With the cold lately, I’ve been seeing a lot of cars come in with no heat concerns. There’s varying reasons why, but it’s had me thinking about whether or not it’s possible to actually test whether or not there’s air in a heater core.

Currently the only thing I know to do is to spend time trying to purge them, but some vehicles (Chevy uplander for example) can take a long time to properly purge the heater cores. That’s not ideal when you’re trying to minimize the time spent on a diagnosis, so I’ve just been trying to think of a way to actually test for it.

 

where you purging it separately?. Unhook the hoses from the pump, fill up the core with a strong hose stream. Plug it back in.

 

no idea how to test for it; so i am subscribing for a answer,

I do sometimes hear air gurgling thru the core when i step of the accelerator. no heat at idle and good heat increase at 2500rpm is a sign of low coolant.

 

I’m not talking about one specific vehicle that I had trouble purging the heater core. I understand there’s plenty of methods for purging them - what I’m trying to do (and for some reason didn’t actually ask the question in my first post) is discuss ideas for a method for accurately testing for the presence of an air pocket in the heater core.

It’s funny how sometimes the most obvious answers are hidden from us. I’ve heard the gurgling sound so many times and didn’t even consider what it meant. The gurgling sound is definitely an indicator of air in the heater core - so that’s one test!!


I had a couple different ideas:

1) Use a thermometer to measure the surface temperature of the heater core in different areas - then, a significantly cooler area would indicate air being trapped. The problem with that is most heater cores in today’s vehicles aren’t accessible enough to actually perform that test while in the vehicle, and obviously, removing it for a bench test would defeat the purpose.

2) Test system pressure to see what it builds up to. As opposed to standard pressure testing, where a pump is used to pressurize the system and check for leaks, see if the system pressure reaches its rating.

For example, if the system rating is 15 PSI, then running the engine to temperature should produce at least 15 PSI of pressure. But if the pressure only builds to 10 PSI, the presence of air is indicated provided there’s no other leaks present.

My theory here is that air can be compressed, but liquid can’t, so an air pocket trapped in the system would effectively lower the overall system pressure because the rated pressure is only achieved by the coolant expanding and exceeding the system volume. If that’s true, then the rated pressure is never reached, and the cap never opens to purge the air. I’ve only developed this theory in the past few days so I’ve had no chance to actually test it, and I’m sure there’s several factors to consider.

 

How about putting a flushing tee in both heater hoses near the firewall, then you could easily flush the core with a hose; could even use a reservoir & pump with coolant as the flush source.

 

I don't think it would. If the system was not pressurized, the water would vaporize, and steam has very high pressure. That's why it drives locomotives, and when you open an overheating radiator, it blasts all over the place.

 

If the coolant is always circulating through the heater core, all you need to do is with the engine cool ,not hot, remove the highest coolant hose from the heater core while the engine is idling, just enough to allow the trapped air to exit. Expect to lose a little coolant, but you only move the hose off so the top is open and if air is in here it will come out.
Then push the hose back on and tighten the hose clamp.

 

Not always true. In the event of sudden pressure release, yes, the coolant will boil; but if the system is never pressurized to begin with, the coolant only boils if the heat isn't adequately transferred to the radiator and cooled off by the fans. I've seen several vehicles with faulty system caps that didn't hold pressure at all and otherwise ran fine and had no problems with boiling coolant. One method for purging cooling systems after major service is to fill the system as full as you can, then run the engine up to temp with the cap off and add coolant once the thermostat opens. This is also how some combustion leak detector kits check for the presence of exhaust gases in the cooling system.

I think the "expertly drawn illustration" below will help demonstrate my point.

For those that may need an explanation of the principles at work here -

Pressure in the cooling system builds when the coolant expands as it's heated, due to the coolant absorbing heat from combustion. Eventually the volume of the fluid expands beyond the volume of the system, at which point, pressure begins to build and when the rated pressure is reached, the excess fluid is expelled through the radiator cap into the overflow container.

The reason pressure is necessary is because it raises the boiling point of the coolant, allowing it to reach much higher temperatures than it would at atmospheric pressure.

Since coolant is a fluid, it's classified as a hydraulic system. One principle of hydraulics is that pressure is equal at every point in the system. So, a rating of 15 PSI means the radiator, block, heads, hoses, and heater core are all at 15 PSI in a normally functioning system.

System rating is determined by many things including the materials used to build the engine, radiator, heater core, and hoses as well as the expected operating temperature of the engine. Most systems today are in the 15-18 PSI range.

One other principle of hydraulics is that liquids can't be compressed.

Air, on the other hand, can be compressed. That's why air in your brake system causes the pedal to feel spongy - because the air is being compressed by the fluid. That's the same principle I'm basing this theory on.


Keep in mind - this is a THEORY based on what I know to be true of how cooling systems operate and the properties of fluids and air. It is far from an in-depth analysis and I've not had the opportunity to test it or collect data. Even if it holds some truth, I don't know how much air in a system would be required for this to occur.

Now, obviously, the situation below couldn't actually happen as it's drawn - I'm only trying to provide visual aid to demonstrate my point.

 

I'm with you so far. It takes more volume of gas to squeeze that air pocket to 15PSI (like a spring), than a liquid. But I think it's easily reached. 15PSI isn't that much.

The temperature inside cylinders can reach thousands of degrees. More than enough energy to vaporize the coolant.

 

another variable will be coolant temp as pressure and temp are proportional.
time to test your theory with experiments!