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On a bone stock, 96 BBB with 137k on the odo. Noticed the other morning that the heat wasn't blowing hot air. At best, it was warm when the fan was on low. When I cranked the fan up to the highest setting, the air got cooler. I popped the hood and noticed that the top heater hose going to the core was lukewarm and the bottom heater hose was hot to the touch. This is after a 30 minute drive to work in the morning. This is the second core in the car - I replaced the original several years ago. No leaks nor any strong coolant smell, so I'm wondering if its time to replace the core once again? Could it be anything else?
 

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With new HC just a few years ago, only thing might be plug-up again from contaminants using non-distilled water???? Recommend hard clamp and disconnect two hoses, gentle flush HC both directions - just to remove that variable. Otherwise, vacuum hose or electric actuator on diverter door????
 

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It's probably not the heatercore's fault.
1) If the search function ever catches up, you'll find myriad posts mentioning that, to seriously flush the system, you must remove [and often replace] the knock sensors which double as freezeplugs for our iron blocks, then BACKflush the engine, the radiator, and the heatercore separately.

There is ALWAYS some amount of sediment at the bottom of our blocks that gets distributed around our cooling system whenever the H2Opump is running.
Problem is, it tends to get stuck in the heatercore due to its inherent flow restriction.

If you BACKflush the heatercore only, heat will be restored temporarily, but sooner or later, detritus in your cooling system accumulates in your heatercore again.

2a) Did the replacement heatercore have foam around the perimeter? Few years ago, mine didn't. Colder it was outside, colder the air coming out.
It's all gotta seal up tight.
2b) Pretty much what 96 Black said - vacuum hose [electric actuator if you installed Buick or Cadillac HVAC controls], or something else is stopping your diverter door from sealing up tight.
 

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Isn't there a one way valve located on the upper hose going to the heater core? If I flush the core, do I have to be careful as to which hose I use for attaching the garden hose to or can I run the water through both hoses?
 

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Nope. No one way valve.
 

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Actually the black plastic bulge in the heater hose (we call it the restrictor valve) can affect flushing with a garden hose. I recommend disconnecting the hose there so you can eliminate it from the circuit while you flush. On most cars, you can just unhook the worm clamp there and flush both directions. I let it flow into a 5-gallon bucket so you can inspect the debris. If nasty stuff comes out, it's time to flush the whole system as others have recommended.

While you are at it, inspect the restrictor valve for cracks and brittle plastic. It is very common for that part to crack and fail suddenly. Consider replacing it. Do not bypass it with a straight pipe. You may need to cut open the factory crimp to remove the one side if it hasn't been replaced before.
 

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Isn't there a one way valve located on the upper hose going to the heater core?
For some reason, I have a vague memory …
In the 91s, wasn't hot coolant flow to the heatercore regulated with some kind of diverter valve?
By 94, the heatercore's coolant flow was based on H2Opump RpM, then regulated by a fragile plastic spring-loaded variable restrictor thingy to protect it from high RpMs - I think?
But I think the stuff is on the LOWER hose.

If I'm wrong about any of the above, we'll all learn something, so please feel free to correct me!
ANYWAY …
If I flush the core, do I have to be careful as to which hose I use for attaching the garden hose to or can I run the water through both hoses?
When you flush the heatercore, you wanna flush it BOTH ways, til it flows clean both ways. Ideally, the same applies to separately flushing the radiator, and the engine.

Problem is one of the heater hoses will be less cooperative than the other because of the relatively delicate plastic thingies attached to it.
I imagine it would be easier to temporarily use the simpler heater hose to attach to either end of the heatercore for the purposes of flushing it both ways.

Can't hurt to add a bottle of either LubeGard KOOL-IT or RMI-25 Coolant Treatment.
 

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Is that plastic valve crimped on one side (to the hose that goes to the water pump)? My fear is if I break it while removing the hose with the clamp, can I replace the valve itself or do I need to buy the hose assembly that has the valve already attached.
 

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On a stock configuration, only one side is crimped if I recall from memory. If someone already replaced it once, then it may be clamped on both sides.

Both configurations are sold: the hose + valve assembly and the valve by itself. Several years ago, the entire hose assy was way more expensive than the valve itself. And if you have green 9C1 hoses, it's NLA. So I always just replaced the valve by itself. Advance Auto and Autozone part number: 74795. Showing for $12.99 as of today.

It's easy to replace the restrictor valve if it's never been done before:

1) You can either cut off the crimp part and stretch the hose and add a new clamp. That method is super fast and easy and can be done with a razor blade to cut the hose. But stretching the hose an extra inch or two comes with its own challenges and visual detractions that I never liked.

2) Cut off the crimp so you don't actually cut any rubber hose length. There are a few methods mentioned over the years. But I use a Dremel with a cut-off wheel, make 2-4 slits in the metal crimp longways and pry it off in pieces. When you cut the crimp, just dig deep enough to score the metal. Once you break through the metal, you will start cutting into the hose (not ideal). Scoring the metal as deep as possible without going through will weaken it enough to crack and pry off the pieces with a screwdriver or other tool. If you don't want to keep the old valve at all, after making the slits, you can just crunch the crimp with pliers to work it loose but it will shatter the plastic nipple in the process. Four or more slits seems to be best if you have the patience for the cuts. But it can be done with only two with some practice.

Good luck!
 
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