97 Blazer No Heat Check engine soon

I flushed and replaced the coolant. I also replaced the Themistat. The truck was running very cool. I still have no heat, and now the service engine soon light it on. I have read all about the heater core and Dexcool. I am not sure where the problem lies. I read GMerlin talking about using the AC for heat. I tried that last winter and it blows the same temp when I use the heater alone . Perhaps that's a clue? If anyone has any thoughts on this please let me know. :loser:

Have you tried the therm. diverting valve it works off of vacum pressure on you control panel. Is it opening to allow the water to run thru the heater core?

As for the service eng. light I do a tune up and it goes off. not that is the issue with yours. Just a thought.

Thanks for replying. The check engine light went off on it's own after driving it for a bit. I let the engine cool down and I really worried now. When I removed the cap there was chunks of something stuck right at the fill tube, and the fluid level was low. I checked for leaks, I found none. I filled the rad, and let the term open , and filled till full. While I was waiting for the term to open, the fluid that came out of the rad was clear. No more debris. I don't know how to tell it the switch is working to change the air flow over the heater core. I took the control panel out, and didn't see anything that looked like a vac. line like I was expecting. The temp switch is a potentiometer. I am getting very slight heat now I think.

See my previous posts on this subject.

As for the chunks, you are likely on the right path when you mention the GM bulletin.

Unlike other vehicles you can only check the radiator fill level when the enginee is cool. The system is under pressure which lowers the boiling temp of dexcool. As soon as you remove the cap the preasure is released and the dexcool will boil spilling out of the radiator. I suggest that you wait until everything is cool. Fill the radiator and overflow to the hot level. Let the engine warm up and then start. Check for pressure in the rad hose first as I suggest in previous posts. Just give it a squeeze. You will feel preasure inside the hose if it is there. If not it will feel mushy and there will be some kind of leak, now to find it. Still might be the cap or crud between the seal and radiator neck. (Just thought of that one!). Ultimately the system should always operate with 15psi in the rad.

Then check to see if you have flow through the heater core. This should be there whether there is an open thermostat or not. I suspect the heater core is plugged and a simple flush may not be enough to unplug. A preasure flush of the core may be required to get all the stuff out. Last thing you want to do is replace the heater core!!!

Since you were low on coolant I cannot help but suspect some kind of leak somewhere. I would check this extensively. Again read my previous posts on this subject.

Goodluck