How to flush the heater core

[buickwagon]

There have been a couple of recent questions about lack of heat, and some of the answers (my own included) suggest flushing the heater core without really explaining to the uninitiated what that entails. Since it's such a quick job, I took a half hour to snap some photos. Note that this is of an LT1 model, the earlier TBI cars are piped a bit different and I'll mention the differences but don't have one at hand to show photos.

The first thing you will need is an adapter to connect a garden hose to the heater hose. You can pick up one of these (or similar) at any auto parts supply shop:

(I added the brass quick-disconnect to match my garden hose). There's some plastic "tee" fittings around too, but they are inserted permanently and I'm not a big fan. The one pictured is aluminium and features a Schrader valve fitting for those really stubborn clogs.

Unless you are draining and refilling the entire cooling system, you can pinch off the engine side of the hoses to minimize coolant loss. You can buy special clamps for this purpose, but Vise-Grips work well too. Three are required for the LT1 -- the two from the engine and the one under the reservoir.

Separate the supply hose by releasing the hose clamp down stream of the black plastic restrictor and gently twisting the hose a bit while pulling it straight back. You shouldn't use a lot of force and you should pull straight back to avoid cracking the plastic nipple. The restrictor is labelled with an arrow showing the normal flow direction.

If you look inside, you can see the 1/4" restriction. It's there to control coolant flow at high rpm. Obviously it would also minimize flow during flushing, so we want to be down stream of it:

The return hose is similarly separated from the tee to the reservoir, and the hose line adapter inserted in the line. Note that we are flushing in the opposite direction from normal flow. Turn on the water and run until you get a nice clear stream. In extreme cases, small shots of shop air through the Schrader valve create a turbulent flow that helps scour the piping and blast out the crud. The air bubbles will come out somewhat explosively though, so make sure the discharge is pointed away from you! Don't introduce shop air unless the water is flowing -- you don't want to overpressurize and damage the heater core!.

When you are happy with the results, shut off the water, disconnect the adapter, drain the heater core (you can gently blow into the return hose to facilitate this) reconnect the two hoses, release the clamps and add about 1 cup of pre-mixed coolant to the reservoir. Crack the bleeder valve on top of the thermostat housing (pliers are probably required) to bleed any air bubbles and run the engine for a couple of minutes before shutting off the bleed valve (CAUTION: if your car is a tow-pack model, your hand will be perilously close to the mechanical fan. DO NOT ATTEMPT TO OPEN/CLOSE THE BLEEDER VALVE WITH THE ENGINE RUNNING.) Close the bleeder valve securely, but don't crank hard on it -- it's pretty tiny and easily damaged by too much force.

The TBI models are similar in principle, however the supply to the heater core comes from the manifold and the restrictor is built in to the hose. The easiest way is to separate the quick connect fitting of the supply hose right at the firewall connection to the heater core. If I Recall Correctly, it's the hose to the passenger side of the car, but you can quickly trace the hose. The nice thing about the TBI is that you don't have to clamp off the hose -- just keep it up above the level of the engine.

The heater core return hose goes straight to the radiator near the fill cap and it has a traditional hose clamp fitting. Simply disconnect the return line at the radiator and insert your flush adapter there. Then flush until clear as described above.

 

[mrnewland1]

Awesome! Thanks for taking the time for the write-up. I plan on flushing mine tomorrow.

Also, what is your opinion on pulling the knock sensors to drain the coolant? (94 RMS base model). Some say DO NOT touch knock sensors, as way too much can go wrong, but other folks seem not to have any problems with them. My '94 was garage kept most of it's life and is relatively rust-free.

 

[buickwagon]

They are kind of fragile. I think if you have a serious problem, like Dexcool sludge, then you should probably risk the damage and pull them, knowing that you may end up having to replace them.

If you're just flushing with plain water when changing the old antifreeze out, then you can safely leave them. But if you leave them, you would be best to mix your own A/F and use a hydrometer after running the engine so you can compensate for the excess water trapped in the block.

 

[dwightlightning]

