OK here is the deal. Not getting any heat. I blew out the heater core with air compressor and use a chemical to flush it out. All kinds of junk came out. Repeated it again and flushed it out with air and water until it was clear. Have to do this every year. Have heat but only when driving above 1500 RPM's. Not sure if I replaced the termorstat last year or year before that. Is it possible that the termostat is not opening and closing if I only get heat while driving and no heat at idle? Thanks for any help.
BTW...Truck info: 1998 F150, V-6, 4.2 XLT.

 

Have you looked into other things in the HVAC system besides heater core/cooling system that control heat?? If you have to do that to your heater core once a year, it sounds to me like you've got some other goofy issues going on with your truck; that is most certainly not normal. Yes, I'd want to look into fixing that problem for sure.

Heat only above 1500 rpm does kinda indicate some type of potential flow problem (water pump not circulating good at lower rpm's maybe), but I'd probably lean away from the thermostat. If this were the case however, seems you'd have overheating problems as well??? If thermostat were stuck closed, truck should potentially run hotter all the time thus giving you heat at any time. If stuck open, this would potentially cause a scenario where the truck would run hotter at idle/traffic/lower rpm's than going down the road; thus you should have heat below 1,500 rpm's.

Just thinking out loud to help come up with ideas.

Whatever you come up with, having to flush out a heater core every year is certainly a problem. If the heater core is getting filled with "junk", that junk is being generated in the engine somehow and carried to the heater core...need to start there to find the source of that problem at least. Now that I think about it, if something is generating enough junk to clog your heater core, it may not be getting enough water circulating through it to generate heat at lower rpm's, which would indiciate a flow problem as I mentioned...again, just a thought out loud.

 

I have to agree with you on the flow problem. I read another post on here that had the same problem as I do. I am leaning towards the water pump. Truck has over 220K on it and the water pump is the original one. In the other post many guys told the other guy to check other things just like you have and it all lead to the water pump failing. Just wasn't pumping hard enough on idle only high RPM's. I plan on flushing the block when I decide to replace the water pump and might as well replace hoses and termostat too. Lucky for me I have a brother in law that owes me so I can get the parts I need cheap or free....LOL.....BTW, the book says that my type of truck has 2 drain plugs on the block one on either side. I know where the one on driver side is but I sure as heck can't seem to find the one on passenger side. Any idea???? Thanks for the info above.

 

Sounds like the impeller is rotting away on the water pump. That could be the source of the gunk and the low flow.

Have you been running straight water in it? That can cause it to rust up pretty bad.

Some trucks have a flow restrictor in the heater inlet hose, so when you flush it make sure to back flush to clean it out.

 

jgger... yeah thats what I figure it was after reading other post. And no, I don't run straight water. I figure cause of the age of the water (original) that the impeller has rotted away. Shoot... a 1998 with over 220K's, I am surprised that the water pump has not leaked yet....LOL....thanks for the info about the restrictor in the inlet hose. I didn't know but I did back flush when I flushed the heater core...Good thing I did....LOL