired about replacement instead, as if the rad was compromised would rather go that route, they said no. I then had to have the gaskets replaced, they also replaced the radiator cap, this cost me a total of $1300, the heat returned wonderfully for approx 2 mths and then went cold, the anti-freeze was fine, I returned to the specialty shop, over the next month they flushed the heater core 3 times as it kept going cold, however after this last time I now have lukewarm heat. The thermostat has been replaced twice during this entire process as well. Am I going to have to replace this heater core anyway? or is there something else I need to look at?

Probably the engine block itself is full of rust, and sediment. If this is the case the engine block will need to be flush out. This could include removing engine block drain plugs, freeze plugs, and or the water pump, to be able to flush out all the crap laying in the bottom of the engine block.

On a Ford Taurus a once owned that used to plug up with rust I removed the water pump, and then shoved a cut end of garden hose down both sides of the water jacket ports deep down into the engine block, Then turned the water on full blast to gave each side a enema. I would also rap a rag around the hose to create a seal to fill the block up with water from the bottom, and then remove the rag to let the water pour out the block. Of course all the antifreeze needs to be properly removed before flushing.

                                                          Good Luck