I have a 92 TBI RMW that I purchased recently for a winter car. To ensure reliability I flushed and changed the engine oil, changed trans fluid and filter, rear diff fluid w/posi additive and ran it for 3 or 4 days with Prestone rad and cooling system flush and water in it. I drained the rad and cleaner, refilled with water, ran it a day, drained it and refilled with water, ran it a day then drained it and reverse flushed the core. The flow was very good. I then installed a 205* thermostat. The old one looked like new and the inside of the intake manifold and rad were very clean, due I guess to the cleaner.
I changed the rad and heater hoses and serpentine belt, then I filled it with 50/50 distilled water and AC Delco anti-freeze (non-DexCool). When I run it with the rad cap off the flow from the heater return hose is very strong into the rad.
My problem is that it takes quite a while for it to warm up and produce interior heat. I'm in Alberta and it's been about 0*F all week. After driving for a while it warms up to about 205* according to the temp gauge and remains there as the T-stat cycles open and closed.
My guess is that the long initial warm-up is due to the heater return hose allowing semi-warm water to exit to the rad as opposed to remaining in the engine behind the closed thermostat. How does the engine replace this coolant that leaves the heater core if the thermostat is closed? I can't seem to wrap my head around this.
Would my engine warm up faster if I replaced my water pump with one that has a fitting on it that I can connect the heater return hose to?
Wouldn't this keep all the coolant in the block until it reached 205*? Where am I going wrong in my logic?

On a side note, I have the auto climate control and when I have it set to "floor" it doesn't flow much air at all. On defrost and dash the flow is fine. Is there an actuator that fails? It shouldn't be the blend door actuator because the temp is hot. I want to pull the dash apart under the glovebox but it's been so cold I haven't yet. My whale is too long to fit in my garage.

Thanks,
Dwight

 

[buickwagon]

I wonder if your restrictor is missing from the heater hose.

 

[dwightlightning]

I wonder if your restrictor is missing from the heater hose.

It must be, because I replaced the heater hoses with regular hose like those which were on it. They weren't the factory original heater hoses.
Does the restrictor slow the flow through the core allowing better heat transfer?
I don't like how the return line from the core connects to the rad. I guess the water pump must just pull ice-cold anti-freeze from the bottom of the rad through the lower rad hose to replace the coolant that goes through the core.
It kind of makes the thermostat moot, until the coolant in the engine as well as the coolant in the rad equalize temperature. Finally, at that point, the thermostat actually does something, it exchanges hot coolant in the engine for cooled coolant in the rad.
The way this is setup, the thermostat is more about engine cooling than engine warm-up. Even in the summer I believe this setup is leaving some MPG and power on the table by not allowing the engine to come up to operating temperature as quickly as possible. Is it possible that the engine even stays in closed loop until the computer's coolant temperature sensor reaches a pre-set value?
I'm going to get a new water pump with the fitting to attach my heater core exit hose to and see what happens. If it works worse or overheats I can just put a pipe plug in the fitting and reattach the hose to the rad nipple. At worst I'll have a new water pump.

 

[HotZ28]

Is it possible that the engine even stays in closed loop until the computer's coolant temperature sensor reaches a pre-set value?

The engine will stay in "open loop" until the engine temp reaches about 140°, then if all sensors are reporting/sending correct data, it will go into closed loop.

 

[buickwagon]

With the thermostat closed, it is necessary to circulate water through the pump and the top of the heads as the engine warms up to prevent damage to both the pump and the engine. The heater core is inserted into that bypass to take advantage of the hottest part of the engine.

So this system takes coolant from the rad to the pump, which pushes it through the top of the engine and from the back of the manifold, through the heater core and back to the rad. (An alternative is to bypass the rad completely, returning the coolant to the pump instead of the rad, but drawing it off the bottom of the rad ensures that no air or vapour is introduced into the system.) To limit the amount of heat removed during the warm-up, a restrictor in the line from the manifold to the core controls the coolant flow.

Without the restrictor, more heat will be removed from the engine but the temperature of the coolant in the line will be lower, so warm-up will be delayed but the air coming out of the HVAC ducts will feel cooler. Once the engine gets up to operating temperature, the thermostat can allow any additional flow required to prevent the engine from overheating.

The restrictor was built in to the factory hose on the 92/93 Roadmasters. If you replaced the hose with straight hose, then you have no restrictor. You can buy and add an aftermarket restrictor, but I don't recall the exact ID required off the top of my head. Here's a photo that google produced, I'm sure there are others out there:
http://jagsthatrun.com/Pages/Parts_Cooling_HH-splicers.html

 

[Bohb]

Thanks for the guidance. Solved my no heat issue.

 

[SKV]

I would just disconnect both heater hoses and lower one end down so the coolant and water flow into a bucket that I'd put on top of a wide heater tank plastic pan to catch the coolant in the beginning. If you raise the ends of the hoses that go towards the engine, I believe the coolant loss from the expansion tank will not occur or will be limited. I used to have this narrow garden hose adapter that would fit inside the heater hose but last time I just put the 'pistol' like garden hose adapter set on stream to the heater hose and pressed to the 'trigger' to open water. It was enough to provide the water flow. Then I would reverse the hoses and let the water flow in the opposite direction. I did this reversal a couple of times. I blew into the heater hose and it forced most of the water out. Then I reconnected the hoses, added coolant, bled the system a couple of times and now I almost have a sauna in the Buick on high heat.