1999 Sirerra New Body Type Heater Inop

[1953poot]

Ok, late last year my heater started blowing likewarm then I noticed the temp gauge was lower then it used to be so I flushed the system, installed a new thermostat and all the hoses. Afterward the temp got up where it should, like around 210 but hard to tell on a factory gauge, and I then had heat. Rocked on for about 3 weeks then the heat went back to lukewarm. I felt both hoses and the "TO" the core hose was so hot I could hold it only a couple of seconds. The "FROM" from hose I could hold for several seconds which told me water was getting to the core, but not out of, or at least not nearly as much as in. I flushed the core then back flushed it and all was well for.......3 weeks, then it went lukewarm. I've done this twice already and yesterday it went lukewarm for the third time. It's got 154K miles on it with a 5.3 and has no other issues, no oil or water leaks. I'm thinking about installing a system flush "T" in the colder of the two hoses to burp it every 3 weeks. Good truck and runs like a stripped behind ape but this problem every 3 weeks is getting frustrating. So what the heck might be causing this? Is it air locking or what?

Thx

Poot

[txab]

Sounds like it's simply plugging up. Might be time for a new core and a really thorough flushing. You never said what the flush water looked like. If it had trash in it or not. Unless you're loosing water, then you're not likely to have it become air-locked. A full system will prevent that from happening