Since the temperature has dropped really dramatically here, I've been noticing that my heat sucks. On warmer days, it seems to be fine, or at least good enough to keep me comfortable. I flushed the heater core this fall, and had good heat after that. If the temp was around 40 degrees, I had to really dial back the heat in order to not get cooked, and based on what I've found on recent warmer days, the performance of the heating system has not changed since I flushed everything, so I believe the heater core is ok. The lower hose for the heater core is a little cooler than the top one, but they both seem to be fairly hot. However, on really cold days, I get almost no heat. Also, I noticed that if the engine is off or at low RPM, it blows ice cold air. Engine temp is right around 195-210, and heats up quickly. T-stat and water pump are both less than two months old, and it had a coolant flush and serpentine belt at the same time. I know I had a very small radiator leak, but I used stop leak and I haven't lost any coolant since. I doubt the stop leak clogged anything it shouldn't since the system was flushed literally the day after I added it. Here's a rough timeline:

1. Heater core flushed Fall 2014, had good heat in the warmer temps
2. Early January 2015, I added stop leak to stop the tiny radiator leak
3. The next day, water pump goes out
4. Water pump, T-stat, and serpentine belt replaced, coolant sytem flushed
5. February 2015, temperature plummets and I start to notice the weak heat

Any thoughts on what could be going on here?

 

What year, engine?

 

2001 4.0

 

Try flushing core again. Especially in through smaller hose. Can't hurt.

 

+1 on flushing the heater core again. I think the PO of mine put stop leak in a some point and despite changing most of the coolant twice while doing repairs I still have to flush my heater core often. I think they are prone to clogging as it is so even small amounts of stop leak probably just speed up the process.

 

It can't hurt to flush it again. The issue is that I have absolutely no access to a hose right now. I'm in school for mechanical engineering, and my college is heavily co-op based. Basically my terms are 11 weeks long, and after each, I go to my co-op and work for 12 weeks. Right now I'm at work and going on my third month in a row of living in a hotel. Any ideas of places that might have a hose I could borrow?

 

Got any friends at work that would give you a hand?

 

I could give it a shot, although I don't know how many people still have hoses out now.

 

If the engine is off, there is no circulation of hot coolant through the heater core and nothing bringing engine heat into the cabin. At idle it should be piping in heat, but since the water pump is driven off the engine, it wouldn't be circulating as fast as at higher RPM. If your heater core is partially clogged, it may not have enough flow at low RPM to keep you warm.

AHA! If you didn't get all that stop leak out, it may have migrated to the heater core. Or some of the stop leak may have taken up residence in the heater core when you first put it in, and you just didn't notice the difference because there was still enough flow for heat in the warmer temperatures.

It does sound like you need to backflush the heater core again. Not sure what to tell you about finding a way to do that. You might just need to bundle up through this winter and fix it when you're in a better position to do so.

 

As long as we don't hit -25 degrees again like last week, I'll be perfectly fine for the rest of the winter. It really is adequate when the temp is up around 20 degrees and higher. I understand the water pump doesn't work if the engine isn't running, but usually they have some residual heat, especially after driving for 25 minutes. I'm just hoping it isn't something more serious than another flush.